228 research outputs found
Alternating electron and proton transfer steps in photosynthetic water oxidation
Water oxidation by cyanobacteria, algae, and plants is pivotal in oxygenic
photosynthesis, the process that powers life on Earth, and is the paradigm for
engineering solar fuelâproduction systems. Each complete reaction cycle of
photosynthetic water oxidation requires the removal of four electrons and four
protons from the catalytic site, a manganeseâcalcium complex and its protein
environment in photosystem II. In time-resolved photothermal beam deflection
experiments, we monitored apparent volume changes of the photosystem II
protein associated with charge creation by light-induced electron transfer
(contraction) and charge-compensating proton relocation (expansion). Two
previously invisible proton removal steps were detected, thereby filling two
gaps in the basic reaction-cycle model of photosynthetic water oxidation. In
the S2 â S3 transition of the classical S-state cycle, an intermediate is
formed by deprotonation clearly before electron transfer to the oxidant
(Graphic). The rate-determining elementary step (Ï, approximately 30 ”s at 20
°C) in the long-distance proton relocation toward the proteinâwater interface
is characterized by a high activation energy (Ea = 0.46 ± 0.05 eV) and strong
H/D kinetic isotope effect (approximately 6). The characteristics of a proton
transfer step during the S0 â S1 transition are similar (Ï, approximately 100
”s; Ea = 0.34 ± 0.08 eV; kinetic isotope effect, approximately 3); however,
the proton removal from the Mn complex proceeds after electron transfer to
Graphic. By discovery of the transient formation of two further intermediate
states in the reaction cycle of photosynthetic water oxidation, a temporal
sequence of strictly alternating removal of electrons and protons from the
catalytic site is established
Identification of YdhV as the first molybdoenzyme binding a Bis-Mo-MPT cofactor in escherichia coli
The oxidoreductase YdhV in Escherichia coli has been predicted to belong to the family of molybdenum/tungsten cofactor (Moco/Wco)-containing enzymes. In this study, we characterized the YdhV protein in detail, which shares amino acid sequence homology with a tungsten-containing benzoyl-CoA reductase binding the bis-W-MPT (for metal-binding pterin) cofactor. The cofactor was identified to be of a bis-Mo-MPT type with no guanine nucleotides present, which represents a form of Moco that has not been found previously in any molybdoenzyme. Our studies showed that YdhV has a preference for bis-Mo-MPT over bis-W-MPT to be inserted into the enzyme. In-depth characterization of YdhV by X-ray absorption and electron paramagnetic resonance spectroscopies revealed that the bis-Mo-MPT cofactor in YdhV is redox active. The bis-Mo-MPT and bis-W-MPT cofactors include metal centers that bind the four sulfurs from the two dithiolene groups in addition to a cysteine and likely a sulfido ligand. The unexpected presence of a bis-Mo-MPT cofactor opens an additional route for cofactor biosynthesis in E. coli and expands the canon of the structurally highly versatile molybdenum and tungsten cofactors
De Mediis Introducendi Philosophiam eclecticam Cogitata : Adjecta est ob affinitatem argumenti Dissertatio Johannis Ludovici Vivis De vita et moribus eruditi
http://tartu.ester.ee/record=b2166557~S1*es
Modelling the coordination environment in α-ketoglutarate dependent oxygenases â a comparative study on the effect of N- vs. O-ligation
In various non-heme iron oxygenases the Fe(II) center is coordinated by 2â
N and 1â
O atoms of the 2-His-2-carboxylate facial triad; however, most artificial model complexes bear only N-based ligands. In an effort to closely mimic the coordination environment in α-ketoglutarate dependent oxygenases, we have now employed the Me2tacnO ligand (4,7-dimethyl-1-oxa-4,7-diazacyclononane) in the synthesis of the complexes [(Me2tacnO)FeCl2]2 (1-NNO), [(Me2tacnO)FeCl3] (1âb-NNO) and [(Me2tacnO)Fe(BF)Cl] (2-NNO; BF=benzoylformate). The weaker donation of the O atom in the ligand was found to result in stronger binding of the ligand in trans-position to the O-atom of the ancillary ligand as compared to the corresponding complexes involving the Me3tacn (1,4,7-trimethyl-1,4,7-triazacyclononane) ligand. Furthermore, by stopped-flow techniques we could detect an intermediate (3-NNO) in the reaction of 2-NNO with O2. The spectroscopic features of 3-NNO agree with the involvement of an Fe(IV)-oxo intermediate and hence this study represents the first detection of such an intermediate in the O2 activation of artificial α-ketoglutarate Fe(II) complexes
Modelling the coordination environment in αâketoglutarate dependent oxygenases â a comparative study on the effect of Nâ vs. Oâligation
In various non-heme iron oxygenases the Fe(II) center is coordinated by 2â
N and 1â
O atoms of the 2-His-2-carboxylate facial triad; however, most artificial model complexes bear only N-based ligands. In an effort to closely mimic the coordination environment in α-ketoglutarate dependent oxygenases, we have now employed the Me2tacnO ligand (4,7-dimethyl-1-oxa-4,7-diazacyclononane) in the synthesis of the complexes [(Me2tacnO)FeCl2]2 (1-NNO), [(Me2tacnO)FeCl3] (1âb-NNO) and [(Me2tacnO)Fe(BF)Cl] (2-NNO; BF=benzoylformate). The weaker donation of the O atom in the ligand was found to result in stronger binding of the ligand in trans-position to the O-atom of the ancillary ligand as compared to the corresponding complexes involving the Me3tacn (1,4,7-trimethyl-1,4,7-triazacyclononane) ligand. Furthermore, by stopped-flow techniques we could detect an intermediate (3-NNO) in the reaction of 2-NNO with O2. The spectroscopic features of 3-NNO agree with the involvement of an Fe(IV)-oxo intermediate and hence this study represents the first detection of such an intermediate in the O2 activation of artificial α-ketoglutarate Fe(II) complexes.Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)Peer Reviewe
Phosphate Coordination in a Water-Oxidizing Cobalt Oxide Electrocatalyst Revealed by X-ray Absorption Spectroscopy at the Phosphorus K-Edge
In the research on water splitting at neutral pH, phosphorus-containing transition metal oxyhydroxides are often employed for catalyzing the oxygen evolution reaction (OER). We investigated a cobaltâphosphate catalyst (CoCat) representing this material class. We found that CoCat films prepared with potassium phosphate release phosphorus in phosphate-free electrolytes within hours, contrasting orders of magnitudeâs faster K+ release. For P speciation and binding mode characterization, we performed technically challenging X-ray absorption spectroscopy experiments at the P K-edge and analyzed the resulting XANES and EXAFS spectra. The CoCat-internal phosphorus is present in the form of phosphate ions. Most phosphate species are likely linked to cobalt ions in CoâOâPO3 motifs, where the connecting oxygen could be a terminal or bridging ligand in Co-oxide fragments (PâCo distance, ~3.1 Ă
), with additional ionic bonds to K+ ions (PâK distance, ~3.3 Ă
). The phosphate coordination bond is stronger than the ionic K+-binding, explaining the strongly diverging ion release rates of phosphate and K+. Our results support a structural role of phosphate in the CoCat, with these ions binding at the margins of Co-oxide fragments, thereby limiting the long-range material ordering. The relations of catalyst-internal phosphate ions to cobaltâs redox-state changes, proton transfer, and catalytic activity are discussed
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