Center of Artificial Photosynthesis for Solar Fuels and Department of Chemistry, School of Science, Westlake University, Hangzhou 310024, China.
Institute of Natural Sciences, Westlake Institute for Advanced Study, Hangzhou 310024, China.
J Am Chem Soc. 2022 Jul 6;144(26):11736-11747. doi: 10.1021/jacs.2c03528. Epub 2022 Jun 24.
Photosynthetic water oxidation is catalyzed by a manganese-calcium oxide cluster, which experiences five "S-states" during a light-driven reaction cycle. The unique "distorted chair"-like geometry of the MnCaO cluster shows structural flexibility that has been frequently proposed to involve "open" and "closed"-cubane forms from the S to S states. The isomers are interconvertible in the S and S states, while in the S state, the open-cubane structure is observed to dominate in (cyanobacteria) samples. In this work, using density functional theory calculations, we go beyond the SY state to the SY → SY step, and report for the first time that the reversible isomerism, which is suppressed in the SY state, is fully recovered in the ensuing SY state due to the proton release from a manganese-bound water ligand. The altered coordination strength of the manganese-ligand facilitates formation of the closed-cubane form, in a dynamic equilibrium with the open-cubane form. This tautomerism immediately preceding dioxygen formation may constitute the rate limiting step for O formation, and exert a significant influence on the water oxidation mechanism in photosystem II.
光合作用水氧化由锰-钙氧化簇催化，在光驱动反应循环中经历五个“S 态”。MnCaO 簇独特的“扭曲椅”状几何形状显示出结构灵活性，这种灵活性经常被提议涉及从 S 态到 S 态的“开”和“闭”立方烷形式。异构体在 S 和 S 态之间可相互转化，而在 S 态，观察到开立方烷结构在（蓝细菌）样品中占主导地位。在这项工作中，我们使用密度泛函理论计算，超越了 SY 态到 SY → SY 步骤，并首次报道，由于锰结合的水分子配体释放质子，可逆异构化在 SY 态被抑制，在随后的 SY 态中完全恢复。锰配体的改变配位强度促进了闭立方烷形式的形成，与开立方烷形式处于动态平衡。在形成氧气之前的这种互变异构可能构成氧气形成的限速步骤，并对光合作用系统 II 中的水氧化机制产生重大影响。
