中文摘要：

基于光合作用自由基耦合的释氧机理,提出了S_4-S_0转化期水结合至放氧复合体Mn_4CaO_4的三种不同模式.使用密度泛函理论计算了释氧后两种可能的自旋态势能面,结果表明它们在热力学上都是容易发生的,差别很小,能共存于S态循环之间的衔接阶段,并且对实验所测的S_1,S_2和S_3态的底物水交换速率没有差异性.然而,水结合的这三种类型理论上具有不同的底物选择性,对当前和之后的循环中底物的归属产生影响（Ca和Mn_4的配体水或周围的结晶水）.但是,O_2的最终来源分别为S_4-S_0和S2-S3结合的两个水分子,关键在于它们演变为底物的时机决定于S_4-S_0的水结合机制.整个S态循环中,锰簇灵活多变的几何结构和Ca,Mn_4及其附近的水通道是实现底物水转移和分解释氧的重要因素.

英文摘要：

It has been acknowledged that molecular oxygen produced in photosynthesis originates from water, rather than carbon dioxide. Dioxygen releases in the S_4-S_0 transition immediately prior to a new water binding to the oxygen-evolving complex, but hardly any investigation has been carried out on the binding mechanism up to date. Based on the open-cubane oxo-oxyl coupling mechanism in the S_4 state of photosynthetic oxygen evolution, in this study we propose three possible pathways of water binding to the oxygen-evolving complex Mn_4 CaO_4 during the S_4-S_0 transition, i.e. water binding to Ca trans to O5, water binding to Ca cis to O5, and water binding to Mn_4 trans to O5. Broken-symmetry density functional theory（BS-DFT） calculations have demonstrated the thermodynamic feasibility for all these possible modes, without an overwhelming inclination for a certain manner. Besides, all these styles do not bring about any difference embodied in the experimental kinetic data on substrate water exchange in the S_1, S_2 and S_3 states, for the basically same structures of the S_0 state derived from these different routes. Therefore, it is considered that the alternative mechanisms could coexist coordinately in the connecting stage between S-state cycles. Importantly, diverse forms of substrate selectivity are deduced according to different water binding ways, which exert obvious influences on the present and later S-cycles. In the long run, however, it can be seen that the two waters binding in the S_4-S_0 and S_2-S_3 periods together constitute the components of the released O_2. What matters is variation of the time to become substrates for different water binding modes during the S_4-S_0 transition, either in the current cycle or in the following cycles. Meanwhile, it is indicated that the dangler Mn_4（Ⅲ）/（Ⅳ） which possesses a five-coordinated pyramidal ligand field in both ＇ ＇0 3S/S states, along with Ca（II） on which the carrousel rearrangement of water ligands can also occur, are essential str