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1.	Keable, Stephen M., et al. (author) Room temperature XFEL crystallography reveals asymmetry in the vicinity of the two phylloquinones in photosystem I 2021 In: Scientific Reports. - : Nature Publishing Group. - 2045-2322. ; 11:1 Journal article (peer-reviewed)abstract Photosystem I (PS I) has a symmetric structure with two highly similar branches of pigments at the center that are involved in electron transfer, but shows very different efficiency along the two branches. We have determined the structure of cyanobacterial PS I at room temperature (RT) using femtosecond X-ray pulses from an X-ray free electron laser (XFEL) that shows a clear expansion of the entire protein complex in the direction of the membrane plane, when compared to previous cryogenic structures. This trend was observed by complementary datasets taken at multiple XFEL beamlines. In the RT structure of PS I, we also observe conformational differences between the two branches in the reaction center around the secondary electron acceptors A1A and A1B. The π-stacked Phe residues are rotated with a more parallel orientation in the A-branch and an almost perpendicular confirmation in the B-branch, and the symmetry breaking PsaB-Trp673 is tilted and further away from A1A. These changes increase the asymmetry between the branches and may provide insights into the preferential directionality of electron transfer.
2.	Alonso-Mori, Roberto, et al. (author) Energy-dispersive X-ray emission spectroscopy using an X-ray free-electron laser in a shot-by-shot mode 2012 In: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 109:47, s. 19103-19107 Journal article (peer-reviewed)abstract The ultrabright femtosecond X-ray pulses provided by X-ray free-electron lasers open capabilities for studying the structure and dynamics of a wide variety of systems beyond what is possible with synchrotron sources. Recently, this probe-before-destroy approach has been demonstrated for atomic structure determination by serial X-ray diffraction of microcrystals. There has been the question whether a similar approach can be extended to probe the local electronic structure by X-ray spectroscopy. To address this, we have carried out femtosecond X-ray emission spectroscopy (XES) at the Linac Coherent Light Source using redox-active Mn complexes. XES probes the charge and spin states as well as the ligand environment, critical for understanding the functional role of redox-active metal sites. K beta(1,3) XES spectra of Mn-II and Mn-2(III,IV) complexes at room temperature were collected using a wavelength dispersive spectrometer and femtosecond X-ray pulses with an individual dose of up to > 100 MGy. The spectra were found in agreement with undamaged spectra collected at low dose using synchrotron radiation. Our results demonstrate that the intact electronic structure of redox active transition metal compounds in different oxidation states can be characterized with this shot-by-shot method. This opens the door for studying the chemical dynamics of metal catalytic sites by following reactions under functional conditions. The technique can be combined with X-ray diffraction to simultaneously obtain the geometric structure of the overall protein and the local chemistry of active metal sites and is expected to prove valuable for understanding the mechanism of important metalloproteins, such as photosystem II.
3.	Alonso-Mori, R., et al. (author) Towards characterization of photo-excited electron transfer and catalysis in natural and artificial systems using XFELs 2016 In: Faraday discussions. - : Royal Society of Chemistry (RSC). - 1359-6640 .- 1364-5498. ; 194, s. 621-638 Journal article (peer-reviewed)abstract The ultra-bright femtosecond X-ray pulses provided by X-ray Free Electron Lasers (XFELs) open capabilities for studying the structure and dynamics of a wide variety of biological and inorganic systems beyond what is possible at synchrotron sources. Although the structure and chemistry at the catalytic sites have been studied intensively in both biological and inorganic systems, a full understanding of the atomic-scale chemistry requires new approaches beyond the steady state X-ray crystallography and X-ray spectroscopy at cryogenic temperatures. Following the dynamic changes in the geometric and electronic structure at ambient conditions, while overcoming X-ray damage to the redox active catalytic center, is key for deriving reaction mechanisms. Such studies become possible by using the intense and ultra-short femtosecond X-ray pulses from an XFEL, where sample is probed before it is damaged. We have developed methodology for simultaneously collecting X-ray diffraction data and X-ray emission spectra, using an energy dispersive spectrometer, at ambient conditions, and used this approach to study the room temperature structure and intermediate states of the photosynthetic water oxidizing metallo-protein, photosystem II. Moreover, we have also used this setup to simultaneously collect the X-ray emission spectra from multiple metals to follow the ultrafast dynamics of light-induced charge transfer between multiple metal sites. A Mn-Ti containing system was studied at an XFEL to demonstrate the efficacy and potential of this method.
4.	Ames, William, et al. (author) Theoretical Evaluation of Structural Models of the S(2) State in the Oxygen Evolving Complex of Photosystem II : Protonation States and Magnetic Interactions 2011 In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 133:49, s. 19743-19757 Journal article (peer-reviewed)abstract Protonation states of water ligands and oxo bridges are intimately involved in tuning the electronic structures and oxidation potentials of the oxygen evolving complex (OEC) in Photosystem II, steering the mechanistic pathway, which involves at least five redox state intermediates S(n) (n = 0-4) resulting in the oxidation of water to molecular oxygen. Although protons are practically invisible in protein crystallography, their effects on the electronic structure and magnetic properties of metal active sites can be probed using spectroscopy. With the twin purpose of aiding the interpretation of the complex electron paramagnetic resonance (EPR) spectroscopic data of the OEC and of improving the view of the cluster at the atomic level, a complete set of protonation configurations for the S(2) state of the OEC were investigated, and their distinctive effects on magnetic properties of the cluster were evaluated. The most recent X-ray structure of Photosystem II at 1.9 Å resolution was used and refined to obtain the optimum structure for the Mn(4)O(5)Ca core within the protein pocket. Employing this model, a set of 26 structures was constructed that tested various protonation scenarios of the water ligands and oxo bridges. Our results suggest that one of the two water molecules that are proposed to coordinate the outer Mn ion (Mn(A)) of the cluster is deprotonated in the S(2) state, as this leads to optimal experimental agreement, reproducing the correct ground state spin multiplicity (S = 1/2), spin expectation values, and EXAFS-derived metal-metal distances. Deprotonation of Ca(2+)-bound water molecules is strongly disfavored in the S(2) state, but dissociation of one of the two water ligands appears to be facile. The computed isotropic hyperfine couplings presented here allow distinctions between models to be made and call into question the assumption that the largest coupling is always attributable to Mn(III). The present results impose limits for the total charge and the proton configuration of the OEC in the S(2) state, with implications for the cascade of events in the Kok cycle and for the water splitting mechanism.
5.	Arafa, Wael A. A., et al. (author) Dinuclear manganese complexes for water oxidation : evaluation of electronic effects and catalytic activity 2014 In: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 16:24, s. 11950-11964 Journal article (peer-reviewed)abstract During recent years significant progress has been made towards the realization of a sustainable and carbon-neutral energy economy. One promising approach is photochemical splitting of H2O into O-2 and solar fuels, such as H-2. However, the bottleneck in such artificial photosynthetic schemes is the H2O oxidation half reaction where more efficient catalysts are required that lower the kinetic barrier for this process. In particular catalysts based on earth-abundant metals are highly attractive compared to catalysts comprised of noble metals. We have now synthesized a library of dinuclear Mn-2 (II,III) catalysts for H2O oxidation and studied how the incorporation of different substituents affected the electronics and catalytic efficiency. It was found that the incorporation of a distal carboxyl group into the ligand scaffold resulted in a catalyst with increased catalytic activity, most likely because of the fact that the distal group is able to promote proton-coupled electron transfer (PCET) from the high-valent Mn species, thus facilitating O-O bond formation.
6.	Bag, Pushan, 1993-, et al. (author) Flavodiiron-mediated O2 photoreduction at photosystem I acceptor-side provides photoprotection to conifer thylakoids in early spring 2023 In: Nature Communications. - : Springer Nature. - 2041-1723. ; 14:1 Journal article (peer-reviewed)abstract Green organisms evolve oxygen (O2) via photosynthesis and consume it by respiration. Generally, net O2 consumption only becomes dominant when photosynthesis is suppressed at night. Here, we show that green thylakoid membranes of Scots pine (Pinus sylvestris L) and Norway spruce (Picea abies) needles display strong O2 consumption even in the presence of light when extremely low temperatures coincide with high solar irradiation during early spring (ES). By employing different electron transport chain inhibitors, we show that this unusual light-induced O2 consumption occurs around photosystem (PS) I and correlates with higher abundance of flavodiiron (Flv) A protein in ES thylakoids. With P700 absorption changes, we demonstrate that electron scavenging from the acceptor-side of PSI via O2 photoreduction is a major alternative pathway in ES. This photoprotection mechanism in vascular plants indicates that conifers have developed an adaptative evolution trajectory for growing in harsh environments.
7.	Barber, J., et al. (author) Using small molecule complexes to elucidate features of photosynthetic water oxidation - Discussion 2008 In: Philosophical Transactions of the Royal Society of London. Biological Sciences. - : ROYAL SOC. - 0962-8436 .- 1471-2970. ; 363:1494, s. 1279-1281 Journal article (other academic/artistic)
8.	Beckmann, K., et al. (author) Formation of stoichiometrically O-18-labelled oxygen from the oxidation of O-18-enriched water mediated by a dinuclear manganese complex : a mass spectrometry and EPR study 2008 In: Energy & Environmental Science. - : Royal Society of Chemistry (RSC). - 1754-5692 .- 1754-5706. ; 1:6, s. 668-676 Journal article (peer-reviewed)abstract Oxygen formation was detected for the oxidations of various multinuclear manganese complexes by oxone (HSO5-) in aqueous solution. To determine to what extent water was the source of the evolved O-2, (H2O)-O-18 isotope-labelling experiments coupled with membrane inlet mass spectrometry (MIMS) were carried out. We discovered that during the reaction of oxone with [Mn-2(OAc)(2)(bpmp)](+) (1), stoichiometrically labelled oxygen (O-18(2)) was formed. This is the first example of a homogeneous reaction mediated by a synthetic manganese complex where the addition of a strong chemical oxidant yields O-18(2) with labelling percentages matching the theoretically expected values for the case of both O-atoms originating from water. Experiments using lead acetate as an alternative oxidant supported this finding. A detailed investigation of the reaction by EPR spectroscopy, MIMS and Clark-type oxygen detection enabled us to propose potential reaction pathways.
9.	Beckmann, Katrin, et al. (author) On-line mass spectrometry : membrane inlet sampling 2009 In: Photosynthesis Research. - : Springer Netherlands. - 0166-8595 .- 1573-5079. ; 102:2-3, s. 511-522 Journal article (peer-reviewed)abstract Significant insights into plant photosynthesis and respiration have been achieved using membrane inlet mass spectrometry (MIMS) for the analysis of stable isotope distribution of gases. The MIMS approach is based on using a gas permeable membrane to enable the entry of gas molecules into the mass spectrometer source. This is a simple yet durable approach for the analysis of volatile gases, particularly atmospheric gases. The MIMS technique strongly lends itself to the study of reaction flux where isotopic labeling is employed to differentiate two competing processes; i.e., O2 evolution versus O2 uptake reactions from PSII or terminal oxidase/rubisco reactions. Such investigations have been used for in vitro studies of whole leaves and isolated cells. The MIMS approach is also able to follow rates of isotopic exchange, which is useful for obtaining chemical exchange rates. These types of measurements have been employed for oxygen ligand exchange in PSII and to discern reaction rates of the carbonic anhydrase reactions. Recent developments have also engaged MIMS for online isotopic fractionation and for the study of reactions in inorganic systems that are capable of water splitting or H2 generation. The simplicity of the sampling approach coupled to the high sensitivity of modern instrumentation is a reason for the growing applicability of this technique for a range of problems in plant photosynthesis and respiration. This review offers some insights into the sampling approaches and the experiments that have been conducted with MIMS.
10.	Benlloch, Reyes, et al. (author) Crystal structure and functional characterization of Photosystem II-associated carbonic anhydrase CAH3 in Chlamydomonas reinhardtii 2015 In: Plant Physiology. - : American Society of Plant Biologists. - 0032-0889 .- 1532-2548. ; 167:3, s. 950-962 Journal article (peer-reviewed)abstract In oxygenic photosynthesis, light energy is stored in the form of chemical energy by converting CO2 and water into carbohydrates.The light-driven oxidation of water that provides the electrons and protons for the subsequent CO2 fixation takes place inphotosystem II (PSII). Recent studies show that in higher plants, HCO3– increases PSII activity by acting as a mobile acceptor ofthe protons produced by PSII. In the green alga Chlamydomonas reinhardtii, a luminal carbonic anhydrase, CrCAH3, was suggested toimprove proton removal from PSII, possibly by rapid reformation of HCO3– from CO2. In this study, we investigated the interplaybetween PSII and CrCAH3 by membrane inlet mass spectrometry and x-ray crystallography. Membrane inlet mass spectrometrymeasurements showed that CrCAH3 was most active at the slightly acidic pH values prevalent in the thylakoid lumen underillumination. Two crystal structures of CrCAH3 in complex with either acetazolamide or phosphate ions were determined at 2.6- and2.7-Å resolution, respectively. CrCAH3 is a dimer at pH 4.1 that is stabilized by swapping of the N-terminal arms, a feature notpreviously observed in a-type carbonic anhydrases. The structure contains a disulfide bond, and redox titration of CrCAH3 functionwith dithiothreitol suggested a possible redox regulation of the enzyme. The stimulating effect of CrCAH3 and CO2/HCO3– on PSIIactivity was demonstrated by comparing the flash-induced oxygen evolution pattern of wild-type and CrCAH3-less PSIIpreparations. We showed that CrCAH3 has unique structural features that allow this enzyme to maximize PSII activity at lowpH and CO2 concentration.
11.	Bergmann, U, et al. (author) High-resolution X-ray spectroscopy of rare events : a different look at local structure and chemistry 2001 In: Journal of Synchrotron Radiation. - Univ Calif Berkeley, Lawrence Berkeley Lab, Phys Biosci Div, Berkeley, CA 94720 USA. Univ Calif Davis, Dept Appl Sci, Davis, CA 95616 USA. CALTECH, Jet Prop Lab, Pasadena, CA 91109 USA. : MUNKSGAARD INT PUBL LTD. - 0909-0495 .- 1600-5775. ; 8, s. 199-203 Journal article (peer-reviewed)abstract The combination of large-acceptance high-resolution X-ray optics with bright synchrotron sources permits quantitative analysis of rare events such as X-ray fluorescence from very dilute systems, weak fluorescence transitions or X-ray Raman scattering. Transition-metal K beta fluorescence contains information about spin and oxidation state; examples of the characterization of the Mn oxidation states in the oxygen-evolving complex of photosystem II and Mn-consuming spores from the marine bacillus SG-1 are presented. Weaker features of the K beta spectrum resulting from valence-level and 'interatomic' ligand to metal transitions contain detailed information on the ligand-atom type, distance and orientation. Applications of this spectral region to characterize the local structure of model compounds are presented. X-ray Raman scattering (XRS) is an extremely rare event, but also represents a unique technique to obtain bulk-sensitive low-energy (<600 eV) X-ray absorption fine structure (XAFS) spectra using hard ( 10 keV) X-rays. A photon is inelastically scattered, losing part of its energy to promote an electron into an unoccupied level. In many cases, the cross section is proportional to that of the corresponding absorption process yielding the same X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) features. XRS finds application for systems that defy XAFS analysis at low energies, e.g. liquids or highly concentrated complex systems, reactive compounds and samples under extreme conditions (pressure, temperature). Recent results are discussed.
12.	Bhowmick, Asmit, et al. (author) Going around the Kok cycle of the water oxidation reaction with femtosecond X-ray crystallography 2023 In: IUCrJ. - : International Union Of Crystallography. - 2052-2525. ; 10:6, s. 642-655 Research review (peer-reviewed)abstract The water oxidation reaction in photosystem II (PS II) produces most of the molecular oxygen in the atmosphere, which sustains life on Earth, and in this process releases four electrons and four protons that drive the downstream process of CO2 fixation in the photosynthetic apparatus. The catalytic center of PS II is an oxygen-bridged Mn4Ca complex (Mn4CaO5) which is progressively oxidized upon the absorption of light by the chlorophyll of the PS II reaction center, and the accumulation of four oxidative equivalents in the catalytic center results in the oxidation of two waters to dioxygen in the last step. The recent emergence of X-ray free-electron lasers (XFELs) with intense femtosecond X-ray pulses has opened up opportunities to visualize this reaction in PS II as it proceeds through the catalytic cycle. In this review, we summarize our recent studies of the catalytic reaction in PS II by following the structural changes along the reaction pathway via room-temperature X-ray crystallography using XFELs. The evolution of the electron density changes at the Mn complex reveals notable structural changes, including the insertion of OX from a new water molecule, which disappears on completion of the reaction, implicating it in the O-O bond formation reaction. We were also able to follow the structural dynamics of the protein coordinating with the catalytic complex and of channels within the protein that are important for substrate and product transport, revealing well orchestrated conformational changes in response to the electronic changes at the Mn4Ca cluster.
13.	Bhowmick, Asmit, et al. (author) Structural evidence for intermediates during O2 formation in photosystem II 2023 In: Nature. - : Springer Nature. - 0028-0836 .- 1476-4687. ; 617:7961, s. 629-636 Journal article (peer-reviewed)abstract In natural photosynthesis, the light-driven splitting of water into electrons, protons and molecular oxygen forms the first step of the solar-to-chemical energy conversion process. The reaction takes place in photosystem II, where the Mn4CaO5 cluster first stores four oxidizing equivalents, the S0 to S4 intermediate states in the Kok cycle, sequentially generated by photochemical charge separations in the reaction center and then catalyzes the O–O bond formation chemistry. Here, we report room temperature snapshots by serial femtosecond X-ray crystallography to provide structural insights into the final reaction step of Kok’s photosynthetic water oxidation cycle, the S3→[S4]→S0 transition where O2 is formed and Kok’s water oxidation clock is reset. Our data reveal a complex sequence of events, which occur over micro- to milliseconds, comprising changes at the Mn4CaO5 cluster, its ligands and water pathways as well as controlled proton release through the hydrogen-bonding network of the Cl1 channel. Importantly, the extra O atom Ox, which was introduced as a bridging ligand between Ca and Mn1 during the S2→S3 transition, disappears or relocates in parallel with Yz reduction starting at approximately 700 μs after the third flash. The onset of O2 evolution, as indicated by the shortening of the Mn1–Mn4 distance, occurs at around 1,200 μs, signifying the presence of a reduced intermediate, possibly a bound peroxide.
14.	Boniolo, Manuel, et al. (author) Electronic and geometric structure effects on one-electron oxidation of first-row transition metals in the same ligand framework 2021 In: Dalton Transactions. - : Royal Society of Chemistry. - 1477-9226 .- 1477-9234. ; 50:2, s. 660-674 Journal article (peer-reviewed)abstract Developing new transition metal catalysts requires understanding of how both metal and ligand properties determine reactivity. Since metal complexes bearing ligands of the Py5 family (2,6-bis-[(2-pyridyl)methyl] pyridine) have been employed in many fields in the past 20 years, we set out here to understand their redox properties by studying a series of base metal ions (M = Mn, Fe, Co, and Ni) within the Py5OH (pyridine-2,6-diylbis[di-(pyridin-2-yl)methanol]) variant. Both reduced (M-II) and the one-electron oxidized (M-III) species were carefully characterized using a combination of X-ray crystallography, X-ray absorption spectroscopy, cyclic voltammetry, and density-functional theory calculations. The observed metal-ligand interactions and electrochemical properties do not always follow consistent trends along the periodic table. We demonstrate that this observation cannot be explained by only considering orbital and geometric relaxation, and that spin multiplicity changes needed to be included into the DFT calculations to reproduce and understand these trends. In addition, exchange reactions of the sixth ligand coordinated to the metal, were analysed. Finally, by including published data of the extensively characterised Py5OMe (pyridine-2,6-diylbis[di-(pyridin-2-yl)methoxymethane])complexes, the special characteristics of the less common Py5OH ligand were extracted. This comparison highlights the non-innocent effect of the distal OH functionalization on the geometry, and consequently on the electronic structure of the metal complexes. Together, this gives a complete analysis of metal and ligand degrees of freedom for these base metal complexes, while also providing general insights into how to control electrochemical processes of transition metal complexes.
15.	Boniolo, Manuel, 1990-, et al. (author) Pentapyridyl Base Metal Complexes: Apical Ligand Exchange, Redox Potential, Stability and Water Oxidation Other publication (other academic/artistic)
16.	Boniolo, Manuel, et al. (author) Spin transition in a ferrous chloride complex supported by a pentapyridine ligand 2020 In: Chemical Communications. - : Royal Society of Chemistry. - 1359-7345 .- 1364-548X. ; 56:18, s. 2703-2706 Journal article (peer-reviewed)abstract Ferrous chloride complexes [FeIILxCl] commonly attain a high-spin state independently of the supporting ligand(s) and temperature. Herein, we present the first report of a complete spin crossover with T1/2 = 80 K in [FeII(Py5OH)Cl]+ (Py5OH = pyridine-2,6-diylbis[di(pyridin-2-yl)methanol]). Both spin forms of the complex are analyzed by X-ray spectroscopy and DFT calculations.
17.	Boniolo, Manuel (author) Structural, Electronic and Reactive Properties of Pentapyridyl - Base Metal Complexes : Relevance for Water Oxidation Catalysis 2021 Doctoral thesis (other academic/artistic)abstract The rationalization of chemical-physical proprieties of transition metal complexes is fundamental in order to understand and tune their reactivity. In this thesis, a systematic investigation of the geometrical and electronic properties of [M(Py5OH)Cl]+ complexes (M= Mn, Fe, Co, Ni) has been performed, and their ability to act as molecular water oxidation catalysts has been probed. Through this scientific journey, new insights into their chemical and physical properties have been revealed. The spin crossover behavior of the ferrous chloride complex ([Fe(Py5OH)Cl]PF6) is the first example of a molecular Fe(II) complex coordinated to a weak-field ligand that can be thermodynamically stable in a low-spin electron configuration (Chapter 3). The spin state also dictates the electrochemical proprieties of the one-electron oxidized state of all the metal complexes investigated in our study (Chapter 4). The atypical rhombicity of the manganese complex ([Mn(Py5OH)Cl]PF6) gives an unusual anisotropic EPR signal for a Mn(II, S = 5/2) complex. This is compared with the analog [Mn(Py5OMe)Cl]PF6 complex providing, in combination with DFT calculations, insight into how the magnetic parameters (i.e., zero field splitting) are affected by small structural changes (Chapter 5). Finally, I investigated the role of water as substrate for water oxidation catalysis with the [M(Py5OH)Cl]+ complexes. The addition of small amounts of water into a non-aqueous medium allowed trapping possible water-bound intermediates for the Fe complex in the M(III) oxidation state but not for the other complexes. Nevertheless, all Py5OH-metal complexes are not particularly active catalysts with a maximum turnover number (TON) of 2. By introducing two methoxy functional groups, we obtained [Fe(Py5OMe)Cl]+ that turns out to facilitate water oxidation catalysis with a TON = 133 in a light-driven experiment. Further electrochemical experiments and post-catalytic solution analysis reveals that the oxygen evolution is generated by iron oxo/hydroxo species formed from the degradation of the methoxy-substituted Fe complex. This study highlights the difficulty of obtaining a stable base metal molecular catalyst and the importance of conducting a multi-technique analysis to attest firmly the nature of the catalysis (Chapter 6).
18.	Boniolo, Manuel, et al. (author) Water Oxidation by Pentapyridyl Base Metal Complexes? : A Case Study 2022 In: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 61:24, s. 9104-9118 Journal article (peer-reviewed)abstract The design of molecular water oxidation catalysts (WOCs) requires a rational approach that considers the intermediate steps of the catalytic cycle, including water binding, deprotonation, storage of oxidizing equivalents, O–O bond formation, and O2 release. We investigated several of these properties for a series of base metal complexes (M = Mn, Fe, Co, Ni) bearing two variants of a pentapyridyl ligand framework, of which some were reported previously to be active WOCs. We found that only [Fe(Py5OMe)Cl]+ (Py5OMe = pyridine-2,6-diylbis[di-(pyridin-2-yl)methoxymethane]) showed an appreciable catalytic activity with a turnover number (TON) = 130 in light-driven experiments using the [Ru(bpy)3]2+/S2O82– system at pH 8.0, but that activity is demonstrated to arise from the rapid degradation in the buffered solution leading to the formation of catalytically active amorphous iron oxide/hydroxide (FeOOH), which subsequently lost the catalytic activity by forming more extensive and structured FeOOH species. The detailed analysis of the redox and water-binding properties employing electrochemistry, X-ray absorption spectroscopy (XAS), UV–vis spectroscopy, and density-functional theory (DFT) showed that all complexes were able to undergo the MIII/MII oxidation, but none was able to yield a detectable amount of a MIV state in our potential window (up to +2 V vs SHE). This inability was traced to (i) the preference for binding Cl– or acetonitrile instead of water-derived species in the apical position, which excludes redox leveling via proton coupled electron transfer, and (ii) the lack of sigma donor ligands that would stabilize oxidation states beyond MIII. On that basis, design features for next-generation molecular WOCs are suggested.
19.	Britt, R D, et al. (author) Recent pulsed EPR studies of the Photosystem II oxygen-evolving complex : implications as to water oxidation mechanisms 2004 In: Biochimica et Biophysica Acta - Bioenergetics. - Univ Calif Davis, Dept Chem, Davis, CA 95616 USA. Univ Calif Berkeley, Lawrence Berkeley Lab, Melvin Calvin Lab, Phys Biosci Div, Berkeley, CA 94720 USA. Univ Calif Berkeley, Dept Chem, Berkeley, CA 94720 USA. : ELSEVIER. - 0005-2728 .- 1879-2650. ; 1655:1-3, s. 158-171 Journal article (peer-reviewed)abstract The pulsed electron paramagnetic resonance (EPR) methods of electron spin echo envelope modulation (ESEEM) and electron spill echo-electron nuclear double resonance (ESE-ENDOR) are used to investigate the structure of the Photosystem II oxygen-evolving complex (OEC), including the paramagnetic manganese cluster and its immediate surroundings. Recent unpublished results from the pulsed EPR laboratory at UC-Davis are discussed, along with aspects of recent publications, with a focus on substrate and cofactor interactions. New data on the proximity of exchangeable deuterons around the Mn cluster poised in the So-state are presented and interpreted. These pulsed EPR results are used in an evaluation of several recently proposed mechanisms for PSII water oxidation. We strongly favor mechanistic models where the substrate waters bind within the OEC early in the S-state cycle. Models in which the O-O bond is formed by a nucleophilic attack by a Ca2+-bound water on a strong S-4-state electrophile provide a good match to the pulsed EPR data. (C) 2004 Elsevier B.V. All rights reserved.
20.	Burén, Stefan, et al. (author) Importance of post-translational modifications for functionality of a chloroplast-localized carbonic anhydrase (CAH1) in Arabidopsis thaliana 2011 In: PLOS ONE. - : Public Library of Science. - 1932-6203. ; 6:6, s. e21021- Journal article (peer-reviewed)abstract BackgroundThe Arabidopsis CAH1 alpha-type carbonic anhydrase is one of the few plant proteins known to be targeted to the chloroplast through the secretory pathway. CAH1 is post-translationally modified at several residues by the attachment of N-glycans, resulting in a mature protein harbouring complex-type glycans. The reason of why trafficking through this non-canonical pathway is beneficial for certain chloroplast resident proteins is not yet known. Therefore, to elucidate the significance of glycosylation in trafficking and the effect of glycosylation on the stability and function of the protein, epitope-labelled wild type and mutated versions of CAH1 were expressed in plant cells.Methodology/Principal FindingsTransient expression of mutant CAH1 with disrupted glycosylation sites showed that the protein harbours four, or in certain cases five, N-glycans. While the wild type protein trafficked through the secretory pathway to the chloroplast, the non-glycosylated protein formed aggregates and associated with the ER chaperone BiP, indicating that glycosylation of CAH1 facilitates folding and ER-export. Using cysteine mutants we also assessed the role of disulphide bridge formation in the folding and stability of CAH1. We found that a disulphide bridge between cysteines at positions 27 and 191 in the mature protein was required for correct folding of the protein. Using a mass spectrometric approach we were able to measure the enzymatic activity of CAH1 protein. Under circumstances where protein N-glycosylation is blocked in vivo, the activity of CAH1 is completely inhibited.Conclusions/SignificanceWe show for the first time the importance of post-translational modifications such as N-glycosylation and intramolecular disulphide bridge formation in folding and trafficking of a protein from the secretory pathway to the chloroplast in higher plants. Requirements for these post-translational modifications for a fully functional native protein explain the need for an alternative route to the chloroplast.
21.	Chatterjee, Ruchira, et al. (author) Structural isomers of the S-2 state in photosystem II : do they exist at room temperature and are they important for function? 2019 In: Physiologia Plantarum. - : Wiley-Blackwell. - 0031-9317 .- 1399-3054. ; 166:1, s. 60-72 Journal article (peer-reviewed)abstract In nature, an oxo‐bridged Mn4CaO5 cluster embedded in photosystem II (PSII), a membrane‐bound multi‐subunit pigment protein complex, catalyzes the water oxidation reaction that is driven by light‐induced charge separations in the reaction center of PSII. The Mn4CaO5 cluster accumulates four oxidizing equivalents to enable the four‐electron four‐proton catalysis of two water molecules to one dioxygen molecule and cycles through five intermediate S‐states, S0 – S4 in the Kok cycle. One important question related to the catalytic mechanism of the oxygen‐evolving complex (OEC) that remains is, whether structural isomers are present in some of the intermediate S‐states and if such equilibria are essential for the mechanism of the O‐O bond formation. Here we compare results from electron paramagnetic resonance (EPR) and X‐ray absorption spectroscopy (XAS) obtained at cryogenic temperatures for the S2state of PSII with structural data collected of the S1, S2 and S3 states by serial crystallography at neutral pH (∼6.5) using an X‐ray free electron laser at room temperature. While the cryogenic data show the presence of at least two structural forms of the S2 state, the room temperature crystallography data can be well‐described by just one S2 structure. We discuss the deviating results and outline experimental strategies for clarifying this mechanistically important question.
22.	Chatterjee, Ruchira, et al. (author) XANES and EXAFS of dilute solutions of transition metals at XFELs 2019 In: Journal of Synchrotron Radiation. - : INT UNION CRYSTALLOGRAPHY. - 0909-0495 .- 1600-5775. ; 26, s. 1716-1724 Journal article (peer-reviewed)abstract This work has demonstrated that X-ray absorption spectroscopy (XAS), both Mn XANES and EXAFS, of solutions with millimolar concentrations of metal is possible using the femtosecond X-ray pulses from XFELs. Mn XAS data were collected using two different sample delivery methods, a Rayleigh jet and a drop-on-demand setup, with varying concentrations of Mn. Here, a new method for normalization of XAS spectra based on solvent scattering that is compatible with data collection from a highly variable pulsed source is described. The measured XANES and EXAFS spectra of such dilute solution samples are in good agreement with data collected at synchrotron sources using traditional scanning protocols. The procedures described here will enable XFEL-based XAS on dilute biological samples, especially metalloproteins, with low sample consumption. Details of the experimental setup and data analysis methods used in this XANES and EXAFS study are presented. This method will also benefit XAS performed at high-repetition-rate XFELs such as the European XFEL, LCLS-II and LCLS-II-HE.
23.	Cheah, Mun Hon, et al. (author) Assessment of the manganese cluster’s oxidation state via photoactivation of photosystem II microcrystals 2020 In: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 117:1, s. 141-145 Journal article (peer-reviewed)abstract Knowledge of the manganese oxidation states of the oxygen-evolving Mn4CaO5 cluster in photosystem II (PSII) is crucial toward understanding the mechanism of biological water oxidation. There is a 4 decade long debate on this topic that historically originates from the observation of a multiline electron paramagnetic resonance (EPR) signal with effective total spin of S = 1/2 in the singly oxidized S2 state of this cluster. This signal implies an overall oxidation state of either Mn(III)3Mn(IV) or Mn(III)Mn(IV)3 for the S2 state. These 2 competing assignments are commonly known as “low oxidation (LO)” and “high oxidation (HO)” models of the Mn4CaO5 cluster. Recent advanced EPR and Mn K-edge X-ray spectroscopy studies converge upon the HO model. However, doubts about these assignments have been voiced, fueled especially by studies counting the number of flash-driven electron removals required for the assembly of an active Mn4CaO5 cluster starting from Mn(II) and Mn-free PSII. This process, known as photoactivation, appeared to support the LO model since the first oxygen is reported to evolve already after 7 flashes. In this study, we improved the quantum yield and sensitivity of the photoactivation experiment by employing PSII microcrystals that retained all protein subunits after complete manganese removal and by oxygen detection via a custom built thin-layer cell connected to a membrane inlet mass spectrometer. We demonstrate that 9 flashes by a nanosecond laser are required for the production of the first oxygen, which proves that the HO model provides the correct description of the Mn4CaO5 cluster’s oxidation states.
24.	Christianson, Helena C, et al. (author) Tumor antigen glycosaminoglycan modification regulates antibody-drug conjugate delivery and cytotoxicity 2017 In: Oncotarget. - : IMPACT JOURNALS LLC. - 1949-2553. ; 8:40, s. 66960-66974 Journal article (peer-reviewed)abstract Aggressive cancers are characterized by hypoxia, which is a key driver of tumor development and treatment resistance. Proteins specifically expressed in the hypoxic tumor microenvironment thus represent interesting candidates for targeted drug delivery strategies. Carbonic anhydrase (CAIX) has been identified as an attractive treatment target as it is highly hypoxia specific and expressed at the cell-surface to promote cancer cell aggressiveness. Here, we find that cancer cell internalization of CAIX is negatively regulated by post-translational modification with chondroitin or heparan sulfate glycosaminoglycan chains. We show that perturbed glycosaminoglycan modification results in increased CAIX endocytosis. We hypothesized that perturbation of CAIX glycosaminoglycan conjugation may provide opportunities for enhanced drug delivery to hypoxic tumor cells. In support of this concept, pharmacological inhibition of glycosaminoglycan biosynthesis with xylosides significantly potentiated the internalization and cytotoxic activity of an antibody-drug conjugate (ADC) targeted at CAIX. Moreover, cells expressing glycosaminoglycan-deficient CAIX were significantly more sensitive to ADC treatment as compared with cells expressing wild-type CAIX. We find that inhibition of CAIX endocytosis is associated with an increased localization of glycosaminoglycan-conjugated CAIX in membrane lipid raft domains stabilized by caveolin-1 clusters. The association of CAIX with caveolin-1 was partially attenuated by acidosis, i.e. another important feature of malignant tumors. Accordingly, we found increased internalization of CAIX at acidic conditions. These findings provide first evidence that intracellular drug delivery at pathophysiological conditions of malignant tumors can be attenuated by tumor antigen glycosaminoglycan modification, which is of conceptual importance in the future development of targeted cancer treatments.
25.	Chrysina, Maria, et al. (author) Five-coordinate Mn-IV intermediate in the activation of nature's water splitting cofactor 2019 In: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 116:34, s. 16841-16846 Journal article (peer-reviewed)abstract Nature's water splitting cofactor passes through a series of catalytic intermediates (S-0-S-4) before O-O bond formation and O-2 release. In the second last transition (S-2 to S-3) cofactor oxidation is coupled to water molecule binding to Mn1. It is this activated, water-enriched all Mn-IV form of the cofactor that goes on to form the O-O bond, after the next light-induced oxidation to S-4. How cofactor activation proceeds remains an open question. Here, we report a so far not described intermediate (S-3') in which cofactor oxidation has occurred without water insertion. This intermediate can be trapped in a significant fraction of centers (> 50%) in (i) chemical-modified cofactors in which Ca2+ is exchanged with Sr2+; the Mn4O5Sr cofactor remains active, but the S-2-S-3 and S-3-S-0 transitions are slower than for the Mn4O5Ca cofactor; and (ii) upon addition of 3% vol/vol methanol; methanol is thought to act as a substrate water analog. The S-3' electron paramagnetic resonance (EPR) signal is significantly broader than the untreated S-3 signal (2.5 T vs. 1.5 T), indicating the cofactor still contains a 5-coordinate Mn ion, as seen in the preceding S-2 state. Magnetic double resonance data extend these findings revealing the electronic connectivity of the S-3' cofactor is similar to the high spin form of the preceding S-2 state, which contains a cuboidal Mn3O4Ca unit tethered to an external, 5-coordinate Mn ion (Mn-4). These results demonstrate that cofactor oxidation regulates water molecule insertion via binding to Mn-4. The interaction of ammonia with the cofactor is also discussed.
26.	Cinco, R M, et al. (author) Orientation of calcium in the Mn4Ca cluster of the oxygen-evolving complex determined using polarized strontium EXAFS of photosystem II membranes 2004 In: Biochemistry. - Univ Calif Berkeley, Lawrence Berkeley Lab, Phys Biosci Div, Melvin Calvin Lab, Berkeley, CA 94720 USA. Univ Calif Berkeley, Dept Chem, Berkeley, CA 94720 USA. : AMER CHEMICAL SOC. - 0006-2960 .- 1520-4995. ; 43:42, s. 13271-13282 Journal article (peer-reviewed)abstract The oxygen-evolving complex of photosystem II (PS II) in green plants and algae contains a cluster of four Mn atoms in the active site, which catalyzes the photoinduced oxidation of water to dioxygen. Along with Mn, calcium and chloride ions are necessary cofactors for proper functioning of the complex. The current Study using polarized Sr EXAFS on oriented Sr-reactivated samples shows that Fourier peak II, which fits best to Mn at 3.5 Angstrom rather than lighter atoms (C, N, O, or Cl), is dichroic, with a larger magnitude at 10degrees (angle between the PS II membrane normal and the X-ray electric field vector) and a smaller magnitude at 80degrees. Analysis of the dichroism of the Sr EXAFS yields a lower and upper limit of 0degrees and 23degrees for the average angle between the Sr-Mn vectors and the membrane normal and an isotropic coordination number (number of Mn neighbors to Sr) of 1 or 2 for these layered PS II samples. The results confirm the contention that Ca (Sr) is proximal to the Mn cluster and lead to refined working models of the heteronuclear Mn4Ca cluster of the oxygen-evolving complex in PS II.
27.	Cinco, R M, et al. (author) Refined model of the oxidation states and structures of the Mn/Ca/Cl cluster of the oxygen evolving complex of photosystem II. 1998 In: PHOTOSYNTHESIS. - Univ Calif Berkeley, Lawrence Berkeley Lab, Phys Biosci Div, Berkeley, CA 94720 USA. : SPRINGER. - 0792355474 ; , s. 1273-1278 Conference paper (peer-reviewed)
28.	Clausen, J, et al. (author) Evidence that bicarbonate is not the substrate in photosynthetic oxygen evolution 2005 In: Plant Physiology. - : AMER SOC PLANT BIOLOGISTS. - 0032-0889 .- 1532-2548. ; 139:3, s. 1444-1450 Journal article (peer-reviewed)abstract It is widely accepted that the oxygen produced by photosystem II of cyanobacteria, algae, and plants is derived from water. Earlier proposals that bicarbonate may serve as substrate or catalytic intermediate are almost forgotten, though not rigorously disproved. These latter proposals imply that CO2 is an intermediate product of oxygen production in addition to O-2. In this work, we investigated this possible role of exchangeable HCO3- in oxygen evolution in two independent ways. (1) We studied a possible product inhibition of the electron transfer into the catalytic Mn4Ca complex during the oxygen-evolving reaction by greatly increasing the pressure of CO2. This was monitored by absorption transients in the near UV. We found that a 3,000-fold increase of the CO2 pressure over ambient conditions did not affect the UV transient, whereas the S-3 -> S-4 -> S-0 transition was half-inhibited by raising the O-2 pressure only 10-fold over ambient, as previously established. (2) The flash-induced O-2 and CO2 production by photosystem II was followed simultaneously with membrane inlet mass spectrometry under approximately 15% H-2 O-18 enrichment. Light flashes that revealed the known oscillatory O-2 release failed to produce any oscillatory CO2 signal. Both types of results exclude that exchangeable bicarbonate is the substrate for (and CO2 an intermediate product of) oxygen evolution by photosynthesis. The possibility that a tightly bound carbonate or bicarbonate is a cofactor of photosynthetic water oxidation has remained.
29.	Conlan, Brendon l, et al. (author) Thomas John Wydrzynski (8 July 1947-16 March 2018) 2019 In: Photosynthesis Research. - : Springer Netherlands. - 0166-8595 .- 1573-5079. ; 140:3, s. 253-261 Journal article (peer-reviewed)abstract With this Tribute, we remember and honor Thomas John (Tom) Wydrzynski. Tom was a highly innovative, independent and committed researcher, who had, early in his career, defined his life-long research goal. He was committed to understand how Photosystem II produces molecular oxygen from water, using the energy of sunlight, and to apply this knowledge towards making artificial systems. In this tribute, we summarize his research journey, which involved working on soft money' in several laboratories around the world for many years, as well as his research achievements. We also reflect upon his approach to life, science and student supervision, as we perceive it. Tom was not only a thoughtful scientist that inspired many to enter this field of research, but also a wonderful supervisor and friend, who is deeply missed (see footnote*).
30.	Conlan, Brendon, et al. (author) Photo-catalytic oxidation of a di-nuclear manganese centre in an engineered bacterioferritin 'reaction centre' 2009 In: Biochimica et Biophysica Acta. - : Elsevier. - 0006-3002 .- 1878-2434 .- 0005-2728 .- 1879-2650. ; 1787:9, s. 1112-1121 Journal article (peer-reviewed)abstract Photosynthesis involves the conversion of light into chemical energy through a series of electron transfer reactions within membrane-bound pigment/protein complexes. The Photosystem II (PSII) complex in plants, algae and cyanobacteria catalyse the oxidation of water to molecular O(2). The complexity of PSII has thus far limited attempts to chemically replicate its function. Here we introduce a reverse engineering approach to build a simple, light-driven photo-catalyst based on the organization and function of the donor side of the PSII reaction centre. We have used bacterioferritin (BFR) (cytochrome b1) from Escherichia coli as the protein scaffold since it has several, inherently useful design features for engineering light-driven electron transport. Among these are: (i.) a di-iron binding site; (ii.) a potentially redox-active tyrosine residue; and (iii.) the ability to dimerise and form an inter-protein heme binding pocket within electron tunnelling distance of the di-iron binding site. Upon replacing the heme with the photoactive zinc-chlorin e(6) (ZnCe(6)) molecule and the di-iron binding site with two manganese ions, we show that the two Mn ions bind as a weakly coupled di-nuclear Mn(2)(II,II) centre, and that ZnCe(6) binds in stoichiometric amounts of 1:2 with respect to the dimeric form of BFR. Upon illumination the bound ZnCe(6) initiates electron transfer, followed by oxidation of the di-nuclear Mn centre possibly via one of the inherent tyrosine residues in the vicinity of the Mn cluster. The light dependent loss of the Mn(II) EPR signals and the formation of low field parallel mode Mn EPR signals are attributed to the formation of Mn(III) species. The formation of the Mn(III) is concomitant with consumption of oxygen. Our model is the first artificial reaction centre developed for the photo-catalytic oxidation of a di-metal site within a protein matrix which potentially mimics WOC photo-assembly.
31.	Conlan, Brendon, et al. (author) Thomas John Wydrzynski (8 July 1947–16 March 2018) 2019 In: Photosynthesis Research. - : Springer. - 0166-8595 .- 1573-5079. ; 140:3, s. 253-261 Journal article (pop. science, debate, etc.)abstract With this Tribute, we remember and honor Thomas John (Tom) Wydrzynski. Tom was a highly innovative, independent and committed researcher, who had, early in his career, defined his life-long research goal. He was committed to understand how Photosystem II produces molecular oxygen from water, using the energy of sunlight, and to apply this knowledge towards making artificial systems. In this tribute, we summarize his research journey, which involved working on 'soft money' in several laboratories around the world for many years, as well as his research achievements. We also reflect upon his approach to life, science and student supervision, as we perceive it. Tom was not only a thoughtful scientist that inspired many to enter this field of research, but also a wonderful supervisor and friend, who is deeply missed (see footnote*).
32.	Cox, Nicholas, et al. (author) Effect of Ca(2+)/Sr(2+) substitution on the electronic structure of the oxygen-evolving complex of photosystem II : a combined multifrequency EPR, (55)Mn-ENDOR, and DFT study of the S(2) State 2011 In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 133:10, s. 3635-3648 Journal article (peer-reviewed)abstract The electronic structures of the native Mn(4)O(x)Ca cluster and the biosynthetically substituted Mn(4)O(x)Sr cluster of the oxygen evolving complex (OEC) of photosystem II (PSII) core complexes isolated from Thermosynechococcus elongatus, poised in the S(2) state, were studied by X- and Q-band CW-EPR and by pulsed Q-band (55)Mn-ENDOR spectroscopy. Both wild type and tyrosine D less mutants grown photoautotrophically in either CaCl(2) or SrCl(2) containing media were measured. The obtained CW-EPR spectra of the S(2) state displayed the characteristic, clearly noticeable differences in the hyperfine pattern of the multiline EPR signal [Boussac et al. J. Biol. Chem.2004, 279, 22809-22819]. In sharp contrast, the manganese ((55)Mn) ENDOR spectra of the Ca and Sr forms of the OEC were remarkably similar. Multifrequency simulations of the X- and Q-band CW-EPR and (55)Mn-pulsed ENDOR spectra using the Spin Hamiltonian formalism were performed to investigate this surprising result. It is shown that (i) all four manganese ions contribute to the (55)Mn-ENDOR spectra; (ii) only small changes are seen in the fitted isotropic hyperfine values for the Ca(2+) and Sr(2+) containing OEC, suggesting that there is no change in the overall spin distribution (electronic coupling scheme) upon Ca(2+)/Sr(2+) substitution; (iii) the changes in the CW-EPR hyperfine pattern can be explained by a small decrease in the anisotropy of at least two hyperfine tensors. It is proposed that modifications at the Ca(2+) site may modulate the fine structure tensor of the Mn(III) ion. DFT calculations support the above conclusions. Our data analysis also provides strong support for the notion that in the S(2) state the coordination of the Mn(III) ion is square-pyramidal (5-coordinate) or octahedral (6-coordinate) with tetragonal elongation. In addition, it is shown that only one of the currently published OEC models, the Siegbahn structure [Siegbahn, P. E. M. Acc. Chem. Res.2009, 42, 1871-1880, Pantazis, D. A. et al. Phys. Chem. Chem. Phys.2009, 11, 6788-6798], is consistent with all data presented here. These results provide important information for the structure of the OEC and the water-splitting mechanism. In particular, the 5-coordinate Mn(III) is a potential site for substrate 'water' (H(2)O, OH(-)) binding. Its location within the cuboidal structural unit, as opposed to the external 'dangler' position, may have important consequences for the mechanism of O-O bond formation.
33.	Cox, Nicholas, et al. (author) Electronic Structure of a Weakly Antiferromagnetically Coupled Mn(II)Mn(III) Model Relevant to Manganese Proteins : A Combined EPR, (55)Mn-ENDOR, and DFT Study 2011 In: Inorganic Chemistry. - : American Chemical Society. - 0020-1669 .- 1520-510X. ; 50:17, s. 8238-8251 Journal article (peer-reviewed)abstract An analysis of the electronic structure of the [Mn(II)Mn(III)(μ-OH)-(μ-piv)(2)(Me(3)tacn)(2)](ClO(4))(2) (PivOH) complex is reported. It displays features that include: (i) a ground 1/2 spin state; (ii) a small exchange (J) coupling between the two Mn ions; (iii) a mono-μ-hydroxo bridge, bis-μ-carboxylato motif; and (iv) a strongly coupled, terminally bound N ligand to the Mn(III). All of these features are observed in structural models of the oxygen evolving complex (OEC). Multifrequency electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) measurements were performed on this complex, and the resultant spectra simulated using the Spin Hamiltonian formalism. The strong field dependence of the (55)Mn-ENDOR constrains the (55)Mn hyperfine tensors such that a unique solution for the electronic structure can be deduced. Large hyperfine anisotropy is required to reproduce the EPR/ENDOR spectra for both the Mn(II) and Mn(III) ions. The large effective hyperfine tensor anisotropy of the Mn(II), a d(5) ion which usually exhibits small anisotropy, is interpreted within a formalism in which the fine structure tensor of the Mn(III) ion strongly perturbs the zero-field energy levels of the Mn(II)Mn(III) complex. An estimate of the fine structure parameter (d) for the Mn(III) of -4 cm(-1) was made, by assuming the intrinsic anisotropy of the Mn(II) ion is small. The magnitude of the fine structure and intrinsic (onsite) hyperfine tensor of the Mn(III) is consistent with the known coordination environment of the Mn(III) ion as seen from its crystal structure. Broken symmetry density functional theory (DFT) calculations were performed on the crystal structure geometry. DFT values for both the isotropic and the anisotropic components of the onsite (intrinsic) hyperfine tensors match those inferred from the EPR/ENDOR simulations described above, to within 5%. This study demonstrates that DFT calculations provide reliable estimates for spectroscopic observables of mixed valence Mn complexes, even in the limit where the description of a well isolated S = 1/2 ground state begins to break down.
34.	Cox, Nicholas, et al. (author) Electronic Structure of a Weakly Antiferromagnetically Coupled MnIIMnIII Model Relevant to Manganese Proteins : A Combined EPR, 55Mn-ENDOR, and DFT Study 2011 In: Inorganic Chemistry. - : AMER CHEMICAL SOC. - 0020-1669 .- 1520-510X. ; 50:17, s. 8238-8251 Journal article (peer-reviewed)abstract An analysis of the electronic structure of the [(MnMnIII)-Mn-II(mu-OH)-(mu-piv)(2)(Me(3)tacn)(2)] (ClO4)(2) (PivOH) complex is reported. It displays features that include: (i) a ground 1/2 spin state; (ii) a small exchange (J) coupling between the two Mn ions; (iii) a mono-mu-hydroxo bridge, bis-mu-carboxylato motif; and (iv) a strongly coupled, terminally bound N ligand to the Mn-III. All of these features are observed in structural models of the oxygen evolving complex (OEC). Multifrequency electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) measurements were performed on this complex, and the resultant spectra simulated using the Spin Hamiltonian formalism. The strong field dependence of the Mn-55-ENDOR constrains the Mn-55 hyperfine tensors such that a unique solution for the electronic structure can be deduced. Large hyperfine anisotropy is required to reproduce the EPR/ENDOR spectra for both the Mn-II and Mn-III ions. The large effective hyperfine tensor anisotropy of the Mn-II, a d(5) ion which usually exhibits small anisotropy, is interpreted within a formalism in which the fine structure tensor of the Mn-III ion strongly perturbs the zero-field energy levels of the (MnMnIII)-Mn-II complex. An estimate of the fine structure parameter (d) for the Mn-III of -4 cm(-1) was made, by assuming the intrinsic anisotropy of the Mn-II ion is small. The magnitude of the fine structure and intrinsic (onsite) hyperfine tensor of the Mn-III is consistent with the known coordination environment of the Mn-III ion as seen from its crystal structure. Broken symmetry density functional theory (DFT) calculations were performed on the crystal structure geometry. DFT values for both the isotropic and the anisotropic components of the onsite (intrinsic) hyperfine tensors match those inferred from the EPR/ENDOR simulations described above, to within 5%. This study demonstrates that DFT calculations provide reliable estimates for spectroscopic observables of mixed valence Mn complexes, even in the limit where the description of a well isolated S = 1/2 ground state begins to break down.
35.	Cox, Nicholas, et al. (author) Reflections on substrate water and dioxygen formation 2013 In: Biochimica et Biophysica Acta - Bioenergetics. - : Elsevier. - 0005-2728 .- 1879-2650. ; 1827:8-9, s. 1020-1030 Research review (peer-reviewed)abstract This brief article aims at presenting a concise summary of all experimental findings regarding substrate water-binding to the Mn4CaO5 cluster in photosystem II. Mass spectrometric and spectroscopic results are interpreted in light of recent structural information of the water oxidizing complex (WOC) obtained by x-ray crystallography, spectroscopy and theoretical modeling. Within this framework current proposals for the mechanism of photosynthetic water-oxidation are evaluated.
36.	D'Amario, Luca, et al. (author) Towards time resolved characterization of electrochemical reactions : electrochemically-induced Raman spectroscopy 2022 In: Chemical Science. - : RSC Publishing. - 2041-6520 .- 2041-6539. ; 13:36, s. 10734-10742 Journal article (peer-reviewed)abstract Structural characterization of transient electrochemical species in the sub-millisecond time scale is the all-time wish of any electrochemist. Presently, common time resolution of structural spectro-electrochemical methods is about 0.1 seconds. Herein, a transient spectro-electrochemical Raman setup of easy implementation is described which allows sub-ms time resolution. The technique studies electrochemical processes by initiating the reaction with an electric potential (or current) pulse and analyses the product with a synchronized laser pulse of the modified Raman spectrometer. The approach was validated by studying a known redox driven isomerization of a Ru-based molecular switch grafted, as monolayer, on a SERS active Au microelectrode. Density-functional-theory calculations confirmed the spectral assignments to sub-ms transient species. This study paves the way to a new generation of time-resolved spectro-electrochemical techniques which will be of fundamental help in the development of next generation electrolizers, fuel cells and batteries.
37.	Dau, H., et al. (author) FTIR detection of water reactions in the oxygen-evolving centre of photosystem II - Discussion 2008 In: Philosophical Transactions of the Royal Society of London. Biological Sciences. - : ROYAL SOC. - 0962-8436 .- 1471-2970. ; 363:1494, s. 1194-1195 Journal article (other academic/artistic)
38.	de Lichtenberg, Casper, et al. (author) Assignment of the slow exchanging substrate water of Nature's water splitting cofactor Other publication (other academic/artistic)
39.	de Lichtenberg, Casper, et al. (author) Assignment of the slowly exchanging substrate water of nature's water-splitting cofactor 2024 In: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences (PNAS). - 0027-8424 .- 1091-6490. ; 121:11 Journal article (peer-reviewed)abstract Identifying the two substrate water sites of nature's water-splitting cofactor (Mn4CaO5 cluster) provides important information toward resolving the mechanism of O-O bond formation in Photosystem II (PSII). To this end, we have performed parallel substrate water exchange experiments in the S1 state of native Ca-PSII and biosynthetically substituted Sr-PSII employing Time-Resolved Membrane Inlet Mass Spectrometry (TR-MIMS) and a Time-Resolved 17O-Electron-electron Double resonance detected NMR (TR-17O-EDNMR) approach. TR-MIMS resolves the kinetics for incorporation of the oxygen-isotope label into the substrate sites after addition of H218O to the medium, while the magnetic resonance technique allows, in principle, the characterization of all exchangeable oxygen ligands of the Mn4CaO5 cofactor after mixing with H217O. This unique combination shows i) that the central oxygen bridge (O5) of Ca-PSII core complexes isolated from Thermosynechococcus vestitus has, within experimental conditions, the same rate of exchange as the slowly exchanging substrate water (WS) in the TR-MIMS experiments and ii) that the exchange rates of O5 and WS are both enhanced by Ca2+→Sr2+ substitution in a similar manner. In the context of previous TR-MIMS results, this shows that only O5 fulfills all criteria for being WS. This strongly restricts options for the mechanism of water oxidation.
40.	de Lichtenberg, Casper, et al. (author) Effects of the D61A and E189Q mutations on the exchange of substrate water in photosystem II Other publication (other academic/artistic)
41.	de Lichtenberg, Casper, et al. (author) Substrate water exchange in the S-2 state of photosystem II is dependent on the conformation of the Mn4Ca cluster 2020 In: Physical Chemistry, Chemical Physics - PCCP. - : ROYAL SOC CHEMISTRY. - 1463-9076 .- 1463-9084. ; 22:23, s. 12894-12908 Journal article (peer-reviewed)abstract In photosynthesis, dioxygen formation from water is catalyzed by the oxygen evolving complex (OEC) in Photosystem II (PSII) that harbours the Mn4Ca cluster. During catalysis, the OEC cycles through five redox states, S-0 to S-4. In the S-2 state, the Mn4Ca cluster can exist in two conformations, which are signified by the low-spin (LS) g = 2 EPR multiline signal and the high-spin (HS) g = 4.1 EPR signal. Here, we employed time-resolved membrane inlet mass spectrometry to measure the kinetics of (H2O)-O-18/(H2O)-O-16 exchange between bulk water and the two substrate waters bound at the Mn4Ca cluster in the S-2(LS), S-2(HS), and the S-3 states in both Ca-PSII and Sr-PSII core complexes from T. elongatus. We found that the slowly exchanging substrate water exchanges 10 times faster in the S-2(HS) than in the S-2(LS) state, and that the S-2(LS) -> S-2(HS) conversion has at physiological temperature an activation barrier of 17 +/- 1 kcal mol(-1). Of the presently suggested S-2(HS) models, our findings are best in agreement with a water exchange pathway involving a S-2(HS) state that has an open cubane structure with a hydroxide bound between Ca and Mn1. We also show that water exchange in the S-3 state is governed by a different equilibrium than in S-2, and that the exchange of the fast substrate water in the S-2 state is unaffected by Ca/Sr substitution. These findings support that (i) O5 is the slowly exchanging substrate water, with W2 being the only other option, and (ii) either W2 or W3 is the fast exchanging substrate. The three remaining possibilities for O-O bond formation in PSII are discussed.
42.	de Lichtenberg, Casper, et al. (author) The D1-V185N mutation alters substrate water exchange by stabilizing alternative structures of the Mn4Ca-cluster in photosystem II 2021 In: Biochimica et Biophysica Acta - Bioenergetics. - : Elsevier. - 0005-2728 .- 1879-2650. ; 1862:1 Journal article (peer-reviewed)abstract In photosynthesis, the oxygen-evolving complex (OEC) of the pigment-protein complex photosystem II (PSII) orchestrates the oxidation of water. Introduction of the V185N mutation into the D1 protein was previously reported to drastically slow O2-release and strongly perturb the water network surrounding the Mn4Ca cluster. Employing time-resolved membrane inlet mass spectrometry, we measured here the H218O/H216O-exchange kinetics of the fast (Wf) and slow (Ws) exchanging substrate waters bound in the S1, S2 and S3 states to the Mn4Ca cluster of PSII core complexes isolated from wild type and D1-V185N strains of Synechocystis sp. PCC 6803. We found that the rate of exchange for Ws was increased in the S1 and S2 states, while both Wf and Ws exchange rates were decreased in the S3 state. Additionally, we used EPR spectroscopy to characterize the Mn4Ca cluster and its interaction with the redox active D1-Tyr161 (YZ). In the S2 state, we observed a greatly diminished multiline signal in the V185N-PSII that could be recovered by addition of ammonia. The split signal in the S1 state was not affected, while the split signal in the S3 state was absent in the D1-V185N mutant. These findings are rationalized by the proposal that the N185 residue stabilizes the binding of an additional water-derived ligand at the Mn1 site of the Mn4Ca cluster via hydrogen bonding. Implications for the sites of substrate water binding are discussed.
43.	de Lichtenberg, Casper, et al. (author) The exchange of the fast substrate water in the S-2 state of photosystem II is limited by diffusion of bulk water through channels - implications for the water oxidation mechanism 2021 In: Chemical Science. - : Royal Society of Chemistry. - 2041-6520 .- 2041-6539. ; 12:38, s. 12763-12775 Journal article (peer-reviewed)abstract The molecular oxygen we breathe is produced from water-derived oxygen species bound to the Mn4CaO5 cluster in photosystem II (PSII). Present research points to the central oxo-bridge O5 as the 'slow exchanging substrate water (W-s)', while, in the S-2 state, the terminal water ligands W2 and W3 are both discussed as the 'fast exchanging substrate water (W-f)'. A critical point for the assignment of W-f is whether or not its exchange with bulk water is limited by barriers in the channels leading to the Mn4CaO5 cluster. In this study, we measured the rates of (H2O)-O-16/(H2O)-O-18 substrate water exchange in the S-2 and S-3 states of PSII core complexes from wild-type (WT) Synechocystis sp. PCC 6803, and from two mutants, D1-D61A and D1-E189Q, that are expected to alter water access via the Cl1/O4 channels and the O1 channel, respectively. We found that the exchange rates of W-f and W-s were unaffected by the E189Q mutation (O1 channel), but strongly perturbed by the D61A mutation (Cl1/O4 channel). It is concluded that all channels have restrictions limiting the isotopic equilibration of the inner water pool near the Mn4CaO5 cluster, and that D61 participates in one such barrier. In the D61A mutant this barrier is lowered so that W-f exchange occurs more rapidly. This finding removes the main argument against Ca-bound W3 as fast substrate water in the S-2 state, namely the indifference of the rate of W-f exchange towards Ca/Sr substitution.
44.	de Lichtenberg, Casper, 1989- (author) Time-resolved Structural and Mechanistic Studies of Water Oxidation in Photosystem II : water here, water there, water everywhere 2020 Doctoral thesis (other academic/artistic)abstract Oxygenic photosynthesis is undisputedly one of the most important chemical processes for human life on earth as it not only fills the atmosphere with the oxygen that we need to breathe, but also sustains the accumulation of biomass, which is not only used as nourishment but is also present in almost every aspect of our lives as building material, textiles in clothes and furniture, or even as living decorations to name a few.The photosynthetic water-splitting mechanism is catalyzed by a water:plastoquinone oxido-reductase by the name of photosystem II (PSII), which is embedded in the thylakoid membranes of plants, algae and cyanobacteria. As it is excited by light, charge separation occurs in the reaction center of the protein and an electron is extracted by oxidation of Mn4Ca-cluster, that constitutes the active site for the water splitting reaction in PSII. When the Mn4Ca-cluster has been oxidized 4 times, it forms an oxygen-oxygen bond between two water derived ligands bound to the Mn4Ca-cluster and returns to the lowest oxidation state of the catalytic cycle. Understanding what ligands of the cluster that are used in the water splitting reaction is the key to unlocking the underlying chemical mechanism.In this thesis I describe investigations, with room temperature X-ray diffraction (XRD) and X-ray emission spectroscopy (XES) on PSII microcrystals, of how the active site looks in all the stable intermediate oxidation states. Furthermore I describe how we uncovered the sequence of events that lead to insertion of an additional water ligand in the S2-S3 state transition of the catalytic cycle.Furthermore, through time-resolved membrane-inlet mass spectrometry (TR-MIMS) measurements of the isotopic equilibration of the substrate waters with the bulk in conditions that induce different electron magnetic resonance (EPR) spectroscopic signatures, I present evidence that the exchange of the slowly exchanging substrate water Ws is controlled by a dynamic equilibrium between conformations in the S2-state that give rise to either the low-spin multiline (LS-ML) signal or the high-spin (HS) signal. Based on the crystal structures and litterature suggestions for the conformation of the HS state different scenarios were presented for the assignment of Ws and how it exchanges. This analysis is discussed in the context of all semi-stable intermediate oxidation states in the Kok cycle.To further the understanding of this equilibrium, I also studied a selection of mutants positioned at strategic places in the vicinity of the different proposed substrates and at points that were suggested to be critical for substrate entry. With the combination of TR-MIMS and EPR, I reached the conclusion that by mutating valine 185 to asparagine, the water bound A-type conformation was stabilized, meanwhile in the mutant where aspartate 61 was mutated to alanine I observed that the barrier of the equilibrium between the exchanging conformations was so high that the interchange between them was arrested at room temperature. Additionally the retardation of the substrate exchange rates in the S3-states fit best with D61 being in the vicinity of the fast exchanging water. With this information we found the data best explained in a scenario where the water insertion of the S2-S3 transition was determining the if O-O bond formation occurred between the waters that were W2 and W3 or W2 and O5 in the S2 state. In addition, by mutation of glutamate 189 to glutamine that this residue is not important for the exchange of substrate waters in the S2 or the S3 states.Finally I use a combination of substrate labelling with TR-MIMS and time resolved labelling of the waters that ligate the Mn4Ca-cluster to show that the briding oxygen O5 is exchanging with a near identical rate to Ws, further supporting the assignment that Ws=O5.In conclusion, O-O bond formation most likely occurs between W2 (Wf) and O5 (Ws) via an oxo-oxyl radical coupling mechanism. The newly inserted water thus represents the slow exchanging water of the following S-state cycle.
45.	Ekspong, Joakim, et al. (author) Solar-driven water splitting at 13.8 % solar-to-hydrogen efficiency by an earth-abundant PV-electrolyzer 2021 In: ACS Sustainable Chemistry and Engineering. - : American Chemical Society (ACS). - 2168-0485. ; 9:42, s. 14070-14078 Journal article (peer-reviewed)abstract We present the synthesis and characterization of an efficient and low cost solar-driven electrolyzer consisting of Earth-abundant materials. The trimetallic NiFeMo electrocatalyst takes the shape of nanometer-sized flakes anchored to a fully carbon-based current collector comprising a nitrogen-doped carbon nanotube network, which in turn is grown on a carbon fiber paper support. This catalyst electrode contains solely Earth-abundant materials, and the carbon fiber support renders it effective despite a low metal content. Notably, a bifunctional catalyst–electrode pair exhibits a low total overpotential of 450 mV to drive a full water-splitting reaction at a current density of 10 mA cm–2 and a measured hydrogen Faradaic efficiency of ∼100%. We combine the catalyst–electrode pair with solution-processed perovskite solar cells to form a lightweight solar-driven water-splitting device with a high peak solar-to-fuel conversion efficiency of 13.8%.
46.	Faunce, Thomas, et al. (author) Artificial photosynthesis as a frontier technology for energy sustainability 2013 In: Energy & Environmental Science. - : Royal Society of Chemistry. - 1754-5692 .- 1754-5706. ; 6:4, s. 1074-1076 Journal article (other academic/artistic)
47.	Fernandez, C, et al. (author) Calcium and chloride cofactors of the oxygen evolving complex - X-ray absorption spectroscopy evidence for a Mn/Ca/Cl heteronuclear cluster. 1998 In: PHOTOSYNTHESIS. - Univ Calif Berkeley, Lawrence Berkeley Natl Lab, Phys Biosci Div, Berkeley, CA 94720 USA. : SPRINGER. - 0792355474 ; , s. 1399-1402 Conference paper (peer-reviewed)
48.	Fransson, Thomas, et al. (author) Effects of x-ray free-electron laser pulse intensity on the Mn K beta(1,3) x-ray emission spectrum in photosystem II-A case study for metalloprotein crystals and solutions 2021 In: Structural Dynamics. - : AIP Publishing. - 2329-7778. ; 8:6 Journal article (peer-reviewed)abstract In the last ten years, x-ray free-electron lasers (XFELs) have been successfully employed to characterize metalloproteins at room temperature using various techniques including x-ray diffraction, scattering, and spectroscopy. The approach has been to outrun the radiation damage by using femtosecond (fs) x-ray pulses. An example of an important and damage sensitive active metal center is the Mn4CaO5 cluster in photosystem II (PS II), the catalytic site of photosynthetic water oxidation. The combination of serial femtosecond x-ray crystallography and K beta x-ray emission spectroscopy (XES) has proven to be a powerful multimodal approach for simultaneously probing the overall protein structure and the electronic state of the Mn4CaO5 cluster throughout the catalytic (Kok) cycle. As the observed spectral changes in the Mn4CaO5 cluster are very subtle, it is critical to consider the potential effects of the intense XFEL pulses on the K beta XES signal. We report here a systematic study of the effects of XFEL peak power, beam focus, and dose on the Mn K beta(1,3) XES spectra in PS II over a wide range of pulse parameters collected over seven different experimental runs using both microcrystal and solution PS II samples. Our findings show that for beam intensities ranging from & SIM;5 x 10(15) to 5 x 10(17) W/cm(2) at a pulse length of & SIM;35 fs, the spectral effects are small compared to those observed between S-states in the Kok cycle. Our results provide a benchmark for other XFEL-based XES studies on metalloproteins, confirming the viability of this approach.& nbsp;
49.	Fransson, Thomas, et al. (author) Effects of x-ray free-electron laser pulse intensity on the Mn K β 1,3x-ray emission spectrum in photosystem II - A case study for metalloprotein crystals and solutions 2021 In: Structural Dynamics. - : American Institute of Physics (AIP). - 2329-7778. ; 8:6 Journal article (peer-reviewed)abstract In the last ten years, x-ray free-electron lasers (XFELs) have been successfully employed to characterize metalloproteins at room temperature using various techniques including x-ray diffraction, scattering, and spectroscopy. The approach has been to outrun the radiation damage by using femtosecond (fs) x-ray pulses. An example of an important and damage sensitive active metal center is the Mn4CaO5 cluster in photosystem II (PS II), the catalytic site of photosynthetic water oxidation. The combination of serial femtosecond x-ray crystallography and Kβ x-ray emission spectroscopy (XES) has proven to be a powerful multimodal approach for simultaneously probing the overall protein structure and the electronic state of the Mn4CaO5 cluster throughout the catalytic (Kok) cycle. As the observed spectral changes in the Mn4CaO5 cluster are very subtle, it is critical to consider the potential effects of the intense XFEL pulses on the Kβ XES signal. We report here a systematic study of the effects of XFEL peak power, beam focus, and dose on the Mn Kβ1,3 XES spectra in PS II over a wide range of pulse parameters collected over seven different experimental runs using both microcrystal and solution PS II samples. Our findings show that for beam intensities ranging from ∼5 × 1015 to 5 × 1017 W/cm2 at a pulse length of ∼35 fs, the spectral effects are small compared to those observed between S-states in the Kok cycle. Our results provide a benchmark for other XFEL-based XES studies on metalloproteins, confirming the viability of this approach.
50.	Fuller, Franklin D, et al. (author) Drop-on-demand sample delivery for studying biocatalysts in action at X-ray free-electron lasers 2017 In: Nature Methods. - : Macmillan Publishers Ltd.. - 1548-7091 .- 1548-7105. ; 14, s. 443-449 Journal article (peer-reviewed)abstract X-ray crystallography at X-ray free-electron laser sources is a powerful method for studying macromolecules at biologically relevant temperatures. Moreover, when combined with complementary techniques like X-ray emission spectroscopy, both global structures and chemical properties of metalloenzymes can be obtained concurrently, providing insights into the interplay between the protein structure and dynamics and the chemistry at an active site. The implementation of such a multimodal approach can be compromised by conflicting requirements to optimize each individual method. In particular, the method used for sample delivery greatly affects the data quality. We present here a robust way of delivering controlled sample amounts on demand using acoustic droplet ejection coupled with a conveyor belt drive that is optimized for crystallography and spectroscopy measurements of photochemical and chemical reactions over a wide range of time scales. Studies with photosystem II, the phytochrome photoreceptor, and ribonucleotide reductase R2 illustrate the power and versatility of this method.

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