| 1. |
- Ahmed, Md Estak, et al.
(författare)
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A Bidirectional Bioinspired [FeFe]-Hydrogenase Model
- 2022
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 144:8, s. 3614-3625
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Tidskriftsartikel (refereegranskat)abstract
- With the price-competitiveness of solar and wind power, hydrogen technologies may be game changers for a cleaner, defossilized, and sustainable energy future. H-2 can indeed be produced in electrolyzers from water, stored for long periods, and converted back into power, on demand, in fuel cells. The feasibility of the latter process critically depends on the discovery of cheap and efficient catalysts able to replace platinum group metals at the anode and cathode of fuel cells. Bioinspiration can be key for designing such alternative catalysts. Here we show that a novel class of iron-based catalysts inspired from the active site of [FeFe]-hydrogenase behave as unprecedented bidirectional electrocatalysts for interconverting H-2 and protons efficiently under near-neutral aqueous conditions. Such bioinspired catalysts have been implemented at the anode of a functional membrane-less H-2/O-2 fuel cell device.
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| 2. |
- Glover, Starla, et al.
(författare)
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Pourbaix Diagram, Proton-Coupled Electron Transfer, and Decay Kinetics of a Protein Tryptophan Radical : Comparing the Redox Properties of W32• and Y32• Generated Inside the Structurally Characterized α3W and α3Y Proteins
- 2018
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 140:1, s. 185-192
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Tidskriftsartikel (refereegranskat)abstract
- Protein-based “hole” hopping typically involves spatially arranged redox-active tryptophan or tyrosine residues. Thermodynamic information is scarce for this type of process. The well-structured α3W model protein was studied by protein film square wave voltammetry and transient absorption spectroscopy to obtain a comprehensive thermodynamic and kinetic description of a buried tryptophan residue. A Pourbaix diagram, correlating thermodynamic potentials (E°′) with pH, is reported for W32 in α3W and compared to equivalent data recently presented for Y32 in α3Y (Ravichandran, K. R.; Zong, A. B.; Taguchi, A. T.; Nocera, D. G.; Stubbe, J.; Tommos, C. J. Am. Chem. Soc. 2017, 139, 2994−3004). The α3W Pourbaix diagram displays a pKOX of 3.4, a E°′(W32(N•+/NH)) of 1293 mV, and a E°′(W32(N•/NH); pH 7.0) of 1095 ± 4 mV versus the normal hydrogen electrode. W32(N•/NH) is 109 ± 4 mV more oxidizing than Y32(O•/OH) at pH 5.4–10. In the voltammetry measurements, W32 oxidation–reduction occurs on a time scale of about 4 ms and is coupled to the release and subsequent uptake of one full proton to and from bulk. Kinetic analysis further shows that W32 oxidation likely involves pre-equilibrium electron transfer followed by proton transfer to a water or small water cluster as the primary acceptor. A well-resolved absorption spectrum of W32• is presented, and analysis of decay kinetics show that W32• persists ∼104 times longer than aqueous W• due to significant stabilization by the protein. The redox characteristics of W32 and Y32 are discussed relative to global and local protein properties.
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| 3. |
- Grigioni, Ivan, et al.
(författare)
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Photoinduced Charge-Tran sfer Dynamics in WO3/BiVO4 Photoanodes Probed through Midinfrared Transient Absorption Spectroscopy
- 2018
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Ingår i: Journal of the American Chemical Society. - : AMER CHEMICAL SOC. - 0002-7863 .- 1520-5126. ; 140:43, s. 14042-14045
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Tidskriftsartikel (refereegranskat)abstract
- The dynamics of photopromoted electrons in BiVO4, WO3, and WO3/BiVO4 heterojunction electrodes has been directly probed by transient absorption (TA) midinfrared (mid-IR) spectroscopy in the picosecond to microsecond time range. By comparison of the dynamics recorded with the two individual oxides at 2050 cm(-1) with that of the heterojunction system after excitation at different wavelengths, electron-transfer processes between selectively excited BiVO4 and WO3 have been directly tracked for the first time. These results support the charge carrier interactions which were previously hypothesized by probing the BiVO4 hole dynamics through TA spectroscopy in the visible range. Nanosecond mid-IR TA experiments confirmed that charge carrier separation occurs in WO3/BiVO4 electrodes under visible-light excitation, persisting up to the microsecond time scale.
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| 4. |
- Liu, Aijie, et al.
(författare)
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Panchromatic Ternary Polymer Dots Involving Sub-Picosecond Energy and Charge Transfer for Efficient and Stable Photocatalytic Hydrogen Evolution
- 2021
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 143:7, s. 2875-2885
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Tidskriftsartikel (refereegranskat)abstract
- Panchromatic ternary polymer dots (Pdots) consisting of two conjugated polymers (PFBT and PFODTBT) based on fluorene and benzothiadiazole groups, and one small molecular acceptor (ITIC) have been prepared and assessed for photocatalytic hydrogen production with the assistance of a Pt cocatalyst. Femtosecond transient absorption spectroscopic studies of the ternary Pdots have revealed both energy and charge transfer processes that occur on the time scale of sub-picosecond between the different components. They result in photogenerated electrons being located mainly at ITIC, which acts as both electron and energy acceptor. Results from cryo-transmission electron microscopy suggest that ITIC forms crystalline phases in the ternary Pdots, facilitating electron transfer from ITIC to the Pt cocatalyst and promoting the final photocatalytic reaction yield. Enhanced light absorption, efficient charge separation, and the ideal morphology of the ternary Pdots have rendered an external quantum efficiency up to 7% at 600 nm. Moreover, the system has shown a high stability over 120 h without obvious degradation of the photocatalysts.
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| 5. |
- Liu, Tianfei, et al.
(författare)
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Elucidating Proton-Coupled Electron Transfer Mechanisms of Metal Hydrides with Free Energy- and Pressure-Dependent Kinetics
- 2019
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Ingår i: Journal of the American Chemical Society. - : AMER CHEMICAL SOC. - 0002-7863 .- 1520-5126. ; 141:43, s. 17245-17259
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Tidskriftsartikel (refereegranskat)abstract
- Proton-coupled electron transfer (PCET) was studied in a series of tungsten hydride complexes with pendant pyridyl arms ([(PyCH2Cp)WH(CO)(3)], PyCH2Cp = pyridyl methyl cyclopentadienyl), triggered by laser flash-generated Ru-III-tris-bipyridine oxidants, in acetonitrile solution. The free energy dependence of the rate constant and the kinetic isotope effects (KIEs) showed that the PCET mechanism could be switched between concerted and the two stepwise PCET mechanisms (electron-first or proton-first) in a predictable fashion. Straightforward and general guidelines for how the relative rates of the different mechanisms depend on oxidant and base are presented. The rate of the concerted reaction should depend symmetrically on changes in oxidant and base strength, that is on the overall Delta G(PCET)(0), and we argue that an "asynchronous" behavior would not be consistent with a model where the electron and proton tunnel from a common transition state. The observed rate constants and KIEs were examined as a function of hydrostatic pressure (1-2000 bar) and were found to exhibit qualitatively different dependence on pressure for different PCET mechanisms. This is discussed in terms of different volume profiles of the PCET mechanisms as well as enhanced proton tunneling for the concerted mechanism. The results allowed for assignment of the main mechanism operating in the different cases, which is one of the critical questions in PCET research. They also show how the rate of a PCET reaction will be affected very differently by changes of oxidant and base strength, depending on which mechanism dominates. This is of fundamental interest as well as of practical importance for rational design of, for example, catalysts for fuel cells and solar fuel formation, which operate in steps of PCET reactions. The mechanistic richness shown by this system illustrates that the specific mechanism is not intrinsic to a specific synthetic catalyst or enzyme active site but depends on the reaction conditions.
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| 6. |
- Nilsen-Moe, Astrid, et al.
(författare)
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Concerted and Stepwise Proton-Coupled Electron Transfer for Tryptophan-Derivative Oxidation with Water as the Primary Proton Acceptor : Clarifying a Controversy
- 2022
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 144:16, s. 7308-7319
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Tidskriftsartikel (refereegranskat)abstract
- Concerted electron-proton transfer (CEPT) reactions avoid charged intermediates and may be energetically favorable for redox and radical-transfer reactions in natural and synthetic systems. Tryptophan (W) often partakes in radical-transfer chains in nature but has been proposed to only undergo sequential electron transfer followed by proton transfer when water is the primary proton acceptor. Nevertheless, our group has shown that oxidation of freely solvated tyrosine and W often exhibit weakly pH-dependent proton-coupled electron transfer (PCET) rate constants with moderate kinetic isotope effects (KIE approximate to 2-5), which could be associated with a CEPT mechanism. These results and conclusions have been questioned. Here, we present PCET rate constants for W derivatives with oxidized Ru- and Zn-porphyrin photosensitizers, extracted from laser flash-quench studies. Alternative quenching/photo-oxidation methods were used to avoid complications of previous studies, and both the amine and carboxylic acid groups of W were protected to make the indole the only deprotonable group. With a suitably tuned oxidant strength, we found an ET-limited reaction at pH < 4 and weakly pH-dependent rates at pH > similar to 5 that are intrinsic to the PCET of the indole group with water (H2O) as the proton acceptor. The observed rate constants are up to more than 100 times higher than those measured for initial electron transfer, excluding the electron-first mechanism. Instead, the reaction can be attributed to CEPT. These conclusions are important for our view of CEPT in water and of PCET-mediated radical reactions with solvent-exposed tryptophan in natural systems.
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| 7. |
- Nilsen-Moe, Astrid, et al.
(författare)
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Proton-Coupled Electron Transfer from Tyrosine in the Interior of a de novo Protein : Mechanisms and Primary Proton Acceptor
- 2020
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 142:26, s. 11550-11559
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Tidskriftsartikel (refereegranskat)abstract
- Proton-coupled electron transfer (PCET) from tyrosine produces a neutral tyrosyl radical (Y-center dot) that is vital to many catalytic redox reactions. To better understand how the protein environment influences the PCET properties of tyrosine, we have studied the radical formation behavior of Y-32 in the alpha Y-3 model protein. The previously solved alpha Y-3 solution NMR structure shows that Y-32 is sequestered similar to 7.7 +/- 0.3 angstrom below the protein surface without any primary proton acceptors nearby. Here we present transient absorption kinetic data and molecular dynamics (MD) simulations to resolve the PCET mechanism associated with Y-32 oxidation. Y-32(center dot). was generated in a bimolecular reaction with [Ru(bpy)(3)](3+) formed by flash photolysis. At pH > 8, the rate constant of Y-32(center dot). formation (k(P)(CET)) increases by one order of magnitude per pH unit, corresponding to a proton-first mechanism via tyrosinate (PTET). At lower pH < 7.5, the pH dependence is weak and shows a previously measured KIE approximate to 2.5, which best fits a concerted mechanism. k(PC)(ET) is independent of phosphate buffer concentration at pH 6.5. This provides clear evidence that phosphate buffer is not the primary proton acceptor. MD simulations show that one to two water molecules can enter the hydrophobic cavity of alpha Y-3 and hydrogen bond to Y-32, as well as the possibility of hydrogen-bonding interactions between Y-32 and E-13, through structural fluctuations that reorient surrounding side chains. Our results illustrate how protein conformational motions can influence the redox reactivity of a tyrosine residue and how PCET mechanisms can be tuned by changing the pH even when the PCET occurs within the interior of a protein.
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| 8. |
- Rodriguez-Jimenez, Santiago, et al.
(författare)
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Self-Assembled Liposomes Enhance Electron Transfer for Efficient Photocatalytic CO2 Reduction
- 2022
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 144:21, s. 9399-9412
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Tidskriftsartikel (refereegranskat)abstract
- Light-driven conversion of CO2 to chemicals provides a sustainable alternative to fossil fuels, but homogeneous systems are typically limited by cross reactivity between different redox half reactions and inefficient charge separation. Herein, we present the bioinspired development of amphiphilic photosensitizer and catalyst pairs that self-assemble in lipid membranes to overcome some of these limitations and enable photocatalytic CO2 reduction in liposomes using precious metal-free catalysts. Using sodium ascorbate as a sacrificial electron source, a membrane-anchored alkylated cobalt porphyrin demonstrates higher catalytic CO production (1456 vs 312 turnovers) and selectivity (77 vs 11%) compared to its watersoluble nonalkylated counterpart. Time-resolved and steady-state spectroscopy revealed that self-assembly facilitates this performance enhancement by enabling a charge-separation state lifetime increase of up to two orders of magnitude in the dye while allowing for a ninefold faster electron transfer to the catalyst. Spectroelectrochemistry and density functional theory calculations of the alkylated Co porphyrin catalyst support a four-electron-charging mechanism that activates the catalyst prior to catalysis, together with key catalytic intermediates. Our molecular liposome system therefore benefits from membrane immobilization and provides a versatile and efficient platform for photocatalysis.
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| 9. |
- Tian, Lei, et al.
(författare)
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Understanding the Role of Surface States on Mesoporous NiO Films
- 2020
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 142:43
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Tidskriftsartikel (refereegranskat)abstract
- Surface states of mesoporous NiO semiconductor films have particular properties differing from the bulk and are able to dramatically influence the interfacial electron transfer and adsorption of chemical species. To achieve a better performance of NiO-based p-type dye-sensitized solar cells (p-DSCs), the function of the surface states has to be understood. In this paper, we applied a modified atomic layer deposition procedure that is able to passivate 72% of the surface states on NiO by depositing a monolayer of Al2O3. This provides us with representative control samples to study the functions of the surface states on NiO films. A main conclusion is that surface states, rather than the bulk, are mainly responsible for the conductivity in mesoporous NiO films. Furthermore, surface states significantly affect dye regeneration (with I–/I3– as redox couple) and hole transport in NiO-based p-DSCs. A new dye regeneration mechanism is proposed in which electrons are transferred from reduced dye molecules to intra-bandgap states, and then to I3– species. The intra-bandgap states here act as catalysts to assist I3– reduction. A more complete mechanism is suggested to understand the particular hole transport behavior in p-DSCs, in which the hole transport time is independent of light intensity. This is ascribed to the percolation hole hopping on the surface states. When the concentration of surface states was significantly reduced, the light-independent charge transport behavior in pristine NiO-based p-DSCs transformed into having an exponential dependence on light intensity, similar to that observed in TiO2-based n-type DSCs. These conclusions on the function of surface states provide new insight into the electronic properties of mesoporous NiO films.
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| 10. |
- Tyburski, Robin, et al.
(författare)
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Proton-Coupled Electron Transfer Guidelines, Fair and Square
- 2021
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 143:2, s. 560-576
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Tidskriftsartikel (refereegranskat)abstract
- Proton-coupled electron transfer (PCET) reactions are fundamental to energy transformation reactions in natural and artificial systems and are increasingly recognized in areas such as catalysis and synthetic chemistry. The interdependence of proton and electron transfer brings a mechanistic richness of reactivity, including various sequential and concerted mechanisms. Delineating between different PCET mechanisms and understanding why a particular mechanism dominates are crucial for the design and optimization of reactions that use PCET. This Perspective provides practical guidelines for how to discern between sequential and concerted mechanisms based on interpretations of thermodynamic data with temperature-, pressure-, and isotope-dependent kinetics. We present new PCET-zone diagrams that show how a mechanism can switch or even be eliminated by varying the thermodynamic (Delta G(PT)degrees and Delta G(ET)degrees) and coupling strengths for a PCET system. We discuss the appropriateness of asynchronous concerted PCET to rationalize observations in organic reactions, and the distinction between hydrogen atom transfer and other concerted PCET reactions. Contemporary issues and future prospects in PCET research are discussed.
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