光合水氧化由锰-钙氧化物簇催化，在光驱动的反应循环中，锰-钙氧化物簇经历五个“S状态”。Mn4CaO5（6）团簇独特的“扭曲椅状”几何形状显示出结构的灵活性，这种灵活性经常被认为涉及从S1到S3状态的“开放”和“封闭”立方烷形式。异构体在S1和S2状态下可相互转换，而在S3状态下，观察到开放的立方烷结构在长聚球藻（蓝藻）样品中占主导地位。
该文中，使用密度泛函理论计算，研究人员超越了S3+Yz状态，到了S3nYz→ S4+Yz步骤，并首次报告由于锰结合水配体的质子释放，在S3+Yz状态下被抑制的可逆异构在随后的S3nYz状态下完全恢复。锰配位体配位强度的改变促进了封闭式立方烷的形成，并与开放式立方烷形成动态平衡。在氧气形成之前的互变异构可能构成氧气形成的速率限制步骤，并对光系统II中的水氧化机制产生重大影响。
附：英文原文
Title: Reversible Structural Isomerization of Nature’s Water Oxidation Catalyst Prior to O–O Bond Formation
Author: Yu Guo, Johannes Messinger, Lars Kloo, Licheng Sun
Issue&Volume: June 24, 2022
Abstract: 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 Mn4CaO5(6) cluster shows structural flexibility that has been frequently proposed to involve “open” and “closed”-cubane forms from the S1 to S3 states. The isomers are interconvertible in the S1 and S2 states, while in the S3 state, the open-cubane structure is observed to dominate inThermosynechococcus elongatus (cyanobacteria) samples. In this work, using density functional theory calculations, we go beyond the S3+Yz state to the S3nYz → S4+Yz step, and report for the first time that the reversible isomerism, which is suppressed in the S3+Yz state, is fully recovered in the ensuing S3nYz 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 O2 formation, and exert a significant influence on the water oxidation mechanism in photosystem II.
DOI: 10.1021/jacs.2c03528
Source: https://pubs.acs.org/doi/10.1021/jacs.2c03528
