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⁺Y_z state to the S₃ⁿY_z^• → S₄⁺Y_z step, and report for the first time that the reversible isomerism, which is suppressed in the S₃⁺Y_z state, is fully recovered in the ensuing S₃ⁿY_z^• 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.

中文翻译：

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天然水氧化催化剂在 O-O 键形成之前的可逆结构异构化

光合水氧化由锰-钙氧化物簇催化，在光驱动反应循环中经历五个“S-状态”。_{Mn 4} CaO ₅₍₆₎簇独特的“扭曲椅子”状几何结构显示出结构灵活性，经常提出涉及从 S ₁到 S ₃状态的“开放”和“封闭”立方烷形式。_{异构体在 S 1}和 S ₂状态下是可相互转换的，而在 S ₃状态下，观察到开放立方结构在细长热聚球藻（蓝细菌）样品中占主导地位。在这项工作中，使用密度泛函理论计算，我们超越了 S ₃⁺ Y _z状态到 S ₃ ⁿ Y _z ^• → S ₄ ⁺ Y _z步骤，并首次报告在 S ₃ ⁺ Y _z状态下被抑制的可逆异构在随后的 S 中完全恢复₃ ⁿ Y _z ^•态是由于质子从锰结合水配体中释放出来的。锰-配体的配位强度的改变促进了闭立方结构的形成，与开立方结构处于动态平衡状态。这种在分子氧形成之前的互变异构可能构成 O 的限速步骤₂的形成，并对光系统 II 中的水氧化机制产生显着影响。