S_4-S_0转化期水结合至放氧复合体可能机制的理论研究  

Possible Mechanisms of Water Binding to the Oxygen-Evolving Complex during the S_4-S_0 Transition:A Theoretical Investigation

作　　者：郭宇[1] 刘瑜 戚娟娟 李慧 赫兰兰[1] 卢丽男[1] 刘翠[1] 宫利东[1] 赵东霞[1] 杨忠志[1] Guo Yu Liu Yu Qi Juanjuan Li Hui He Lanlan Lu Linan Liu Cui Gong Lidong Zhao Dongxia＊ Yang Zhongzhi＊(School of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian 116029)

机构地区：[1]辽宁师范大学化学化工学院,大连116029

出　　处：《化学学报》2017年第9期914-921,共8页Acta Chimica Sinica

基　　金：国家自然科学基金(Nos.21473083;21133005);辽宁省自然科学基金(No.2014020150)资助~~

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.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

关 键 词：光合作用 放氧复合体 S4-S0转化 水结合机制 水通道 对称性破损密度泛函理论 热力学 底物选择性 

分 类 号：TQ116.14[化学工程—无机化工]