摘要：
It has been acknowledged that molecular oxygen produced in photosynthesis originates from water,rather than carbon dioxide.Dioxygen releases in the S4-S0 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 S4 state of photosynthetic oxygen evolution,in this study we propose three possible pathways of water binding to the oxygen-evolving complex Mn4CaO4 during the S4-S0 transition,i.e.water binding to Ca trans to O5,water binding to Ca cis to O5,and water binding to Mn4 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 S1,S2 and S3 states,for the basically same structures of the So state derived from these different routes.Therefore,it is considered that the altemative 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 54-S0 and S2oS3 periods together constitute the components of the released O2.What matters is variation of the time to become substrates for different water binding modes during the S4-S0 transition,either in the current cycle or in the following cycles.Meanwhile,it is indicated that the dangler Mn4(Ⅲ)/(Ⅳ) which possesses a five-coordinated pyramidal ligand field in both (S)0/(S)3 states,along with Ca(Ⅱ) on which the carrousel rearrangement of water ligands can also occur,are essential structural elements of the S-state advancement and oxygen evolution.Thus,Mn4 and Ca may be in charge of water delivery to the active sites of Mn4CaO5 from the nearby external water channels formed by crystal waters in hydrogen-bond interactions.On the whole,the geometric flexibility of the Mn cluster plays an important role in photosynthetic water oxidation.In the respects of water binding modes in the S4-S0 transition and corresponding substrate identifications for a specific S-cycle,we are looking forward to further confirmations or supplements from the experimental evidences of the targeted isotope labeling combined with mass spectrometry,infrared spectroscopy and site directed mutagenesis,etc.Our investigation may provide useful information and references for the mechanistic elucidations on photosynthetic water oxidation,especially in substrate water identifications.%基于光合作用自由基耦合的释氧机理,提出了S4-50转化期水结合至放氧复合体Mn4CaO4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S1,S2和S3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Mn4的配体水或周围的结晶水).但是,O2的最终来源分别为S4-So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S4-S0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,Mn4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.
