| 1. |
- Li, Xichen, et al.
(författare)
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Alternative mechanisms for O-2 release and O-O bond formation in the oxygen evolving complex of photosystem II
- 2015
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Owner Societies 2015. - 1463-9076 .- 1463-9084. ; 17:18, s. 12168-12174
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Tidskriftsartikel (refereegranskat)abstract
- In a previous detailed study of all the steps of water oxidation in photosystem II, it was surprisingly found that O-2 release is as critical for the rate as O-O bond formation. A new mechanism for O-2 release has now been found, which can be described as an opening followed by a closing of the interior of the oxygen evolving complex. A transition state for peroxide rotation forming a superoxide radical, missed in the previous study, and a structural change around the outside manganese are two key steps in the new mechanism. However, O-2 release may still remain rate-limiting. Additionally, for the step forming the O-O bond, an alternative, experimentally suggested, mechanism was investigated. The new model calculations can rule out the precise use of that mechanism. However, a variant with a rotation of the ligands around the outer manganese by about 301 will give a low barrier, competitive with the old DFT mechanism. Both these mechanisms use an oxyl-oxo mechanism for O-O bond formation involving the same two manganese atoms and the central oxo group (O5).
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| 2. |
- Li, Xichen, et al.
(författare)
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Simulation of the isotropic EXAFS spectra for the S-2 and S-3 structures of the oxygen evolving complex in photosystem II
- 2015
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 112:13, s. 3979-3984
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Tidskriftsartikel (refereegranskat)abstract
- Most of the main features of water oxidation in photosystem II are now well understood, including the mechanism for O-O bond formation. For the intermediate S-2 and S-3 structures there is also nearly complete agreement between quantum chemical modeling and experiments. Given the present high degree of consensus for these structures, it is of high interest to go back to previous suggestions concerning what happens in the S-2-S-3 transition. Analyses of extended X-ray adsorption fine structure (EXAFS) experiments have indicated relatively large structural changes in this transition, with changes of distances sometimes larger than 0.3 angstrom and a change of topology. In contrast, our previous density functional theory (DFT)(B3LYP) calculations on a cluster model showed very small changes, less than 0.1 angstrom. It is here found that the DFT structures are also consistent with the EXAFS spectra for the S2 and S3 states within normal errors of DFT. The analysis suggests that there are severe problems in interpreting EXAFS spectra for these complicated systems.
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| 3. |
- Li, Xichen, et al.
(författare)
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Water Oxidation for Simplified Models of the Oxygen-Evolving Complex in Photosystem II
- 2015
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Ingår i: Chemistry - A European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 21:51, s. 18821-18827
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Tidskriftsartikel (refereegranskat)abstract
- For the main parts of the mechanism for water oxidation in photosystem II there has recently been very strong experimental support for the mechanism suggested by theoretical model studies. The question addressed in the present study is to what extent this knowledge can be used for the design of artificial catalysts. A major requirement for a useful artificial catalyst is that it is small enough to be synthesized. Small catalysts also have the big advantage that they could improve the catalysis per surface area. To make the mechanism found for PSII useful in this context, it needs to be analyzed in detail. A small model system was therefore used and the ligands were replaced one by one by water-derived ligands. Only the main chemical step of O-O bond formation was investigated in this initial study. The energetics for this small model and the larger one previously used for PSII are remarkably similar, which is the most important result of the present study. This shows that small model complexes have a potential for being very good water oxidation catalysts. It was furthermore found that there is a clear correlation between the barrier height for O-O bond formation and the type of optimal structure for the S-3 state. The analysis shows that a flexible central part of the complex is the key for efficient water oxidation.
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