Possible Mechanisms of Water Splitting Reaction Based on Proton and Electron Release Pathways Revealed for CaMn_4O_5 Cluster of PSII Refined to 1.9 Å X-Ray Resolution
Possible Mechanisms of Water Splitting Reaction Based on Proton and Electron Release Pathways Revealed for CaMn_4O_5 Cluster of PSII Refined to 1.9 Å X-Ray Resolution
复制标题
基于 PSII 精细至 1.9 Å X 射线分辨率的 CaMn_4O_5 簇揭示的质子和电子释放途径的水分解反应的可能机制
DOI：
10.1002/qua.23218
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发表时间：
2012
影响因子：
2.2
通讯作者：
Toru Saito
中科院分区：
文献类型：
--
作者：
大曲新矢;鈴木逸良;花田賢志;吉武剛;Toru Saito
最近，乌梅纳等人。揭示了光系统 II (PSII) 的析氧络合物 (OEC) 中 CaMn4O5 团簇的 X 射线衍射结构，分辨率精化至 1.9 Å。它们的 X 射线结构首次阐明了 PSII 的 OEC 处的氢键网络和质子释放途径。在这里，水分解反应的几个工作假设（启发式原理）源自其 X 射线结构，用于理论建模。这些假设表明水如何在 PSII 的 OEC 处被氧化：即该反应的可能反应机制。为了证实它们，我们还根据 XRD 结构对活性位点模型进行了破缺对称 (BS) UB3LYP 计算。对具有不稳定 dπ-pπ 键的形式 Mn(V)O 的键长进行优化，以阐明通常作为 PSII 的 OEC 水分解反应的催化 (Kok) 循环中的关键中间体引入的物质的可能作用。氧-氧（OO）键形成的过渡结构的定位也通过能量优化技术进行。对 UB3LYP 溶液进行自然轨道 (NO) 分析，以获得自然分子轨道及其占据数，这对于定域 d 电子、不稳定化学键和闭壳（价）轨道的分类非常有用。 NO 分析所表征的局域 d 电子是 ENDOR 和其他磁性实验揭示的磁性的起源。另一方面，基于有效键序 (b)、双自由基特征 (y) 和自旋密度 (Q) 指数等化学指数（使用破缺对称轨道之间的轨道重叠计算），研究了负责 OO 键形成的不稳定（软）dπ-pπ 键的性质。这些化学指数已针对 PSII 的 OEC 处 OO 键形成的过渡结构进行了计算。结合导出的假设和可用的累积实验结果讨论了当前计算结果的含义。 © 2011 Wiley periodicals, Inc. Int J Quantum Chem，2012
Recently, Umena et al. have revealed the X‐ray diffraction structure of the CaMn4O5cluster in the oxygen evolving complex (OEC) of photosystem II (PSII) refined to 1.9 Å resolution. Their X‐ray structure has first elucidated hydrogen‐bonding networks and proton release pathways at OEC of PSII. Here, several working hypotheses (heuristic principles) for water splitting reaction are derived from their X‐ray structure for theoretical modeling. These hypotheses suggest how water can be oxidized at OEC of PSII: namely possible reaction mechanisms for the reaction. To confirm them, we have also performed broken‐symmetry (BS) UB3LYP calculations for active site models based on their XRD structure. The bond lengths of formal Mn(V)O with labile dπ‐pπ bonds are optimized to clarify possible roles of the species that are often introduced as a key intermediate in the catalytic (Kok) cycle for water splitting reaction at OEC of PSII. Location of the transition structure for the oxygen‐oxygen (OO) bond formation is also performed by the energy optimization technique. The natural orbital (NO) analysis of the UB3LYP solutions has been performed to obtain the natural molecular orbitals and their occupation numbers that have been useful for classification of localized d‐electrons, labile chemical bonds and closed‐shell (valence) orbitals. The localized d‐electrons characterized by the NO analysis are the origins for the magnetism revealed by ENDOR and other magnetic experiments. On the other hand, the nature of labile (soft) dπ‐pπ bonds responsible for the OO bond formation has been investigated on the basis of chemical indices such as effective bond order (b), diradical character (y), and spin density (Q) indices that are calculated using the orbital overlap between broken‐symmetry orbitals. These chemical indices have been calculated for the transition structure of the OO bond formation at OEC of PSII. Implications of present computational results are discussed in relation to the derived hypotheses and available accumulated experimental results. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012