| 1. |
- Alfredsson, Viveka, et al.
(författare)
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The Dynamic Association Processes Leading from a Silica Precursor to a Mesoporous SBA-15 Material.
- 2015
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 1520-4898 .- 0001-4842. ; 48:7, s. 1891-1900
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Forskningsöversikt (refereegranskat)abstract
- During the last two decades, the synthesis of silica with an ordered mesoporous structure has been thoroughly explored. The basis of the synthesis is to let silica monomers polymerize in the presence of an amphiphilic template component. In the first studies, cationic surfactants were used as structure inducer. Later it was shown that pluronic copolymers also could have the role. One advantage with the pluronics copolymers is that they allow for a wider variation in the radius of pores in the resulting silica material. Another advantage lies in the higher stability resulting from the thicker walls between the pores. Mesoporous silica has a very high area to volume ratio, and the ordered structure ensures surface homogeneity. There are a number of applications of this type of material. It can be used as support for catalysts, as templates to produces other mesoporous inorganic materials, or in controlled release applications. The synthesis of mesoporous silica is, from a practical point of view, simple, but there are significant possibilities to vary synthesis conditions with a concomitant effect on the properties of the resulting material. It is clear that the structural properties on the nanometer scale are determined by the self-assembly properties of the amphiphile, and this knowledge has been used to optimize pore geometry and pore size. To have a practical functional material it is desirable to also control the structure on a micrometer scale and larger. In practice, one has largely taken an empirical approach in optimizing reaction conditions, paying less attention to underlying chemical and physicochemical mechanisms that lead from starting conditions to the final product. In this Account, we present our systematic studies of the processes involved not only in the formation of the mesoporous structure as such, but also of the formation of structures on the micrometer scale. The main point is to show how the ongoing silica polymerization triggers a sequence of structural changes through the action of colloidal interactions. Our approach is to use a multitude of experimental methods to characterize the time evolution of the same highly reproducible synthesis process. It is the silica polymerization reactions that set the time scale, and the block copolymer self-assembly responds to the progress of the polymerization through a basically hydrophobic interaction between silica and ethylene oxide units. The progression of the silica polymerization leads to an increased hydrophobicity triggering an aggregation process resulting in the formation of silica-copolymer composite particles of increasing size. The particle growth occurs in a stepwise way caused by intricate shifts between colloidal stability and instability. By tuning reaction conditions one can have an end product of hexagonal prism composite particles with single crystal 2D hexagonal order.
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| 2. |
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| 3. |
- Artero, Vincent, et al.
(författare)
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From Enzyme Maturation to Synthetic Chemistry : The Case of Hydrogenases
- 2015
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 48:8, s. 2380-2387
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Forskningsöversikt (refereegranskat)abstract
- CONSPECTUS: Water splitting into oxygen and hydrogen is one of the most attractive strategies for storing solar energy and electricity. Because the processes at work are multielectronic, there is a crucial need for efficient and stable catalysts, which in addition have to be cheap for future industrial developments (electrolyzers, photoelectrochemicals, and fuel cells). Specifically for the water/hydrogen interconversion, Nature is an exquisite source of inspiration since this chemistry contributes to the bioenergetic metabolism of a number of living organisms via the activity of fascinating metalloenzymes, the hydrogenases. In this Account, we first briefly describe the structure of the unique dinuclear organometallic active sites of the two classes of hydrogenases as well as the complex protein machineries involved in their biosynthesis, their so-called maturation processes. This knowledge allows for the development of a fruitful bioinspired chemistry approach, which has already led to a number of interesting and original catalysts mimicking the natural active sites. More specifically, we describe our own attempts to prepare artificial hydrogenases. This can be achieved via the standard bioinspired approach using the combination of a synthetic bioinspired catalyst and a polypeptide scaffold. Such hybrid complexes provide the opportunity to optimize the system by manipulating both the catalyst through chemical synthesis and the protein component through mutagenesis. We also raise the possibility to reach such artificial systems via an original strategy based on mimicking the enzyme maturation pathways. This is illustrated in this Account by two examples developed in our laboratory. First, we show how the preparation of a lysozyme-{Mn-I(CO)(3)} hybrid and its clean reaction with a nickel complex led us to generate a new class of binuclear Ni-Mn H-2-evolving catalysts mimicking the active site of [NiFe]-hydrogenases. Then we describe how we were able to rationally design and prepare a hybrid system, displaying remarkable structural similarities to an [FeFe]-hydrogenase, and we show here for the first time that it is catalytically active for proton reduction. This system is based on the combination of HydF, a protein involved in the maturation of [FeFe]-hydrogenase (HydA), and a close mimic of the active site of this class of enzymes. Moreover, the synthetic [Fe-2(adt)(CO)(4)(CN)(2)](2-) (adt(2-) = aza-propanedithiol) mimic, alone or within a HydF hybrid system, was shown to be able to maturate and activate a form of HydA itself lacking its diiron active site. We discuss the exciting perspectives this "synthetic maturation" opens regarding the "invention" of novel hydrogenases by the chemists.
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| 4. |
- Belda, Oscar, et al.
(författare)
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Molybdenum-catalyzed asymmetric allylic alkylations
- 2004
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 37:3, s. 159-167
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Tidskriftsartikel (refereegranskat)abstract
- The highly regio- and enantioselective molybdenum-catalyzed allylic alkylation reaction has become a powerful synthetic tool during the past few years. This Account describes the achievements gained so far in the area, with special attention directed to the different chiral ligands that have been used for inducing chirality in the products, the range of allylic substrates and nucleophiles employed, mechanistic studies, and applications of the reaction in asymmetric syntheses.
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| 5. |
- Bergström, Lennart, et al.
(författare)
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Mesocrystals in Biominerals and Colloidal Arrays
- 2015
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 48:5, s. 1391-1402
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Forskningsöversikt (refereegranskat)abstract
- Mesocrystals, which originally was a term to designate superstructures of nanocrystals with a common crystallographic orientation, have now evolved to a materials concept. The discovery that many biominerals are mesocrystals generated a large research interest, and it was suggested that mesocrystals result in better mechanical performance and optical properties compared to single crystalline structures. Mesocrystalline biominerals are mainly found in spines or shells, which have to be mechanically optimized for protection or as a load-bearing skeleton. Important examples include red coral and sea urchin spine as well as bones. Mesocrystals can also be formed from purely synthetic components. Biomimetic mineralization and assembly have been used to produce mesocrystals, sometimes with complex hierarchical structures. Important examples include the fluorapatite mesocrystals with gelatin as the structural matrix, and mesocrystalline calcite spicules with impressive strength and flexibility that could be synthesized using silicatein protein fibers as template for calcium carbonate deposition. Self-assembly of nanocrystals can also result in mesocrystals if the nanocrystals have a well-defined size and shape and the assembly conditions are tuned to allow the nanoparticles to align crystallographically. Mesocrystals formed by assembly of monodisperse metallic, semiconducting, and magnetic nanocrystals are a type of colloidal crystal with a well-defined structure on both the atomic and mesoscopic length scale. Mesocrystals typically are hybrid materials between crystalline nanoparticles and interspacing amorphous organic or inorganic layers. This structure allows to combine disparate materials like hard but brittle nanocrystals with a soft and ductile amorphous material, enabling a mechanically optimized structural design as realized in the sea urchin spicule. Furthermore, rnesocrystals can combine the properties of individual nanocrystals like the optical quantum size effect, surface plasmon resonance, and size dependent magnetic properties with a mesostructure and morphology tailored for specific applications. Indeed, mesocrystals composed of crystallographically aligned polyhedral or rodlike nanocrystals with anisotropic properties can be materials with strongly directional properties and novel collective emergent properties. An additional advantage of mesocrystals is that they can combine the properties of nanoparticles with a structure on the micro- or macroscale allowing for much easier handling. In this Account, we propose that mesocrystals are defined as a nanostructured material with a defined long-range order on the atomic scale, which can be inferred from the existence of an essentially sharp wide-angle diffraction pattern (with sharp Bragg peaks) together with clear evidence that the material consists of individual nanoparticle building units. We will give several examples of mesocrystals and discuss the structural characteristics for biominerals, biomimetic materials, and colloidal arrays of nanocrystals. The potential of the mesocrystal materials concept in other areas will be discussed and future developments envisioned.
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| 6. |
- Björneholm, Olle, et al.
(författare)
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Superficial Tale of Two Functional Groups : On the Surface Propensity of Aqueous Carboxylic Acids, Alkyl Amines, and Amino Acids
- 2022
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 55:23, s. 3285-3293
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Tidskriftsartikel (refereegranskat)abstract
- ConspectusThe gas-liquid interface of water is environmentally relevant due to the abundance of aqueous aerosol particles in the atmosphere. Aqueous aerosols often contain a significant fraction of organics. As aerosol particles are small, surface effects are substantial but not yet well understood. One starting point for studying the surface of aerosols is to investigate the surface of aqueous solutions. We review here studies of the surface composition of aqueous solutions using liquid-jet photoelectron spectroscopy in combination with theoretical simulations. Our focus is on model systems containing two functional groups, the carboxylic group and the amine group, which are both common in atmospheric organics. For alkanoic carboxylic acids and alkyl amines, we find that the surface propensity of such amphiphiles can be considered to be a balance between the hydrophilic interactions of the functional group and the hydrophobic interactions of the alkyl chain. For the same chain length, the neutral alkyl amine has a lower surface propensity than the neutral alkanoic carboxylic acid, whereas the surface propensity of the corresponding alkyl ammonium ion is higher than that of the alkanoic carboxylate ion. This different propensity leads to a pH-dependent surface composition which differs from the bulk, with the neutral forms having a much higher surface propensity than the charged ones. In aerosols, alkanoic carboxylic acids and alkyl amines are often found together. For such mixed systems, we find that the oppositely charged molecular ions form ion pairs at the surface. This cooperative behavior leads to a more organic-rich and hydrophobic surface than would be expected in a wide, environmentally relevant pH range. Amino acids contain a carboxylic and an amine group, and amino acids of biological origin are found in aerosols. Depending on the side group, we observe surface propensity ranging from surface-depleted to enriched by a factor of 10. Cysteine contains one more titratable group, which makes it exhibit more complex behavior, with some protonation states found only at the surface and not in the bulk. Moreover, the presence of molecular ions at the surface is seen to affect the distribution of inorganic ions. As the charge of the molecular ions changes with protonation, the effects on the inorganic ions also exhibit a pH dependence. Our results show that for these systems the surface composition differs from the bulk and changes with pH and that the results obtained for single-component solutions may be modified by ion-ion interactions in the case of mixed solutions.
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| 7. |
- Boschloo, Gerrit, et al.
(författare)
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Characteristics of the Iodide/Triiodide Redox Mediator in Dye-Sensitized Solar Cells
- 2009
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 42:11, s. 1819-1826
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Forskningsöversikt (refereegranskat)abstract
- Dye-sensitized solar cells (DSCs) have gained widespread interest because of their potential for low-cost solar energy conversion. Currently, the certified record efficiency of these solar cells is 11.1%, and measurements of their durability and stability suggest lifetimes exceeding 10 years under operational conditions, The DSC is a photoelectrochemical system: a monolayer of sensitizing dye is adsorbed onto a mesoporous TiO2 electrode, and the electrode is sandwiched together with a counter electrode. An electrolyte containing a redox couple fills the gap between the electrodes. The redox couple is a key component of the DSC. The reduced part of the couple regenerates the photo-oxidized dye. The formed oxidized species diffuses to the counter electrode, where it is reduced. The photovoltage of the device depends on the redox couple because it sets the electrochemical potential at the counter electrode. The redox couple also affects the electrochemical potential of the TiO2 electrode through the recombination kinetics between electrons in TiO2 and oxidized redox species. This Account focuses on the special properties of the iodide/triiodide (I-/I-3(-)) redox couple in dye-sensitized solar cells. It has been the preferred redox couple since the beginning of DSC development and still yields the most stable and efficient DSCs. Overall, the iodide/triiodide couple has good solubility, does not absorb too much light, has a suitable redox potential, and provides rapid dye regeneration. But what distinguishes I-/I-3(-) from most redox mediators is the very slow recombination kinetics between electrons in TiO2 and the oxidized part of the redox couple, triiodide. Certain dyes adsorbed at TiO2 catalyze this recombination reaction, presumably by binding iodine or triiodide. The standard potential of the iodide/triiodide redox couple is 0.35 V (versus the normal hydrogen electrode, NHE), and the oxidation potential of the standard DSC-sensitizer (Ru(dcbpy)(2)(NCS)(2)) is 1.1 V. The driving force for reduction of oxidized dye is therefore as large as 0.75 V. This process leads to the largest internal potential loss in DSC devices. We expect that overall efficiencies above 15% might be achieved if half of this internal potential loss could be gained. The regeneration of oxidized dye with iodide leads to the formation of the diiodide radical (I-2(-center dot)). The redox potential of the I-2(-center dot)/I- couple must therefore be considered when determining the actual driving force for dye regeneration. The formed I-2(-center dot) disproportionates to I-3(-) and I-, which leads to a large loss in potential energy.
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| 8. |
- Cahill, P. L., et al.
(författare)
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Creating New Antifoulants Using the Tools and Tactics of Medicinal Chemistry
- 2024
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Ingår i: Accounts of Chemical Research. - : American Chemical Society. - 0001-4842 .- 1520-4898. ; 57:3, s. 399-
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Tidskriftsartikel (refereegranskat)abstract
- Conspectus The unwanted accumulation of marine micro- and macroorganisms such as algae and barnacles on submerged man-made structures and vessel hulls is a major challenge for any marine operation. Known as biofouling, this problem leads to reduced hydrodynamic efficiency, significantly increased fuel usage, microbially induced corrosion, and, if not managed appropriately, eventual loss of both performance and structural integrity. Ship hull biofouling in the international maritime transport network conservatively accounts for 0.6% of global carbon emissions, highlighting the global scale and the importance of this problem. Improved antifouling strategies to limit surface colonization are paramount for essential activities such as shipping, aquaculture, desalination, and the marine renewable energy sector, representing both a multibillion dollar cost and a substantial practical challenge. From an ecological perspective, biofouling is a primary contributor to the global spread of invasive marine species, which has extensive implications for the marine environment. Historically, heavy metal-based toxic biocides have been used to control biofouling. However, their unwanted collateral ecological damage on nontarget species and bioaccumulation has led to recent global bans. With expanding human activities within aquaculture and offshore energy, it is both urgent and apparent that environmentally friendly surface protection remains key for maintaining the function of both moving and stationary marine structures. Biofouling communities are typically a highly complex network of both micro- and macroorganisms, representing a broad section of life from bacteria to macrophytes and animals. Given this diversity, it is unrealistic to expect that a single antifouling “silver bullet” will prevent colonization with the exception of generally toxic biocides. For that reason, modern and future antifouling solutions are anticipated to rely on novel coating technologies and “combination therapies” where mixtures of narrow-spectrum bioactive components are used to provide coverage across fouling species. In contrast to the existing cohort of outdated, toxic antifouling strategies, such as copper- and tributyltin-releasing paints, modern drug discovery techniques are increasingly being employed for the rational design of effective yet safe alternatives. The challenge for a medicinal chemistry approach is to effectively account for the large taxonomic diversity among fouling organisms combined with a lack of well-defined conserved molecular targets within most taxa. The current Account summarizes our work employing the tools of modern medicinal chemistry to discover, modify, and develop optimized and scalable antifouling solutions based on naturally occurring antifouling and repelling compounds from both marine and terrestrial sources. Inspiration for rational design comes from targeted studies on allelopathic natural products, natural repelling peptides, and secondary metabolites from sessile marine organisms with clean exteriors, which has yielded several efficient and promising antifouling leads.
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| 9. |
- Darmadi, Iwan, 1990, et al.
(författare)
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Bulk-Processed Plasmonic Plastic Nanocomposite Materials for Optical Hydrogen Detection
- 2023
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Ingår i: Accounts of Chemical Research. - 0001-4842 .- 1520-4898. ; 56:13, s. 1850-1861
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Tidskriftsartikel (refereegranskat)abstract
- Conspectus Sensors are ubiquitous, andtheir importanceis only going to increaseacross many areas of modern technology. In this respect, hydrogengas (H-2) sensors are no exception since they allow mitigationof the inherent safety risks associated with mixtures of H-2 and air. The deployment of H-2 technologies is rapidlyaccelerating in emerging energy, transport, and green steel-makingsectors, where not only safety but also process monitoring sensorsare in high demand. To meet this demand, cost-effective and scalableroutes for mass production of sensing materials are required. Here,the state-of-the-art often resorts to processes derived from the microelectronicsindustry where surface-based micro- and nanofabrication are the methodsof choice and where (H-2) sensor manufacturing is no exception. In this Account, we discuss how our recent efforts to develop sensorsbased on plasmonic plastics may complement the current state-of-the-art.We explore a new H-2 sensor paradigm, established througha series of recent publications, that combines (i) the plasmonic opticalH(2) detection principle and (ii) bulk-processed nanocompositematerials. In particular, plasmonic plastic nanocomposite sensingmaterials are described that comprise plasmonic H-2-sensitivecolloidally synthesized nanoparticles dispersed in a polymer matrixand enable the additive manufacturing of H-2 sensors ina cost-effective and scalable way. We first discuss the concept ofplasmonic plastic nanocomposite materials for the additive manufacturingof an active plasmonic sensing material on the basis of the threekey components that require individual and concerted optimization:(i) the plasmonic sensing metal nanoparticles, (ii) the surfactant/stabilizermolecules on the nanoparticle surface from colloidal synthesis, and(iii) the polymer matrix. We then introduce the working principleof plasmonic H-2 detection, which relies on the selectiveabsorption of H species into hydride-forming metal nanoparticles that,in turn, induces distinct changes in their optical plasmonic signaturein proportion to the H-2 concentration in the local atmosphere.Subsequently, we assess the roles of the key components of a plasmonicplastic for H-2 sensing, where we have established that(i) alloying Pd with Au and Cu eliminates hysteresis and introducesintrinsic deactivation resistance at ambient conditions, (ii) surfactant/stabilizermolecules can significantly accelerate and decelerate H-2 sorption and thus sensor response, and (iii) polymer coatings acceleratesensor response, reduce the limit of detection (LoD), and enable molecularfiltering for sensor operation in chemically challenging environments.Based on these insights, we discuss the rational development and detailedcharacterization of bulk-processed plasmonic plastics based on glassyand fluorinated matrix polymers and on tailored flow-chemistry-basedsynthesis of Pd and PdAu alloy colloidal nanoparticles with optimizedstabilizer molecules. In their champion implementation, they enablehighly stable H-2 sensors with response times in the 2 srange and an LoD of few 10 ppm of H-2. To put plasmonicplastics in a wider perspective, we also report their implementationusing different polymer matrix materials that can be used for 3D printingand (an)isotropic Au nanoparticles that enable the manufacturing ofmacroscopic plasmonic objects with, if required, dichroic opticalproperties and in amounts that can be readily upscaled. We advertisethat melt processing of plasmonic plastic nanocomposites is a viableroute toward the realization of plasmonic objects and sensors, producedby scalable colloidal synthesis and additive manufacturing techniques.
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| 10. |
- Das, Biswanath, et al.
(författare)
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The Impact of Ligand Carboxylates on Electrocatalyzed Water Oxidation
- 2021
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 54:17, s. 3326-3337
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Forskningsöversikt (refereegranskat)abstract
- Fossil fuel shortage and severe climate changes due to global warming have prompted extensive research on carbon-neutral and renewable energy resources. Hydrogen gas (H-2), a clean and high energy density fuel, has emerged as a potential solution for both fulfilling energy demands and diminishing the emission of greenhouse gases. Currently, water oxidation (WO) constitutes the bottleneck in the overall process of producing H-2 from water. As a result, the design of efficient catalysts for WO has become an intensively pursued area of research in recent years. Among all the molecular catalysts reported to date, ruthenium-based catalysts have attracted particular attention due to their robust nature and higher activity compared to catalysts based on other transition metals. Over the past two decades, we and others have studied a wide range of ruthenium complexes displaying impressive catalytic performance for WO in terms of turnover number (TON) and turnover frequency (TOF). However, to produce practically applicable electrochemical, photochemical, or photo-electrochemical WO reactors, further improvement of the catalysts' structure to decrease the overpotential and increase the WO rate is of utmost importance. WO reaction, that is, the production of molecular oxygen and protons from water, requires the formation of an O-O bond through the orchestration of multiple proton and electron transfers. Promotion of these processes using redox noninnocent ligand frameworks that can accept and transfer electrons has therefore attracted substantial attention. The strategic modifications of the ligand structure in ruthenium complexes to enable proton-coupled electron transfer (PCET) and atom proton transfer (APT; in the context of WO, it is the oxygen atom (metal oxo) transfer to the oxygen atom of a water molecule in concert with proton transfer to another water molecule) to facilitate the O-O bond formation have played a central role in these efforts. In particular, promising results have been obtained with ligand frameworks containing carboxylic acid groups that either are directly bonded to the metal center or reside in the close vicinity. The improvement of redox and chemical properties of the catalysts by introduction of carboxylate groups in the ligands has proven to be quite general as demonstrated for a range of mono- and dinudear ruthenium complexes featuring ligand scaffolds based on pyridine, imidazole, and pyridazine cores. In the first coordination sphere, the carboxylate groups are firmly coordinated to the metal center as negatively charged ligands, improving the stability of the complexes and preventing metal leaching during catalysis. Another important phenomenon is the reduction of the potentials required for the formation of higher valent intermediates, especially metal-oxo species, which take active part in the key O-O bond formation step. Furthermore, the free carboxylic acid/carboxylate units in the proximity to the active center have shown exciting proton donor/acceptor properties (through PCET or APT, chemically noninnocent) that can dramatically improve the rate as well as the overpotential of the WO reaction.
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| 11. |
- Dieguez, Montserrat, et al.
(författare)
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Self-Adaptable Tropos Catalysts
- 2021
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 54:16, s. 3252-3263
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Forskningsöversikt (refereegranskat)abstract
- Biological systems have often served as inspiration for the design of synthetic catalysts. The lock and key analogy put forward by Emil Fischer in 1894 to explain the high substrate specificity of enzymes has been used as a general guiding principle aimed at enhancing the selectivity of chemical processes by optimizing attractive and repulsive interactions in molecular recognition events. However, although a perfect fit of a substrate to a catalytic site may enhance the selectivity of a specific catalytic reaction, it inevitably leads to a narrow substrate scope, exduding substrates with different sizes and shapes from efficient binding. An ideal catalyst should instead be able to accommodate a wide range of substrates-it has indeed been recognized that enzymes also are often highly promiscuous as a result of their ability to change their conformation and shape in response to a substrate-and preferentially be useful in various types of processes. In biological adaptation, the process by which species become fitted to new environments is crucial for their ability to cope with changing environmental conditions. With this in mind, we have been exploring catalytic systems that can adapt their size and shape to the environment with the goal of developing synthetic catalysts with wide scope. In this Account, we describe our studies aimed at elucidating how metal catalysts with flexible structural units adapt their binding pockets to the reacting substrate. Throughout our studies, ligands equipped with tropos biaryl units have been explored, and the palladium-catalyzed allylic alkylation reaction has been used as a suitable probe to study the adaptability of the catalytic systems. The conformations of catalytically active metal complexes under different conditions have been studied by both experimental and theoretical methods. By the design of ligands incorporating two flexible units, the symmetry properties of metal complexes could be used to facilitate conformational analysis and thereby provide valuable insight into the structures of complexes involved in the catalytic cycle. The importance of flexibility was convincingly demonstrated when a phosphine group in a privileged ligand that is well-known for its versatility in a number of processes was exchanged for a tropos biaryl phosphite unit: the result was a truly self-adaptive ligand with dramatically increased scope.
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| 12. |
- Duan, Lele, et al.
(författare)
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Highly Efficient Bioinspired Molecular Ru Water Oxidation Catalysts with Negatively Charged Backbone Ligands
- 2015
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 48:7, s. 2084-2096
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Forskningsöversikt (refereegranskat)abstract
- The oxygen evolving complex (OEC) of the natural photosynthesis system II (PSII) oxidizes water to produce oxygen and reducing equivalents (protons and electrons). The oxygen released from PSII provides the oxygen source of our atmosphere; the reducing equivalents are used to reduce carbon dioxide to organic products, which support almost all organisms on the Earth planet. The first photosynthetic organisms able to split water were proposed to be cyanobacteria-like ones appearing ca. 2.5 billion years ago. Since then, nature has chosen a sustainable way by using solar energy to develop itself. Inspired by nature, human beings started to mimic the functions of the natural photosynthesis system and proposed the concept of artificial photosynthesis (AP) with the view to creating energy-sustainable societies and reducing the impact on the Earth environments. Water oxidation is a highly energy demanding reaction and essential to produce reducing equivalents for fuel production, and thereby effective water oxidation catalysts (WOCs) are required to catalyze water oxidation and reduce the energy loss. X-ray crystallographic studies on PSII have revealed that the OEC consists of a Mn4CaO5 cluster surrounded by oxygen rich ligands, such as oxyl, oxo, and carboxylate ligands. These negatively charged, oxygen rich ligands strongly stabilize the high valent states of the Mn cluster and play vital roles in effective water oxidation catalysis with low overpotential. This Account describes our endeavors to design effective Ru WOCs with low overpotential, large turnover number, and high turnover frequency by introducing negatively charged ligands, such as carboxylate. Negatively charged ligands stabilized the high valent states of Ru catalysts, as evidenced by the low oxidation potentials. Meanwhile, the oxygen production rates of our Ru catalysts were improved dramatically as well. Thanks to the strong electron donation ability of carboxylate containing ligands, a seven-coordinate Ru-IV species was isolated as a reaction intermediate, shedding light on the reaction mechanisms of Ru-catalyzed water oxidation chemistry. Auxiliary ligands have dramatic effects on the water oxidation catalysis in terms of the reactivity and the reaction mechanism. For instance, Ru-bda (H(2)bda = 2,2'-bipyridine-6,6'-dicarboxylic acid) water oxidation catalysts catalyze Ce-IV-driven water oxidation extremely fast via the radical coupling of two Ru-V=O species, while Ru-pda (H(2)pda = 1,10-phenanthroline-2,9-dicarboxylic acid) water oxidation catalysts catalyze the same reaction slowly via water nucleophilic attack on a Ru-V-O species. With a number of active Ru catalysts in hands, light driven water oxidation was accomplished using catalysts with low catalytic onset potentials. The structures of molecular catalysts could be readily tailored to introduce additional functional groups, which favors the fabrication of state-of-the-art Ru-based water oxidation devices, such as electrochemical water oxidation anodes and photo-electrochemical anodes. The development of efficient water oxidation catalysts has led to a step forward in the sustainable energy system.
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| 13. |
- Ek, Martin, et al.
(författare)
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Atomic-Scale Choreography of Vapor-Liquid-Solid Nanowire Growth
- 2018
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 51:1, s. 118-126
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Tidskriftsartikel (refereegranskat)abstract
- ConspectusFunctional materials and devices require nanoscale control of morphology, crystal structure, and composition. Vapor-liquid-solid (VLS) crystal growth and its related growth modes enable the synthesis of 1D nanostructures, commonly called "nanowires", where the necessary nanoscale heterogeneity can be encoded axially. During the VLS process, a seed particle collects atoms and directs the nucleation of crystalline material. Modulating the delivery of growth species or conditions permits compositional and/or structural encoding. A range of materials and devices (e.g., for electronics, photonics, thermal transport, and bioprobes) have been produced by VLS growth, but plenty of challenges remain: many desirable structures cannot currently be made, and even for those structures that can be made, the parameter window - in terms of, e.g., temperatures and pressures - is often narrow. Moreover, we are quite far from ab initio determination of which growth conditions should be used or even if a desired structure is fundamentally achievable within the VLS framework.To fully understand the challenges and promises of VLS growth, the governing physicochemical processes must be explored and understood at the atomic scale. This final level of detail is being unraveled with the help of in situ characterization techniques. The picture that is emerging is of a highly dynamical process with several deeply interconnected and highly fundamental components that are difficult to detect with postgrowth ex situ interrogation. For example, recent in situ microscopy and spectroscopy studies have shown that the growth front can undergo cyclical reshaping involving dissolution as well as crystallization and that the state of the nanowire surface, which changes with growth conditions as a result of a competition between adsorption and desorption of passivating species, plays a crucial role in determining the transport to/from and the stability of the seed particle. The available in situ observations currently constitute a somewhat disparate list, but if they can be connected to each other and to the outstanding challenges, they promise meaningful advances in our understanding of VLS growth. In this Account, we review the state of the art regarding the atomic-scale thermodynamic and kinetic phenomena that control VLS growth. Rather than cataloging all of the outstanding contributions to the field, we give priority to in situ observations that have revealed unexpected effects as well as those that hint at incongruities in our current knowledge. As such, our discussion should be viewed as an opportunity to gain deeper understanding and control of the fundamental processes at play, which will be crucial in future scale-up efforts and expansion to completely new materials systems and application areas.
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| 14. |
- Ekedahl, Lars-Gunnar, et al.
(författare)
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Hydrogen sensing mechanisms of metal insulator interfaces
- 1998
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Ingår i: Accounts of Chemical Research. - 0001-4842 .- 1520-4898. ; 31:5, s. 249-256
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Tidskriftsartikel (refereegranskat)abstract
- The hydrogen sensitivity of palladium-silicon dioxidesilicon (Pd-MOS) structures was demonstrated about 25 years ago. One of the most interesting features of the Pd-MOS device as a hydrogen sensor is its very large dynamic pressure range. Such devices are now used in several practical applications and in commercially available equipment, both as single sensors and in sensor arrays. We recall that the hydrogen sensitivity of the device occurs due to a hydrogen induced polarization at the Pd-SiO2 interface as schematically shown in Figure 1. During the years, several types of devices have been developed, both with insulators other than silicon dioxide and catalytic metals other than palladium. Furthermore, it has been demonstrated that sensors with thin, discontinuous catalytic metals can detect molecules, like ammonia, which are not detected by sensors with thick continuous palladium gates. 1-3 Although several insulators have been used in hydrogen sensitive Pd-insulator-semiconductor
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| 15. |
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| 16. |
- Gianni, Stefano, et al.
(författare)
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Fuzziness and Frustration in the Energy Landscape of Protein Folding, Function, and Assembly
- 2021
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 54:5, s. 1251-1259
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Forskningsöversikt (refereegranskat)abstract
- Are all protein interactions fully optimized? Do suboptimal interactions compromise specificity? What is the functional impact of frustration? Why does evolution not optimize some contacts? Proteins and their complexes are best described as ensembles of states populating an energy landscape. These ensembles vary in breadth from narrow ensembles clustered around a single average X-ray structure to broader ensembles encompassing a few different functional "taxonomic" states on to near continua of rapidly interconverting conformations, which are called "fuzzy" or even "intrinsically disordered". Here we aim to provide a comprehensive framework for confronting the structural and dynamical continuum of protein assemblies by combining the concepts of energetic frustration and interaction fuzziness. The diversity of the protein structural ensemble arises from the frustrated conflicts between the interactions that create the energy landscape. When frustration is minimal after folding, it results in a narrow ensemble, but residual frustrated interactions result in fuzzy ensembles, and this fuzziness allows a versatile repertoire of biological interactions. Here we discuss how fuzziness and frustration play off each other as proteins fold and assemble, viewing their significance from energetic, functional, and evolutionary perspectives. We demonstrate, in particular, that the common physical origin of both concepts is related to the ruggedness of the energy landscapes, intramolecular in the case of frustration and intermolecular in the case of fuzziness. Within this framework, we show that alternative sets of suboptimal contacts may encode specificity without achieving a single structural optimum. Thus, we demonstrate that structured complexes may not be optimized, and energetic frustration is realized via different sets of contacts leading to multiplicity of specific complexes. Furthermore, we propose that these suboptimal, frustrated, or fuzzy interactions are under evolutionary selection and expand the biological repertoire by providing a multiplicity of biological activities. In accord, we show that non-native interactions in folding or interaction landscapes can cooperate to generate diverse functional states, which are essential to facilitate adaptation to different cellular conditions. Thus, we propose that not fully optimized structures may actually be beneficial for biological activities of proteins via an alternative set of suboptimal interactions. The importance of such variability has not been recognized across different areas of biology. This account provides a modern view on folding, function, and assembly across the protein universe. The physical framework presented here is applicable to the structure and dynamics continuum of proteins and opens up new perspectives for drug design involving not fully structured, highly dynamic protein assemblies.
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| 17. |
- Grommet, Angela Beth, 1991, et al.
(författare)
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Molecular Photoswitching in Confined Spaces
- 2020
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 53:11, s. 2600-2610
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Forskningsöversikt (refereegranskat)
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| 18. |
- Hammarström, Leif
(författare)
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Accumulative Charge Separation for Solar Fuels Production : Coupling Light-Induced Single Electron Transfer to Multielectron Catalysis
- 2015
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 48:3, s. 840-850
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Forskningsöversikt (refereegranskat)abstract
- CONSPECTUS: The conversion and storage of solar energy into a fuel holds promise to provide a significant part of the future renewable energy demand of our societies. Solar energy technologies today generate heat or electricity, while the large majority of our energy is used in the form of fuels. Direct conversion of solar energy to a fuel would satisfy our needs for storable energy on a large scale. Solar fuels can be generated by absorbing light and converting its energy to chemical energy by electron transfer leading to separation of electrons and holes. The electrons are used in the catalytic reduction of a cheap substrate with low energy content into a high-energy fuel. The holes are filled by oxidation of water, which is the only electron source available for large scale solar fuel production. Absorption of a single photon typically leads to separation of a single electron hole pair. In contrast, fuel production and water oxidation are multielectron, multiproton reactions. Therefore, a system for direct solar fuel production must be able to accumulate the electrons and holes provided by the sequential absorption of several photons in order to complete the catalytic reactions. In this Account, the process is termed accumulative charge separation. This is considerably more complicated than charge separation on a single electron level and needs particular attention. Semiconductor materials and molecular dyes have for a long time been optimized for use in photovoltaic devices. Efforts are made to develop new systems for light harvesting and charge separation that are better optimized for solar fuel production than those used in the early devices presented so far. Significant progress has recently been made in the discovery and design of better homogeneous and heterogeneous catalysts for solar fuels and water oxidation. While the heterogeneous ones perform better today, molecular catalysts based on transition metal complexes offer much greater tunability of electronic and structural properties, they are typically more amenable to mechanistic analysis, and they are small and therefore require less material. Therefore, they have arguably greater potential as future efficient catalysts but must be efficiently coupled to accumulative charge separation. This Account discusses accumulative charge separation with focus on molecular and molecule semiconductor hybrid systems. The coupling between charge separation and catalysis involves many challenges that are often overlooked, and they are not always apparent when studying water oxidation and fuel formation as separate half-reactions with sacrificial agents. Transition metal catalysts, as well as other multielectron donors and acceptors, cycle through many different states that may quench the excited sensitizer by nonproductive pathways. Examples where this has been shown, often with ultrafast rates, are reviewed. Strategies to avoid these competing energy-loss reactions and still obtain efficient coupling of charge separation to catalysis are discussed. This includes recent examples of dye-sensitized semiconductor devices with molecular catalysts and dyes that realize complete water splitting, albeit with limited efficiency.
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| 19. |
- Inganäs, Olle, et al.
(författare)
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Alternating Polyfluorenes Collect Solar Light in Polymer Photovoltaics
- 2009
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Ingår i: ACCOUNTS OF CHEMICAL RESEARCH. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 42:11, s. 1731-1739
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Tidskriftsartikel (refereegranskat)abstract
- The effort to improve the energy conversion efficiency of polymer solar cells has led to the design of novel donor polymers. To improve open circuit photovoltages (OCVs) and the spectral coverage of the solar spectrum, researchers have looked for materials with high HOMO values, an easily modified electronic structure, and sufficient electronic transport within the polymers. One advance in design from our laboratories has been the development of a class of alternating polyfluorene copolymers (APFOs), which can be combined with fullerenes to make bulk heterojunction materials for photovoltaic conversion. This Account describes copolymers of fluorene that we designed to expand the range the optical absorption of solar cells to include wavelengths out to 1000 nm. In most cases, we combine these polymers with acceptors from the fullerene family, typically the phenyl C-61 butyric acid methyl ester (PCBM) molecule, to generate solar cell materials. The synthesis of alternating copolymers of fluorene with various donor-acceptor-don or elements provides the opportunity to shift both HOMO and LUMO, which we have followed by electrochemical spectroscopy. Moving the LUMO of the APFOs farther from the vacuum level eventually leads to a situation where the driving force for photo-induced charge transfer from polymer donor to fullerene acceptor goes to zero, resulting in inefficient charge generation. Moving the HOMO level closer to the vacuum level reduces the OCV of devices made from bulk heterojunction blends. As we move the bandgap toward lower energies and increase the overlap of optical absorption with the solar spectrum, both these events eventually occur. In devices based on these APFO/fullerene blends, the performance depends on the OCV, the photocurrent under solar illumination, and the fill factor. The fill factor is influenced by electrical transport and charge generation. Optimizing these parameters requires new solutions to the perennial conflict between optically thin devices, where electrical extraction of charge is not a limitation, and the optically thick devices, where extraction of charge is hampered by trapping and recombination. As a result, we have developed methods to trap light in optically thin devices. When the thin film flexible solar cells are folded, multiple reflection between adjacent solar cells leads to a longer path length for the photon through the devices and considerable improvement of the optical dissipation in the active material. These optical tricks also enable an alternative route to tandem devices, where two different bandgap materials are located on adjacent folds. Thus light not absorbed in one cell is reflected onto the next cell to produce an effective optical series arrangement. Using experiments and simulations of the light trapping effects, we demonstrate power conversion efficiency enhancements of up to a factor of 1.8.
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| 20. |
- Kaila, Ville R. I.
(författare)
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Resolving Chemical Dynamics in Biological Energy Conversion : Long-Range Proton-Coupled Electron Transfer in Respiratory Complex I
- 2021
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 54:24, s. 4462-4473
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Forskningsöversikt (refereegranskat)abstract
- Biological energy conversion is catalyzed by membrane-bound proteins that transduce chemical or light energy into energy forms that power endergonic processes in the cell. At a molecular level, these catalytic processes involve elementary electron-, proton-, charge-, and energy-transfer reactions that take place in the intricate molecular machineries of cell respiration and photosynthesis. Recent developments in structural biology, particularly cryo-electron microscopy (cryoEM), have resolved the molecular architecture of several energy transducing proteins, but detailed mechanistic principles of their charge transfer reactions still remain poorly understood and a major challenge for modern biochemical research. To this end, multiscale molecular simulations provide a powerful approach to probe mechanistic principles on a broad range of time scales (femtoseconds to milliseconds) and spatial resolutions (10(1)-10(6) atoms), although technical challenges also require balancing between the computational accuracy, cost, and approximations introduced within the model. Here we discuss how the combination of atomistic (aMD) and hybrid quantum/classical molecular dynamics (QM/MM MD) simulations with free energy (FE) sampling methods can be used to probe mechanistic principles of enzymes responsible for biological energy conversion. We present mechanistic explorations of long-range proton-coupled electron transfer (PCET) dynamics in the highly intricate respiratory chain enzyme Complex I, which functions as a redox-driven proton pump in bacterial and mitochondrial respiratory chains by catalyzing a 300 angstrom fully reversible PCET process. This process is initiated by a hydride (H-) transfer between NADH and FMN, followed by long-range (>100 A) electron transfer along a wire of 8 FeS centers leading to a quinone biding site. The reduction of the quinone to quinol initiates dissociation of the latter to a second membrane-bound binding site, and triggers proton pumping across the membrane domain of complex I, in subunits up to 200 angstrom away from the active site. Our simulations across different size and time scales suggest that transient charge transfer reactions lead to changes in the internal hydration state of key regions, local electric fields, and the conformation of conserved ion pairs, which in turn modulate the dynamics of functional steps along the reaction cycle. Similar functional principles, which operate on much shorter length scales, are also found in some unrelated proteins, suggesting that enzymes may employ conserved principles in the catalysis of biological energy transduction processes.
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| 21. |
- Klappenberger, Florian, et al.
(författare)
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On-Surface Synthesis of Carbon-Based Scaffolds and Nanomaterials Using Terminal Alkynes
- 2015
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Ingår i: Accounts of Chemical Research. - : American Chemical Society. - 0001-4842 .- 1520-4898. ; 48:7, s. 2140-2150
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Forskningsöversikt (refereegranskat)abstract
- CONSPECTUS: The covalent linking of acetylene compounds is an important synthetic tool to control carbon carbon bond formation and has been extensively studied for more than a century. Notably, Glaser coupling and subsequently developed refined procedures present an important route for the fabrication of distinct carbon-based scaffolds incorporating units with both sp(2)- and sp-hybridizations, such as carbyne chains, or two-dimensional (2D) graphyne or graphdiyne networks. However, the realization of the envisioned regular low-dimensional compounds and nanoarchitectures poses formidable challenges when following conventional synthesis protocols in solution, which we briefly overview. Now, recent developments in on-surface synthesis establish novel means for the construction of tailored covalent nanostructures under ultrahigh vacuum conditions. Here we focus on the exploration of pathways utilizing interfacial synthesis with terminal alkynes toward the atomically precise fabrication of low-dimensional carbon-rich scaffolds and nanomaterials. We review direct, molecular-level investigations, mainly relying on scanning probe microscopy, providing atomistic insights into thermally activated reaction schemes, their special pathways and products. Using custom-made molecular units, the employed homocoupling, cyclotrimerization, cycloaddition, and radical cyclization processes indeed yield distinct compounds, extended oligomers or 2D networks. Detailed insights into surface interactions such as bonding sites or conformational adaptation, and specific reaction mechanisms, including hierarchic pathways, were gained by sophisticated density functional theory calculations, complemented by X-ray spectroscopy measurements. For the fabrication of regular nanostructures and architectures, it is moreover imperative to cope with spurious side reactions, frequently resulting in chemical diversity. Accordingly, we highlight measures for increasing chemo- and regioselectivity by smart precursor design, substrate templating, and external stimuli. The ensuing preorganization of functional groups and control of side reactions increases product yields markedly. Finally, the electronic band structures of selected cases of novel low-dimensional hydrocarbon materials accessible with the monomers employed to date are discussed with a specific focus on their differences to theoretically established graphyne- and graphdiyne-related scaffolds. The presented methodology and gained insights herald further advancements in the field, heading toward novel molecular compounds, low-dimensional nanostructures, and coherently reticulated polymeric layers, eventually presenting well-defined arrangements with specific carbon carbon bond sequencing and electronic characteristics. The functional properties of these or other foreseeable scaffolds and architectures bear significant prospects for a wide range of applications, for example, in nanoelectronics, photonics, or carbon-based technologies.
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| 22. |
- Kärkäs, Markus D., et al.
(författare)
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Artificial Photosynthesis : Photosynthesis: From Nanosecond Electron Transfer to Catalytic Water Oxidation
- 2014
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 47:1, s. 100-111
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Forskningsöversikt (refereegranskat)abstract
- Human society faces a fundamental challenge as energy consumption is projected to increase due to population and economic growth as fossil fuel resources decrease. Therefore the transition to alternative and sustainable energy sources is of the Utmost importance. The conversion of solar energy into chemical energy, by splitting H2O to generate molecular O-2 and H-2, could contribute to solving the global energy problem. Developing such a system will require the combination of several complicated processes, such as light-harvesting, charge separation, electron transfer, H2O oxidation, and reduction of the generated protons. The primary processes of charge separation and catalysis, which occur in the natural photosynthetic machinery, provide us with an excellent blueprint for the design of such systems. This Account describes our efforts to construct supramolecular assemblies capable of carrying out photoinduced electron transfer and to develop artificial water oxidation catalysts (WOCs). Early work in our group focused on linking a ruthenium chromophore to a manganese-based oxidation catalyst. When we incorporated a tyrosine unit into these supramolecular assemblies, we could observe fast intramolecular electron transfer from the manganese centers, via the tyrosine moiety, to the photooxidized ruthenium center, which clearly resembles the processes occurring in the natural system. Although we demonstrated multi-electron transfer in our artificial systems, the bottleneck proved to be the stability of the WOCs. Researchers have developed a number of WOCs, but the majority can only catalyze H2O oxidation in the presence of strong oxidants such as Ce-IV, which is difficult to generate photochemically. By contrast, illumination of ruthenium(II) photosensitizers in the presence of a sacrificial acceptor generates [Ru(bpy)(3)](3+)-type oxidants. Their oxidation potentials are significantly lower than that of Ce-IV, but our group recently showed that incorporating negatively charged groups into the ligand backbone could decrease the oxidation potential of the catalysts and, at the same time, decrease the potential for H2O oxidation. This permitted us to develop both ruthenium- and manganese-based WOCs that can operate under neutral conditions, driven by the mild oxidant [Ru(bpy)(3)](3+). Many hurdles to the development of viable systems for the production of solar fuels remain. However, the combination of important features from the natural photosynthetic machinery and novel artificial components adds insights into the complicated catalytic processes that are involved in splitting H2O.
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| 23. |
- Larhed, M., et al.
(författare)
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Microwave-accelerated homogeneous catalysis in organic chemistry
- 2002
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Ingår i: Accounts of Chemical Research. - Uppsala Univ, BMC, Dept Organ Pharmaceut Chem, SE-75123 Uppsala, Sweden. Royal Inst Technol, Dept Chem, SE-10044 Stockholm, Sweden. : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 35:9, s. 717-727
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Forskningsöversikt (refereegranskat)abstract
- The efficiency of microwave flash heating in accelerating organic transformations (reaction times reduced from days and hours to minutes and seconds) has recently-been proven in several different fields of organic chemistry. This specific account mainly summarizes our own experiences in developing rapid, robust, and selective microwave-assisted transition metal-catalyzed homogeneous reactions. Applications include selective Heck couplings, cross-couplings, and asymmetric substitutions. The science of green chemistry was developed to meet the increasing demand for environmentally benign chemical processes. We believe the combination of metal catalysis and microwave heating will be of importance in the search for green laboratory-scale synthesis.
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| 24. |
- Li, Man-Bo, et al.
(författare)
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Efficient Heterogeneous Palladium Catalysts in Oxidative Cascade Reactions
- 2021
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 54:9, s. 2275-2286
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Forskningsöversikt (refereegranskat)abstract
- CONSPECTUS: Palladium-catalyzed oxidations involving cascade processes provide a versatile platform for streamlined conversion of simple feedstocks into functional molecules with high atom and step economy. However, the achievement of high palladium efficiency and selectivity in Pd-catalyzed oxidative cascade reactions is still challenging in many cases, as a result of the aggregation of active palladium species to Pd black and the possible side reactions during each bond-forming step. The two current solutions for addressing these issues are either to utilize oxidant-stable ligands or to use electron transfer mediators (ETMs). The former solution, which includes the use of amines, pyridines, sulfoxides, and carbene derivatives, inhibits aggregation of Pd-0 during the catalytic cycle, while the latter solution facilitates reoxidation of Pd-0 to Pd-II to improve the activity and selectivity. Following our long-standing interest in Pd-catalyzed oxidations, very recently we developed heterogeneous catalysts to resolve the issues mentioned above in oxidative cascade reactions. The heterogeneous palladium catalysts (Pd-AmP-MCF or Pd-AmP-CNC) comprise palladium nanoclusters (1-2 nm) immobilized on amino-functionalized siliceous mesocellular foam (MCF) or on crystalline nanocellulose (CNC), exhibiting high activity, selectivity as well as excellent recycling ability. In this Account, we will discuss the synthesis and characterizations of the heterogeneous palladium catalysts, as well as their catalytic behaviors, and the mechanisms involved in their reactions. An important aspect of these catalysts in oxidation reactions is the generation of active Pd(II) species within the heterogeneous phase. Typical oxidative cascade reactions of our recent research on this topic include oxidative carbocyclization-carbonylation, oxidative carbocyclization-borylation, oxidative alkynylation-cyclization, oxidative carbonylation-cyclization, and oxidative carbocyclization-alkynylation. These reactions provide access to important compounds attractive in medicinal chemistry and functional materials, such as gamma-lactone/gamma-lactam-based poly rings, cyclobutenols, highly substituted furans, and oxaboroles. During these processes, the heterogeneous catalysts exhibited much higher turnover numbers (TONs) than their homogeneous counterparts (e.g., Pd(OAc)(2)) as well as unique selectivity that cannot be achieved by homogeneous palladium catalysts. The origin of the high efficiency and unique selectivity of the heterogeneous catalysts was also investigated. Asymmetric syntheses for the construction of optically pure compounds were realized based on the excellent selectivity in these heterogeneous processes. Kinetic studies revealed that the rate and yield of the reactions were essentially maintained during recycling, which demonstrates that Pd-AmP-MCF and Pd-AmP-CNC are robust and highly active in these oxidative cascade reactions. In addition, inductively coupled plasma optical emisson spectroscopy (ICP-OES) analysis and hot filtration test suggest that these processes most likely proceed via a heterogeneous pathway. Recent progress in our group has shown that the activity of Pd-AmP-MCF and Pd-AmP-CNC could be improved even further by the addition of Ag+ to generate cationic Pd(II). Furthermore, intriguing solvent effects were observed in a Pd-AmP-MCF-catalyzed oxidative cascade process, and solvent-controlled chemoselective transformations were developed based on this property of the catalyst. The heterogeneous strategy of this Account provides solutions to palladium deactivation and selectivity issues in Pd(II)catalyzed oxidative cascade reactions and enables efficient catalyst recycling, which will open up new opportunities in oxidative cascade reactions.
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| 25. |
- Li, Xianchan, 1982, et al.
(författare)
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Quantitative Chemical Measurements of Vesicular Transmitters with Electrochemical Cytometry
- 2016
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 49:10, s. 2347-2354
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Tidskriftsartikel (refereegranskat)abstract
- Electrochemical cytometry adds a new dimension to our ability to study the chemistry and chemical storage of transmitter molecules stored in nanometer vesicles. The approach involves the adsorption and subsequent rupture of vesicles on an electrode surface during which the electroactive contents are quantitatively oxidized (or reduced). The measured current allows us to count the number of molecules in the vesicles using Faraday's law and to correlate this-to the amount of molecules released when single exocytosis events take place at communicating cells. The original format for this method involved a capillary electrophoresis separation step to singly address each vesicle, but we have more recently discovered that cellular vesicles tend to adsorb to carbon electrodes and spontaneously as well as stochastically rupture to give mostly single vesicle events. This approach, called impact electrochemical cytometry, even though the impact is perhaps not the important part of this process, has been studied and the vesicle rupture appears to be at the interface between the vesicle and the electrode and is probably driven by electroporation. The pore size and rate of content electrolysis are a function of the pore diameter and the presence of a protein core in the vesicles. In model liposomes with no protein, events appear extremely rapidly as the soft nanoparticles impact the electrode and the contents are oxidized. It appears that the proteins decorating the surface of the vesicle are important in maintaining a gap from the electrode and when this gap is closed electroporation takes place. Models of the event response times suggest the pores formed are small enough so we can carry out these measurements at nanotip electrodes and we have used this to quantify the vesicle content in living cells in a mode we call intracellular impact electrochemical cytometry. The development of electrochemical cytometry allows comparison between vesicle content and vesicular release and we have found that only part of the vesicle content is released in typical exocytotic cases measured by amperometry. This has led to the novel hypothesis that most exocytosis from dense core vesicles is via mechanism where vesicles fuse with the cell membrane, some content is released and then close again to be reloaded and reused. It leaves open the possibility that cells regulate release during individual events. This might be important in learning and memory and be a nonreceptor pharmaceutical target for brain related disorders. Indeed, the concept of the chemo-brain observed in cisplatin-treated cancer patients appears to be at least in part the result of changing the fraction of transmitter released and we have been able to show this by using the combined amperometric measurement of release and electrochemical cytometry at model cells.
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| 26. |
- Lidin, Sven, et al.
(författare)
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In Situ Synthesis and Single Crystal Synchrotron X-ray Diffraction Study of ht-Sn3Sb2 : An Example of How Complex Modulated Structures Are Becoming Generally Accessible
- 2018
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 51:2, s. 223-229
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Tidskriftsartikel (refereegranskat)abstract
- ConspectusRecent developments in X-ray sources and detectors and the parallel development of software for nonstandard crystallography has made analysis of very complex structural problems accessible to nonexperts. Here, we report the successful solution of the structure of ht-Sn3Sb2, an analysis that presents several challenges but that is still manageable in a relatively straightforward way. This compound exists only in a narrow temperature regime and undergoes an unquenchable phase transformation on cooling to room temperature; it contains two elements with close to identical scattering factors, and the structure is incommensurately modulated with four symmetry dependent modulation wave vectors.In this study, an attempt was first made to synthesize the title compound by in-house crystal growth in the stability region of ht-Sn3Sb2, followed by cooling to room temperature. This is known to produce mutiply twinned stistaite and elemental tin, and this sample, freshly prepared, was then reheated in situ at the single crystal materials beamline Crystal at the synchrotron Soleil. This method was unsuccessful as reheating the sample led to loss of Sn from stistaite as revealed by a change in the measured modulation wave vector.The compound was instead successfully synthesized in situ at the beamline by the topochemical reaction of single crystalline stistaite and liquid tin. A well-formed crystal of stistaite was enclosed in a quartz capillary together with a large excess of tin and heated above the melting point of tin but below the melting point of ht-Sn3Sb2. The structure was probed by sychrotron X-ray diffraction using a wavelength close to the absorption edge of Sn to maximize elemental contrast.In the diffraction patterns, first order satellites were observed, making the structure of ht-Sn3Sb2 incommensurately modulated. Further analysis exposes four q-vectors running along the body diagonals of the cubic unit cell (q1′ = α α α, q2′ = -α α -α, q3′ = -α -α α, q4′ = α -α -α).To facilitate the analysis, the q vectors were instead treated as axial (q1 = α 0 0, q2 = 0 α 0, q3 = 0 0 α) and an F-type extinction condition for satellites was introduced so that only reflections with hklmnp, mnp all odd or all even, were considered. Further, the modulation functions F(qi) were set to zero, and only modulation functions of the type F(qi′) were refined. The final model uses the four modulation functions, F(q1′), F(q2′), F(q3′), and F(q4′), to model occupancy Sn/Sb and positional modulation. The model shows a structure that comprises small NaCl type clusters, typically 7 × 7 × 7 atoms in extension, interspersed between single layers of elemental tin. The terminating layers of tin are slightly puckered, emulating an incipient deformation toward the structure of the layers perpendicular to the [001] direction in elemental tin. It is notable that this model is complementary to that of stistaite. In stistaite, two-dimensionally infinite slabs of rock salt are interspersed between layers of antimony along the trigonal [001] direction, so that the terminating Sb layers are the puckered bilayers typical for elemental Sb. Since all modulation functions are simple first-order harmonics, the structural model describes a locally disordered and most probably dynamic ordering.
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| 27. |
- Liu, Li-Hong, et al.
(författare)
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Perfluorophenyl Azides : New Applications in Surface Functionalization and Nanomaterial Synthesis
- 2010
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 43:11, s. 1434-1443
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Forskningsöversikt (refereegranskat)abstract
- A major challenge in materials science is the ongoing search for coupling agents that are readily synthesized, capable of versatile chemistry, able to easily functionalize materials and surfaces, and efficient in covalently linking organic and inorganic entities. A decade ago, we began a research program investigating perfluorophenylazides (PFPA) as the coupling agents in surface functionalization and nanomaterial synthesis. The p-substituted PFPAs are attractive heterobifunctional coupling agents because of their two distinct and synthetically distinguishable reactive centers: (i) the fluorinated phenylazide, which is capable of forming stable covalent adducts, and (ii) the functional group R, which can be tailored through synthesis. Two approaches have been undertaken for material synthesis and surface functionalization. The first method involves synthesizing PFPA bearing the first molecule or material with a functional linker R and then attaching the resulting PFPA to the second material by activating the azido group. In the second approach, the material surface is first functionalized with PFPA via functional center R, and coupling of the second molecule or material is achieved with the surface azido groups. In this Account, we review the design and protocols of the two approaches, providing examples in which PFPA derivatives were successfully used in material surface functionalization, ligand conjugation, and the synthesis of hybrid nanomaterials. The methods developed have proved to be general and versatile, and they are applicable to a wide range of materials (especially those that lack reactive functional groups or are difficult to derivatize) and to various substrates of polymers, oxides, carbon materials, and metal films. The coupling chemistry can be initiated by light, heat, and electrons. Patterned structures can be generated by selectively activating the areas of interest. Furthermore, the process is easy to perform, and light activation occurs in minutes, greatly facilitating the efficiency of the reaction. PFPAs indeed demonstrate many benefits as versatile surface coupling agents and offer opportunities for further exploration.
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| 28. |
- Liu, Si-hua, et al.
(författare)
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Smart Hydrogen Atoms in Heterocyclic Cations of 1,2,4-Triazolium-Type Poly(ionic liquid)s
- 2022
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 55:24, s. 3675-3687
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Forskningsöversikt (refereegranskat)abstract
- Discovering and constructing molecular functionality platforms for materials chemistry innovation has been a persistent target in the fields of chemistry, materials, and engineering. Around this task, basic scientific questions can be asked, novel functional materials can be synthesized, and efficient system functionality can be established. Poly(ionic liquid)s (PILs) have attracted growing interest far beyond polymer science and are now considered an interdisciplinary crossing point between multiple research areas due to their designable chemical structure, intriguing physicochemical properties, and broad and diverse applications. Recently, we discovered that 1,2,4-triazolium-type PILs show enhanced performance profiles, which are due to stronger and more abundant supramolecular interactions ranging from hydrogen bonding to metal coordination, when compared with structurally similar imidazolium counterparts. This phenomenon in our view can be related to the smart hydrogen atoms (SHAs), that is, any proton that binds to the carbon in the N-heterocyclic cations of 1,2,4-triazolium-type PILs. The replacement of one carbon by an electron-withdrawing nitrogen atom in the broadly studied heterocyclic imidazolium ring will further polarize the C–H bond (especially for C5–H) of the resultant 1,2,4-triazolium cation and establish new chemical tools for materials design. For instance, the H-bond-donating strength of the SHA, as well as its Bro̷nsted acidity, is increased. Furthermore, polycarbene complexes can be readily formed even in the presence of weak or medium bases, which is by contrast rather challenging for imidazolium-type PILs. The combination of SHAs with the intrinsic features of heterocyclic cation-functionalized PILs (e.g., N-coordination capability and polymeric multibinding effects) enables new phenomena and therefore innovative materials applications.In this Account, recent progress on SHAs is presented. SHA-related applications in several research branches are highlighted together with the corresponding materials design at size scales ranging from nano- to micro- and macroscopic levels. At a nanoscopic level, it is possible to manipulate the interior and outer shapes and surface properties of PIL nanocolloids by adjusting the hydrogen bonds (H-bonds) between SHAs and water. Owing to the interplay of polycarbene structure, N-coordination, and the polymer multidentate binding of 1,2,4-triazolium-type PILs, metal clusters with controllable size at sub-nanometer scale were successfully synthesized and stabilized, which exhibited record-high catalytic performance in H2 generation via methanolysis of ammonia borane. At the microscopic level, SHAs are found to efficiently catalyze single crystal formation of structurally complex organics. Free protons in situ released from the SHAs serve as organocatalysts to activate formation of C–N bonds at room temperature in a series of imine-linked crystalline porous organics, such as organic cages, macrocycles and covalent organic frameworks; meanwhile the concurrent “salting-out” effect of PILs as polymers in solution accelerated the crystallization rate of product molecules by at least 1 order of magnitude. At the macroscopic scale, by finely regulating the supramolecular interactions of SHAs, a series of functional supramolecular porous polyelectrolyte membranes (SPPMs) with switchable pores and gradient cross-sectional structures were manufactured. These membranes demonstrate impressive figures of merit, ranging from chiral separation and proton recognition to switchable optical properties and real-time chemical reaction monitoring. Although the concept of SHAs is in the incipient stage of development, our successful examples of applications portend bright prospects for materials chemistry innovation.
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| 29. |
- Liu, Yizhu, et al.
(författare)
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Fe N-Heterocyclic Carbene Complexes as Promising Photosensitizers
- 2016
-
Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 49:8, s. 1477-1485
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Tidskriftsartikel (refereegranskat)abstract
- ConspectusThe photophysics and photochemistry of transition metal complexes (TMCs) has long been a hot field of interdisciplinary research. Rich metal-based redox processes, together with a high variety in electronic configurations and excited-state dynamics, have rendered TMCs excellent candidates for interconversion between light, chemical, and electrical energies in intramolecular, supramolecular, and interfacial arrangements. In specific applications such as photocatalytic organic synthesis, photoelectrochemical cells, and light-driven supramolecular motors, light absorption by a TMC-based photosensitizer and subsequent excited-state energy or electron transfer constitute essential steps. In this context, TMCs based on rare and expensive metals, such as ruthenium and iridium, are frequently employed as photosensitizers, which is obviously not ideal for large-scale implementation. In the search for abundant and environmentally benign solutions, six-coordinate FeII complexes (FeIIL6) have been widely considered as highly desirable alternatives. However, not much success has been achieved due to the extremely short-lived triplet metal-to-ligand charge transfer (3MLCT) excited state that is deactivated by low-lying metal-centered (MC) states on a 100 fs time scale. A fundamental strategy to design useful Fe-based photosensitizers is thus to destabilize the MC states relative to the 3MLCT state by increasing the ligand field strength, with special focus on making eg σ∗ orbitals on the Fe center energetically less accessible. Previous efforts to directly transplant successful strategies from RuIIL6 complexes unfortunately met with limited success in this regard, despite their close chemical kinship. In this Account, we summarize recent promising results from our and other groups in utilizing strongly σ-donating N-heterocyclic carbene (NHC) ligands to make strong-field FeIIL6 complexes with significantly extended 3MLCT lifetimes. Already some of the first homoleptic bis(tridentate) complexes incorporating (CNHC^Npyridine^CNHC)-type ligands gratifyingly resulted in extension of the 3MLCT lifetime by more than 2 orders of magnitude compared to the parental [Fe(tpy)2]2+ (tpy = 2,2′:6′,2″-terpyridine) complex. Quantum chemical (QC) studies also revealed that the 3MC instead of the 5MC state likely dictates the deactivation of the 3MLCT state, a behavior distinct from traditional FeIIL6 complexes but rather resembling Ru analogues. A heteroleptic FeII NHC complex featuring mesoionic bis(1,2,3-triazol-5-ylidene) (btz) ligands also delivered a 100-fold elongation of the 3MLCT lifetime relative to its parental [Fe(bpy)3]2+ (bpy = 2,2′-bipyridine) complex. Again, a Ru-like deactivation mechanism of the 3MLCT state was indicated by QC studies. With a COOH-functionalized homoleptic complex, a record 3MLCT lifetime of 37 ps was recently observed on an Al2O3 nanofilm. As a proof of concept, it was further demonstrated that the significant improvement in the 3MLCT lifetime indeed benefits efficient light harvesting with FeII NHC complexes. For the first time, close-to-unity electron injection from the lowest-energy 3MLCT state to a TiO2 nanofilm was achieved by a stable FeII complex. This is in complete contrast to conventional FeIIL6-derived photosensitizers that could only make use of high-energy photons. These exciting results significantly broaden the understanding of the fundamental photophysics and photochemistry of d6 FeII complexes. They also open up new possibilities to develop solar energy-converting materials based on this abundant, inexpensive, and intrinsically nontoxic element.
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| 30. |
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| 31. |
- Lundgren, Edvin, et al.
(författare)
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Novel in Situ Techniques for Studies of Model Catalysts
- 2017
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 50:9, s. 2326-2333
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Tidskriftsartikel (refereegranskat)abstract
- ConspectusMotivated mainly by catalysis, gas-surface interaction between single crystal surfaces and molecules has been studied for decades. Most of these studies have been performed in well-controlled environments and have been instrumental for the present day understanding of catalysis, providing information on surface structures, adsorption sites, and adsorption and desorption energies relevant for catalysis. However, the approach has been criticized for being too far from a catalyst operating under industrial conditions at high temperatures and pressures. To this end, a significant amount of effort over the years has been used to develop methods to investigate catalysts at more realistic conditions under operating conditions. One result from this effort is a vivid and sometimes heated discussion concerning the active phase for the seemingly simple CO oxidation reaction over the Pt-group metals in the literature.In recent years, we have explored the possibilities to perform experiments at conditions closer to those of a technical catalyst, in particular at increased pressures and temperatures. In this contribution, results from catalytic CO oxidation over a Pd(100) single crystal surface using Near Ambient Pressure X-ray Photo emission Spectroscopy (NAPXPS), Planar Laser-Induced Fluorescence (PLIF), and High Energy Surface X-ray Diffraction (HESXRD) are presented, and the strengths and weaknesses of the experimental techniques are discussed. Armed with structural knowledge from ultrahigh vacuum experiments, the presence of adsorbed molecules and gas-phase induced surface structures can be identified and related to changes in the reactivity or to reaction induced gas-flow limitations. In particular, the application of PLIF to catalysis allows one to visualize how the catalyst itself changes the gas composition close to the model catalyst surface upon ignition, and relate this to the observed surface structures. The effect obscures a straightforward relation between the active phase and the activity, since in the case of CO oxidation, the gas-phase close to the model catalyst surface is shown to be significantly more oxidizing than far away from the catalyst. We show that surface structural knowledge from UHV experiments and the composition of the gas phase close to the catalyst surface are crucial to understand structure-function relationships at semirealistic conditions. In the particular case of Pd, we argue that the surface structure of the PdO(101) has a significant influence on the activity, due to the presence of Coordinatively Unsaturated Sites (CUS) Pd atoms, similar to undercoordinated Ru and Ir atoms found for RuO2(110) and IrO2(110), respectively.
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| 32. |
- Magnuson, Ann, et al.
(författare)
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Biomimetic and Microbial Approaches to Solar Fuel Generation
- 2009
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 42:12, s. 1899-1909
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Forskningsöversikt (refereegranskat)abstract
- Photosynthesis is performed by a multitude of organisms, but in P nearly all cases, it is variations on a common theme: absorption of light followed by energy transfer to a reaction center where charge separation takes place, This initial form of chemical energy is stabilized by the biosynthesis of carbohydrates. To produce these energy-rich products, a substrate is needed that feeds in reductive equivalents, When photosynthetic microorganisms learned to use water as a substrate some 2 billion years ago, a fundamental barrier against unlimited use of solar energy was overcome. The possibility of solar energy use has inspired researchers to construct artificial photosynthetic systems that show analogy to parts of the intricate molecular machinery of photosynthesis. Recent years have seen a reorientation of efforts toward creating integrated light-to-fuel systems that can use solar energy for direct synthesis of energy-rich compounds, so-called solar fuels. Sustainable production of solar fuels is a long awaited development that promises extensive solar energy use combined with long-term storage. The stoichiometry of water splitting into molecular oxygen, protons, and electrons is deceptively simple; achieving it by chemical catalysis has proven remarkably difficult. The reaction center Photosystem II couples light-induced charge separation to an efficient molecular water-splitting catalyst, a Mn4Ca complex, and is thus an important template for biomimetic chemistry. In our aims to design biomimetic manganese complexes for light-driven water oxidation, we link photosensitizers and charge-separation motifs to potential catalysts in supramolecular assemblies. In photosynthesis, production of carbohydrates demands the delivery of multiple reducing equivalents to CO2. In contrast, the two-electron reduction of protons to molecular hydrogen is much less demanding. Virtually all microorganisms have enzymes called hydrogenases that convert protons to hydrogen, many of them with good catalytic efficiency. The catalytic sites of hydrogenases are now the center of attention of biomimetic efforts, providing prospects for catalytic hydrogen production with inexpensive metals. Thus, we might complete the water-to-fuel conversion: light + 2H(2)O -> 2H(2) + O-2 This reaction formula is to some extent already elegantly fulfilled by cyanobacteria and green algae, water-splitting photosynthetic microorganisms that under certain conditions also can produce hydrogen. An alternative route to hydrogen from solar energy is therefore to engineer these organisms to produce hydrogen more efficiently. This Account describes our original approach to combine research in these two fields: mimicking structural and functional principles of both Photosystem II and hydrogenases by synthetic chemistry and engineering cyanobacteria to become better hydrogen producers and ultimately developing new routes toward synthetic biology.
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| 33. |
- Moberg, Christina
(författare)
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Recycling in Asymmetric Catalysis
- 2016
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Ingår i: ACCOUNTS OF CHEMICAL RESEARCH. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 49:12, s. 2736-2745
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Forskningsöversikt (refereegranskat)abstract
- CONSPECTUS: Cyclic reaction networks consisting of an enantioselective product forming step and a reverse reaction of the undesired enantiomer back to starting reactant are important for the generation of compounds with high enantiomeric purity. In order to avoid an equilibrium racemic state, a unidirectional cyclic process where product formation and regeneration of starting reactant proceed through different mechanistic pathways is required. Such processes must necessarily include a thermodynamically unfavorable step, since the product of the forward reaction is the reactant of the reverse reaction and vice versa. Thermodynamically uphill processes are ubiquitous to the function of living systems. Such systems gain the required energy by coupling to thermodynamically downhill reactions. In the same way, artificial cyclic reaction networks can be realized in systems open to mass or energy flow, and an out-of equilibrium nonracemic steady state can be maintained as long as the system is supplied with energy. In contrast to a kinetic resolution, a recycling process where the minor enantiomer is converted to starting reactant can result in a quantitative yield, but the enantiomeric purity of the product is limited by the selectivity of the catalysts used for the reactions. On the other hand, in a kinetic resolution, the slowly reacting enantiomer can always be obtained in an enantiomerically pure state, although the yield will suffer. In cyclic reaction systems which use chiral catalysts for both the forward and the reverse processes, a reinforcing effect results, and selectivities higher than those achieved by a single chiral catalyst are observed. A dynamic kinetic resolution can in principle also lead to a quantitative yield, but lacks the reinforcing effect of two chiral catalysts. Most examples of cyclic reaction networks reported in the literature are deracemizations of racemic mixtures, which proceed via oxidation of one enantiomer followed by reduction to the opposite enantiomer. We have developed cyclic reaction networks comprising a carbon carbon bond formation. In these processes, the product is generated by the addition of a cyanide reagent to a prochiral aldehyde. This is followed by hydrolysis of the minor enantiomer of the product to generate the starting aldehyde. A unidirectional cycle is maintained by coupling to the exergonic transformation of the high potential cyanide reagent to a low potential compound, either a carboxylate or carbon dioxide. The products, which are obtained with high enantiomeric purity, serve as valuable starting materials for a variety of biologically and pharmaceutically active compounds.
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| 34. |
- Nakase, Ikuhiko, et al.
(författare)
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Efficient Intracellular Delivery of Nucleic Acid Pharmaceuticals Using Cell-Penetrating Peptides
- 2012
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 45:7, s. 1132-1139
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Forskningsöversikt (refereegranskat)abstract
- Over the last 20 years, researchers have designed or discovered peptides that can permeate membranes and deliver exogenous molecules inside a cell. These peptides, known as cell-penetrating peptides (CPPs), typically consist of 6-30 residues, including HIV TAT peptide, penetratin, oligoarginine, transportan, and TP10. Through chemical conjugation or noncovalent complex formation, these structures successfully deliver bioactive and membrane-impermeable molecules into cells. CPPs have also gained attention as an attractive vehicle for the delivery of nucleic add pharmaceuticals (NAPs), including genes/plasmids, short oligonucleotides, and small interference RNAs and their analogues, due to their high internalization efficacy, low cytotoxicity, and flexible structural design. In this Account, we survey the potential of CPPs for the design and optimization of NAP delivery systems. First, we describe the impact of the N-terminal stearylation of CPPs. Endocytic pathways make a major contribution to the cellular uptake of NAPS. Stearylation at the N-terminus of CPPs with stearyl-octaarginine (R8), stearyl-(RxR)(4), and stearyl-TP10 prompts the formation of a self-assembled core shell nanoparticle with NAPS, a compact structure that promotes cellular uptake. Researchers have designed modifications such as the addition of trifluoromethylquinoline moieties to lysine residues to destabilize endosomes, as exemplified by PepFect 6, and these changes further improve biological responsiveness. Alternatively, stearylation also allows implantation of CPPs onto the surface of liposomes. This feature facilitates programmed packaging to establish multifunctional envelope-type nanodevices (MEND). The R8-MEND showed high transfection efficiency comparable to that of adenovirus in non-dividing cells. Understanding the cellular uptake mechanisms of CPPs will further improve CPP-mediated NAP delivery. The cellular uptake of CPPs and their NAP complex involves various types of endocytosis. Macropinocytosis, a mechanism which is also activated in response to stimuli such as growth factors or viruses, is a primary pathway for arginine-rich CPPs because high cationic charge density promotes this endocytic pathway. The use of larger endosomes (known as macropinosomes) rather than clathrin- or caveolae-mediated endocytosis has been reported in macropinocytosis which would also facilitate the endocytosis of NAP nanoparticles into cells.
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| 35. |
- Norberg, J, et al.
(författare)
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Molecular dynamics applied to nucleic acids
- 2002
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Ingår i: Accounts of chemical research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 35:6, s. 465-472
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Tidskriftsartikel (refereegranskat)
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| 36. |
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| 37. |
- Oliveberg, Mikael
(författare)
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Alternative Explanations for "Multistate" Kinetics in Protein Folding: Transient Aggregation and Changing Transition-State Ensembles
- 1998
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 1520-4898 .- 0001-4842. ; 31:11, s. 765-772
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Tidskriftsartikel (refereegranskat)abstract
- Deviations from classical two-state kinetics in protein folding need not always be explained by the presence of rapidly formed intermediates. In some cases, such deviations are caused by short-lived aggregates whereas in other cases they arise from changes of the position of the transition state. These are two new facets of the mechanism of two-state folding. The first part of this account describes the effect of aggregates which form transiently in the first few milliseconds of the refolding reaction. The aggregates show many similarities with folding intermediates, but may be identified by their disappearance at low concentrations of protein where the two-state conversion of monomeric protein becomes predominant. In the second part, the focus is directed to two-state folding and movements of the transition state ensemble. The movements are used to derive information about the shape of the free-energy profile for folding. It emerges from a comparison of the kinetic behaviour of several small model proteins that the free-energy barrier for folding could be generally broad and level. An attractive feature of broad barriers is that, depending on minor variations in the fine-structure of the free-energy profile, they account for a wide range of seemingly unrelated folding data, including deviations from classical two-state kinetics determined by free-energy extrapolations.
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| 38. |
- Orrego Hernandez, Jessica, 1987, et al.
(författare)
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Engineering of Norbornadiene/Quadricyclane Photoswitches for Molecular Solar Thermal Energy Storage Applications
- 2020
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 53:8, s. 1478-1487
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Tidskriftsartikel (refereegranskat)abstract
- ConspectusRenewable energy resources are mostly intermittent and not evenly distributed geographically; for this reason, the development of new technologies for energy storage is in high demand.Molecules that undergo photoinduced isomerization reactions that are capable of absorbing light, storing it as chemical energy, and releasing it as thermal energy on demand are referred to as molecular solar thermal energy storage (MOST) or solar thermal fuels (STF). Such molecules offer a promising solution for solar energy storage applications. Different molecular systems have been investigated for MOST applications, such as norbornadienes, azobenzenes, stilbenes, ruthenium derivatives, anthracenes, and dihydroazulenes. The polycyclic strained molecule norbornadiene (NBD), which photoconverts to quadricyclane (QC), is of great interest because it has a high energy storage density and the potential to store energy for a very long time. Unsubstituted norbornadiene has some limitations in this regard, such as poor solar spectrum match and low quantum yield. In the past decade, our group has developed and tested new NBD systems with improved characteristics. Moreover, we have demonstrated their function in laboratory-scale test devices for solar energy harnessing, storage, and release.This Account describes the most impactful recent findings on how to engineer key properties of the NBD/QC system (photochemistry, energy storage, heat release, stability, and synthesis) as well as examples of test devices for solar energy capture and heat release. While it was known that introducing donor-acceptor groups allows for a red-shifted absorption that better matches the solar spectrum, we managed to introduce donor and acceptor groups with very low molecular weight, which allowed for an unprecedented solar spectrum match combined with high energy density. Strategic steric hindrance in some of these systems dramatically increases the storage time of the photoisomer QC, and dimeric systems have independent energies barriers that lead to an improved solar spectrum match, prolonged storage times, and higher energy densities. These discoveries offer a toolbox of possible chemical modifications that can be used to tune the properties of NBD/QC systems and make them suitable for the desired applications, which can be useful for anyone wanting to take on the challenge of designing efficient MOST systems.Several test devices have been built, for example, a hybrid MOST device that stores sunlight energy and heat water at the same time. Moreover, we developed a device for monitoring catalyzed QC to NBD conversion resulting in the possibility to quantify a significant macroscopic heat generation. Finally, we tested different formulations of polymeric composites that can absorb light during the day and release the energy as heat during the night for possible use in future window coating applications. These lab-scale realizations are formative and contribute to pushing the field forward toward the real-life application of MOST systems.
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| 39. |
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| 40. |
- Santoro, Stefano, et al.
(författare)
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Elucidation of Mechanisms and Selectivities of Metal-Catalyzed Reactions using Quantum Chemical Methodology
- 2016
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 49:5, s. 1006-1018
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Forskningsöversikt (refereegranskat)abstract
- Quantum chemical techniques today are indispensable for the detailed mechanistic understanding of catalytic reactions. The development of modem density functional theory approaches combined with the enormous growth in computer power have made it possible to treat quite large systems at a reasonable level of accuracy. Accordingly, quantum chemistry has been applied extensively to a wide variety of catalytic systems. A huge number of problems have been solved successfully, and vast amounts of chemical insights have been gained. In this Account, we summarize some of our recent work in this field. A number of examples concerned with transition metal-catalyzed reactions are selected, with emphasis on reactions with various kinds of selectivities. The discussed cases are (1) copper-catalyzed C-H bond amidation of indoles, (2) iridium-catalyzed C(sp(3))-H borylation of chlorosilanes, (3) vanadium-catalyzed Meyer-Schuster rearrangement and its combination with aldol- and Mannich-type additions, (4) palladium-catalyzed propargylic substitution with phosphorus nucleophiles, (5) rhodium-catalyzed 1:2 coupling of aldehydes and allenes, and finally (6) copper-catalyzed coupling of nitrones and alkynes to produce beta-lactams (Kinugasa reaction). First, the methodology adopted in these studies is presented briefly. The electronic structure method in the great majority of these kinds of mechanistic investigations has for the last two decades been based on density functional theory. In the cases discussed here, mainly the B3LYP functional has been employed in conjunction with Grimme's empirical dispersion correction, which has been shown to improve the calculated energies significantly. The effect of the surrounding solvent is described by implicit solvation techniques, and the thermochemical corrections are included using the rigid-rotor harmonic oscillator approximation. The reviewed examples are chosen to illustrate the usefulness and versatility of the adopted methodology in solving complex problems and proposing new detailed reaction mechanisms that rationalize the experimental findings. For each of the considered reactions, a consistent mechanism is presented, the experimentally observed selectivities are reproduced, and their sources are identified. Reproducing selectivities requires high accuracy in computing relative transition state energies. As demonstrated by the results summarized in this Account, this accuracy is possible with the use of the presented methodology, benefiting of course from a large extent of cancellation of systematic errors. It is argued that as the employed models become larger, the number of rotamers and isomers that have to be considered for every stationary point increases and a careful assessment of their energies is therefore necessary in order to ensure that the lowest energy conformation is located. This issue constitutes a bottleneck of the investigation in some cases and is particularly important when analyzing selectivities, since small energy differences need to be reproduced.
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| 41. |
- Sheng, Xiang, et al.
(författare)
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The Quantum Chemical Cluster Approach in Biocatalysis
- 2023
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 56:8, s. 938-947
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Forskningsöversikt (refereegranskat)abstract
- CONSPECTUS: The quantum chemical cluster approach has been used for modeling enzyme active sites and reaction mechanisms for more than two decades. In this methodology, a relatively small part of the enzyme around the active site is selected as a model, and quantum chemical methods, typically density functional theory, are used to calculate energies and other properties. The surrounding enzyme is modeled using implicit solvation and atom fixing techniques. Over the years, a large number of enzyme mechanisms have been solved using this method. The models have gradually become larger as a result of the faster computers, and new kinds of questions have been addressed. In this Account, we review how the cluster approach can be utilized in the field of biocatalysis. Examples from our recent work are chosen to illustrate various aspects of the methodology. The use of the cluster model to explore substrate binding is discussed first. It is emphasized that a comprehensive search is necessary in order to identify the lowest-energy binding mode(s). It is also argued that the best binding mode might not be the productive one, and the full reactions for a number of enzyme–substrate complexes have therefore to be considered to find the lowest-energy reaction pathway. Next, examples are given of how the cluster approach can help in the elucidation of detailed reaction mechanisms of biocatalytically interesting enzymes, and how this knowledge can be exploited to develop enzymes with new functions or to understand the reasons for lack of activity toward non-natural substrates. The enzymes discussed in this context are phenolic acid decarboxylase and metal-dependent decarboxylases from the amidohydrolase superfamily. Next, the application of the cluster approach in the investigation of enzymatic enantioselectivity is discussed. The reaction of strictosidine synthase is selected as a case study, where the cluster calculations could reproduce and rationalize the selectivities of both the natural and non-natural substrates. Finally, we discuss how the cluster approach can be used to guide the rational design of enzyme variants with improved activity and selectivity. Acyl transferase from Mycobacterium smegmatis serves as an instructive example here, for which the calculations could pinpoint the factors controlling the reaction specificity and enantioselectivity. The cases discussed in this Account highlight thus the value of the cluster approach as a tool in biocatalysis. It complements experiments and other computational techniques in this field and provides insights that can be used to understand existing enzymes and to develop new variants with tailored properties.
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| 42. |
- Siegbahn, Per E. M.
(författare)
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Structures and Energetics for O-2 Formation in Photosystem II
- 2009
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 42:12, s. 1871-1880
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Forskningsöversikt (refereegranskat)abstract
- Water oxidation, forming O-2 from water and sunlight, is a fundamental process for life on earth. In nature, the enzyme photosystem II (PSII) catalyzes this reaction. The oxygen evolving complex (OEC) the complex within PSII that catalyzes the actual formation of the O-O bond, contains four manganese atoms and one calcium atom connected by oxo bonds. Seven amino acid side chains in the structure, mostly carboxylates, are ligated to the metal atoms. In the study of many enzyme mechanisms, theoretical modeling using density functional theory has served as an indispensable tool. This Account summarizes theoretical research to elucidate,the mechanism for water oxidation in photosynthesis, including the most recent findings. The development of successively larger models, ranging from 50 atoms in the active site up to the present model size of 170 atoms, has revealed the mechanism Of O-2 formation with increasing detail. The X-ray crystal structures of PSII have provided a framework for optimizing the theoretical models. By constraint of the backbone atoms to be at the same positions as those in the X-ray structures, the theoretical structures are in good agreement with both the measured electron density and extended X-ray absorption fine structure (EXAFS) interpretations. By following the structural and energetic changes in those structures through the different steps in the catalytic process, we have modeled the oxidation of the catalytic complex, the binding of the two substrate water molecules, and the subsequent deprotonations of those substrate molecules. In these models, the OEC forms a basin into which the water molecules naturally fit. These findings demonstrate that the binding of the second water molecule causes a reconstruction, results that are consistent with earlier EXAFS measurements. Most importantly, this Account describes a low-barrier mechanism for formation of the O-O bond, involving an oxygen radical that reacts with a mu-oxo ligand of the OEC. Further research revealed that the oxygen radical is bound in the Mn3Ca cube rather than to the outside manganese. This Account provides detailed diagrams of the energetics of the different S-transitions both without and with a membrane gradient. An interesting detail of these reactions concerns the role of the tyrosine (Tyr(z)), which appears as an intermediate radical in the oxidation of the OEC. By simple electrostatic arguments, these results show that the initial oxidation of Tyr(z) is downhill for the first two transitions but uphill for the final ones. In these later transitions, the oxidation of the OEC is coupled to deprotonations of water.
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| 43. |
- Sirohiwal, Abhishek, et al.
(författare)
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Reaction Center Excitation in Photosystem II : From Multiscale Modeling to Functional Principles
- 2023
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Ingår i: Accounts of Chemical Research. - 0001-4842 .- 1520-4898. ; 56:21, s. 2921-2932
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Forskningsöversikt (refereegranskat)abstract
- ConspectusOxygenic photosynthesis is the fundamental energy-converting process that utilizes sunlight to generate molecular oxygen and the organic compounds that sustain life. Protein–pigment complexes harvest light and transfer excitation energy to specialized pigment assemblies, reaction centers (RC), where electron transfer cascades are initiated. A molecular-level understanding of the primary events is indispensable for elucidating the principles of natural photosynthesis and enabling development of bioinspired technologies. The primary enzyme in oxygenic photosynthesis is Photosystem II (PSII), a membrane-embedded multisubunit complex, that catalyzes the light-driven oxidation of water. The RC of PSII consists of four chlorophyll a and two pheophytin a pigments symmetrically arranged along two core polypeptides; only one branch participates in electron transfer. Despite decades of research, fundamental questions remain, including the origin of this functional asymmetry, the nature of primary charge-transfer states and the identity of the initial electron donor, the origin of the capability of PSII to enact charge separation with far-red photons, i.e., beyond the “red limit” where individual chlorophylls absorb, and the role of protein conformational dynamics in modulating charge-separation pathways.In this Account, we highlight developments in quantum-chemistry based excited-state computations for multipigment assemblies and the refinement of protocols for computing protein-induced electrochromic shifts and charge-transfer excitations calibrated with modern local correlation coupled cluster methods. We emphasize the importance of multiscale atomistic quantum-mechanics/molecular-mechanics and large-scale molecular dynamics simulations, which enabled direct and accurate modeling of primary processes in RC excitation at the quantum mechanical level.Our findings show how differential protein electrostatics enable spectral tuning of RC pigments and generate functional asymmetry in PSII. A chlorophyll pigment on the active branch (ChlD1) has the lowest site energy in PSII and is the primary electron donor. The complete absence of low-lying charge-transfer states within the central pair of chlorophylls excludes a long-held assumption about the initial charge separation. Instead, we identify two primary charge separation pathways, both with the same pheophytin acceptor (PheoD1): a fast pathway with ChlD1 as the primary electron donor (short-range charge-separation) and a slow pathway with PD1PD2 as the initial donor (long-range charge separation). The low-energy spectrum is dominated by two states with significant charge-transfer character, ChlD1δ+PheoD1δ− and PD1δ+PheoD1δ−. The conformational dynamics of PSII allows these charge-transfer states to span wide energy ranges, pushing oxygenic photosynthesis beyond the “red limit”. These results provide a quantum mechanical picture of the primary events in the RC of oxygenic photosynthesis, forming a solid basis for interpreting experimental observations and for extending photosynthesis research in new directions.
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| 44. |
- Smetana, Volodymyr, et al.
(författare)
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Gold Polar Intermetallics : Structural Versatility through Exclusive Bonding Motifs
- 2017
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 50:11, s. 2633-2641
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Forskningsöversikt (refereegranskat)abstract
- The design of new materials with desired chemical and physical characteristics requires thorough understanding of the underlying composition structure property relationships and the experimental possibility of their modification through the controlled involvement of new components. From this point of view, intermetallic phases, a class of compounds formed by two or more metals, present an endless field of combinations that produce several chemical compound classes ranging from simple alloys to true ionic compounds. Polar intermetallics (PICs) belong to the class that is electronically situated in the middle, between Hume-Rothery phases and Zintl compounds and possessing e/a (valence electron per atom) values around 2. In contrast to the latter, where logical rules of formation and classification systems were developed decades ago, polar intermetallics remain a dark horse with a huge diversity of crystal structures but unclear mechanisms of their formation. Partial incorporation of structural and bonding features from both nonpolar and Zintl compounds is commonly observed here. A decent number of PICs can be described in terms of complex metallic alloys (CMAs) following the Hume-Rothery electron-counting schemes but exhibit electronic structure changes that cannot be explained by the latter. Our research is aimed at the discovery and synthesis of new polar intermetallic compounds, their structural characterization, and investigation of their properties in line with the analysis of the principles connecting all of these components. Understanding of the basic structural tendencies is one of the most anticipated outcomes of this analysis, and systematization of the available knowledge is the initial and most important step.In this Account, we focus on a well-represented but rather small section of PICs: ternary intermetallic compounds of gold with electropositive and post-transition metals of groups 12 to 15. The strong influence of relativistic effects in its chemical bonding results in special, frequently unique structural motifs, while at the same time gold participates in common structure types as an ordinary transition element. Enhanced bonding strength leads to the formation and stabilization of complex homo-and heteroatomic clusters and networks that are compositionally restricted to just a few options throughout the periodic table. Because it has the highest absolute electronegativity among metals, comparable to those of some halogens, gold usually plays the role of an anion, even being able to form true salts with the most electropositive metals. We discuss the occurrence of the structure types and show the place of gold intermetallics in the general picture. Among the structures considered are ones as common as AIB(2) or BaAl4 types, in line with the recently discovered diamond-like homoatomic metal networks, formation of local fivefold symmetry, different types of tunneled structures, and more complex intergrown multicomponent structures.
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| 45. |
- Soares, Ruben R. G., et al.
(författare)
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Rolling Circle Amplification in Integrated Microsystems : An Uncut Gem toward Massively Multiplexed Pathogen Diagnostics and Genotyping
- 2021
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 54:21, s. 3979-3990
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Forskningsöversikt (refereegranskat)abstract
- The development of robust methods allowing the precise detection of specific nucleic acid sequences is of major societal relevance, paving the way for significant advances in biotechnology and biomedical engineering. These range from a better understanding of human disease at a molecular level, allowing the discovery and development of novel biopharmaceuticals and vaccines, to the improvement of biotechnological processes providing improved food quality and safety, efficient green fuels, and smart textiles. Among these applications, the significance of pathogen diagnostics as the main focus of this Account has become particularly clear during the recent SARS-CoV-2 pandemic. In this context, while RT-PCR is the gold standard method for unambiguous detection of genetic material from pathogens, other isothermal amplification alternatives circumventing rapid heating-cooling cycles up to similar to 95 degrees C are appealing to facilitate the translation of the assay into point-of-care (PoC) analytical platforms. Furthermore, the possibility of routinely multiplexing the detection of tens to hundreds of target sequences with single base pair specificity, currently not met by state-of-the-art methods available in clinical laboratories, would be instrumental along the path to tackle emergent viral variants and antimicrobial resistance genes. Here, we advocate that padlock probes (PLPs), first reported by Nilsson et al. in 1994, coupled with rolling circle amplification (RCA), termed here as PLP-RCA, is an underexploited technology in current arena of isothermal nucleic acid amplification tests (NAATs) providing an unprecedented degree of multiplexing, specificity, versatility, and amenability to integration in miniaturized PoC platforms. Furthermore, the intrinsically digital amplification of PLP-RCA retains spatial information and opens new avenues in the exploration of pathogenesis with spatial multiomics analysis of infected cells and tissue. The Account starts by introducing PLP-RCA in a nutshell focusing individually on the three main assay steps, namely, (1) PLP design and ligation mechanism, (2) RCA after probe ligation, and (3) detection of the RCA products. Each subject is touched upon succinctly but with sufficient detail for the reader to appreciate some assay intricacies and degree of versatility depending on the analytical challenge at hand. After familiarizing the reader with the method, we discuss specific examples of research in our group and others using PLP-RCA for viral, bacterial, and fungal diagnostics in a variety of clinical contexts, including the genotyping of antibiotic resistance genes and viral subtyping. Then, we dissect key developments in the miniaturization and integration of PLPRCA to minimize user input, maximize analysis throughput, and expedite the time to results, ultimately aiming at PoC applications. These developments include molecular enrichment for maximum sensitivity, spatial arrays to maximize analytical throughput, automation of liquid handling to streamline the analytical workflow in miniaturized devices, and seamless integration of signal transduction to translate RCA product titers (and ideally spatial information) into a readable output. Finally, we position PLP-RCA in the current landscape of NAATs and furnish a systematic Strengths, Weaknesses, Opportunities and Threats analysis to shine light upon unpolished edges to uncover the gem with potential for ubiquitous, precise, and unbiased pathogen diagnostics.
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| 46. |
- Stubelius, Alexandra, 1983, et al.
(författare)
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The Chemistry of Boronic Acids in Nanomaterials for Drug Delivery
- 2019
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 52:11, s. 3108-3119
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Forskningsöversikt (refereegranskat)abstract
- Interest in increasing drug delivery efficiency has risen over the past decade both as a means to improve efficacy of already clinically available drugs and due to the increased difficulties of approving new drugs. As a functional group for targeted drug delivery, boronic acids (BAs) have been incorporated in polymeric particles both as a stimuli-responsive functional group and as a targeting ligand. Here, BA chemistry presents a wealth of opportunities for biological applications. It not only reacts with several chemical markers of disease such as reactive oxygen species (ROS), adenosine triphosphate (ATP), glucose, and reduced pH, but it also acts as ligands for diols such as sialic acid. These stimuli-responsive drug delivery systems optimize delivery of therapeutics based on rational design and precise molecular engineering. When designing materials containing BA, the unique chemical properties are important to take into consideration such as its vacant p-orbital, its molecular geometry, and the designed acid's pK(a). Instead of behaving as most carboxylic acids that donate protons, BAs instead primarily act as Lewis acids that accept electrons. In aqueous solution, most polymers containing BA exist in an equilibrium between their triangular hydrophobic form and a tetrahedral hydrophilic form. The most common pK(a)'s are in the nonphysiological range of 8-10, and much ongoing research focuses on modifying BAs into materials sensitive to a more physiologically relevant pH range. So far, BA moieties have been incorporated into a stunning array of materials, ranging from small molecules that can self-assemble into higher order structures such as micelles and polymeric micelles, via larger polymeric assemblies, to large scale hydrogels. With the abundance of biological molecules containing diols and polyhydroxy motifs, BA-containing materials have proven valuable in several biomedical applications such as treatment of cancer, diabetes, obesity, and bacterial infections. Both materials functionalized with BA and boronic esters display good safety profiles in vitro and in vivo; thus, BA-containing materials represent promising carriers for responsive delivery systems with great potential for clinical translation. The intention of this Account is to showcase the versatility of BA for biomedical applications. We first discuss the chemistry of BA and what to consider when designing BA-containing materials. Further, we review how its chemistry recently has been applied to nanomaterials for enhanced delivery efficiency, both as a stimuli-responsive group and as a targeting ligand. Lastly, we discuss the current limitations and further perspectives of BA in biomaterials, based on the great benefits that can come from utilizing the unique BA chemistry to enhance drug delivery efficiency.
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| 47. |
- Svendsen, John, et al.
(författare)
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Very Short and Stable Lactoferricin-Derived Antimicrobial Peptides : Design Principles and Potential Uses
- 2019
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 52:3, s. 749-759
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Tidskriftsartikel (refereegranskat)abstract
- ConspectusThe alarming rate at which micro-organisms are developing resistance to conventional antibiotics represents one of the global challenges of our time. There is currently ample space in the antibacterial drug pipeline, and scientists are trying to find innovative and novel strategies to target the microbial enemies. Nature has remained a source of inspiration for most of the antibiotics developed and used, and the immune molecules produced by the innate defense systems, as a first line of defense, have been heralded as the next source of antibiotics. Most living organisms produce an arsenal of antimicrobial peptides (AMPs) to rapidly fend off intruding pathogens, and several different attempts have been made to transform this versatile group of compounds into the next generation of antibiotics. However, faced with the many hurdles of using peptides as drugs, the success of these defense molecules as therapeutics remains to be realized. AMPs derived from the proteolytic degradation of the innate defense protein lactoferrin have been shown to display several favorable antimicrobial properties. In an attempt to investigate the biological and pharmacological properties of these much shorter AMPs, the sequence dependence was investigated, and it was shown, through a series of truncation experiments, that these AMPs in fact can be prepared as tripeptides, with improved antimicrobial activity, via the incorporation of unnatural hydrophobic residues and terminal cappings. In this Account, we describe how this class of promising cationic tripeptides has been developed to specifically address the main challenges limiting the general use of AMPs. This has been made possible through the identification of the antibacterial pharmacophore and via the incorporation of a range of unnatural hydrophobic and cationic amino acids. Incorporation of these residues at selected positions has allowed us to extensively establish how these compounds interact with the major proteolytic enzymes trypsin and chymotrypsin and also the two major drug-binding plasma proteins serum albumin and α-1 glycoprotein. Several of the challenges associated with using AMPs relate to their size, susceptibility to rapid proteolytic degradation, and poor oral bioavailability. Our studies have addressed these issues in detail, and the results have allowed us to effectively design and prepare active and metabolically stable AMPs that have been evaluated in a range of functional settings. The optimized short AMPs display inhibitory activities against a plethora of micro-organisms at low micromolar concentrations, and they have been shown to target resistant strains of both bacteria and fungi alike with a very rapid mode of action. Our Account further describes how these compounds behave in in vivo experiments and highlights both the challenges and possibilities of the intriguing compounds. In several areas, they have been shown to exhibit comparable or superior activity to established antibacterial, antifungal, and antifouling commercial products. This illustrates their ability to effectively target and eradicate various microbes in a variety of settings ranging from the ocean to the clinic.
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| 48. |
- Thornton, Joel A., et al.
(författare)
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Evaluating Organic Aerosol Sources and Evolution with a Combined Molecular Composition and Volatility Framework Using the Filter Inlet for Gases and Aerosols (FIGAERO)
- 2020
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 53:8, s. 1415-1426
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Forskningsöversikt (refereegranskat)abstract
- The complex array of sources and transformations of organic carbonaceous material that comprises an important fraction of atmospheric fine particle mass, known as organic aerosol, has presented a long running challenge for accurate predictions of its abundance, distribution, and sensitivity to anthropogenic activities. Uncertainties about changes in atmospheric aerosol particle sources and abundance over time translate to uncertainties in their impact on Earth's climate and their response to changes in air quality policy. One limitation in our understanding of organic aerosol has been a lack of comprehensive measurements of its molecular composition and volatility, which can elucidate sources and processes affecting its abundance. Herein we describe advances in the development and application of the Filter Inlet for Gases and Aerosols (FIGAERO) coupled to field-deployable High-Resolution Time-of-Flight Chemical Ionization Mass Spectrometers (HRToF-CIMS). The FIGAERO HRToFCIMS combination broadly probes gas and particulate OA molecular composition by using programmed thermal desorption of particles collected on a Teflon filter with subsequent detection and speciation of desorbed vapors using inherently quantitative selected-ion chemical ionization. The thermal desorption provides a means to obtain quantitative insights into the volatility of particle components and thus the physicochemical nature of the organic material that will govern its evolution in the atmosphere. In this Account, we discuss the design and operation of the FIGAERO, when coupled to the HRToF-CIMS, for quantitative characterization of the molecular-level composition and effective volatility of organic aerosol in the laboratory and field. We provide example insights gleaned from its deployment, which improve our understanding of organic aerosol sources and evolution. Specifically, we connect thermal desorption profiles to the effective equilibrium saturation vapor concentration and enthalpy of vaporization of detected components. We also show how application of the FIGAERO HRToF-CIMS to environmental simulation chamber experiments and the field provide new insights and constraints on the chemical mechanisms governing secondary organic aerosol formation and dynamic evolution. We discuss the associated challenges of thermal decomposition during desorption and calibration of both the volatility axis and signal. We also illustrate how the FIGAERO HRToF-CIMS can provide additional insights into organic aerosol through isothermal evaporation experiments as well as for detection of ultrafine particulate composition. We conclude with a description of future opportunities and needs for its ability to further organic aerosol science.
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| 49. |
- Vazdar, Mario, et al.
(författare)
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Arginine "magic" : Guanidinium Like-Charge Ion Pairing from Aqueous Salts to Cell Penetrating Peptides
- 2018
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 51:6, s. 1455-1464
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Tidskriftsartikel (refereegranskat)abstract
- It is a textbook knowledge that charges of the same polarity repel each other. For two monovalent ions in the gas phase at a close contact this repulsive interaction amounts to hundreds of kilojoules per mole. In aqueous solutions, however, this Coulomb repulsion is strongly attenuated by a factor equal to the dielectric constant of the medium. The residual repulsion, which now amounts only to units of kilojoules per mole, may be in principle offset by attractive interactions. Probably the smallest cationic pair, where a combination of dispersion and cavitation forces overwhelms the Coulomb repulsion, consists of two guanidinium ions in water. Indeed, by a combination of molecular dynamics with electronic structure calculations and electrophoretic, as well as spectroscopic, experiments, we have demonstrated that aqueous guanidinium cations form (weakly) thermodynamically stable like-charge ion pairs.The importance of pairing of guanidinium cations in aqueous solutions goes beyond a mere physical curiosity, since it has significant biochemical implications. Guanidinium chloride is known to be an efficient and flexible protein denaturant. This is due to the ability of the orientationally amphiphilic guanidinium cations to disrupt various secondary structural motifs of proteins by pairing promiscuously with both hydrophobic and hydrophilic groups, including guanidinium-containing side chains of arginines.The fact that the cationic guanidinium moiety forms the dominant part of the arginine side chain implies that the like-charge ion pairing may also play a role for interactions between peptides and proteins. Indeed, arginine-arginine pairing has been frequently found in structural protein databases. In particular, when strengthened by a presence of negatively charged glutamate, aspartate, or C-terminal carboxylic groups, this binding motif helps to stabilize peptide or protein dimers and is also found in or near active sites of several enzymes.The like-charge pairing of the guanidinium side-chain groups may also hold the key to the understanding of the arginine "magic", that is, the extraordinary ability of arginine-rich polypeptides to passively penetrate across cellular membranes. Unlike polylysines, which are also highly cationic but lack the ease in crossing membranes, polyarginines do not exhibit mutual repulsion. Instead, they accumulate at the membrane, weaken it, and might eventually cross in a concerted, "train-like" manner. This behavior of arginine-rich cell penetrating peptides can be exploited when devising smart strategies how to deliver in a targeted way molecular cargos into the cell.
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| 50. |
- Warner, Madeleine C., et al.
(författare)
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Mechanistic Aspects on Cyclopentadienylruthenium Complexes in Catalytic Racemization of Alcohols
- 2013
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Ingår i: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 46:11, s. 2545-2555
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Forskningsöversikt (refereegranskat)abstract
- Cyclopentadienylruthenium complexes commonly serve as efficient transition metal catalysts in the racemization of alcohols. The combination of the racemization reaction with enzymatic resolution leads to dynamic kinetic resolution (DKR). In DKR, a theoretical yield of 100% is possible, making it a powerful tool for enantioselective synthesis. In this Account, we summarize the most important mechanistic aspects of racemization of alcohols reported over the past decade based on both experimental and computational results. Precatalyst activation is often necessary, either by heating the reaction or by adding an alkoxide-type base. The subsequent alcohol-alkoxide exchange is rapid and introduces the substrate into the catalytic cycle. This exchange requires a free coordination site, which may be created via several different mechanisms. Following alkoxide formation, racemization occurs via beta-hydride elimination and subsequent readdition. In cyclopentadienyldicarbonylruthenium alkoxide complexes, which are 18-electron complexes, researchers originally considered two mechanisms for the creation of the free coordination site required for beta-hydride elimination: a change in hapticity of the cyclopentadienyl ligand from eta 5 to eta 3 and dissociation of a CO ligand. Based on computational and experimental results, we have found strong support for the pathway involving CO dissociation. Researchers had also wondered if the substrate remains coordinated to the metal center (the inner-sphere mechanism) during the hydrogen transfer step(s). Using competition and crossover experiments, we found strong evidence for an inner-sphere mechanism. In summary, we have obtained a detailed picture of the racemization of alcohols by cyclopentadienylruthenium catalysts, leading to the development of more efficient catalytic systems for racemization.
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