Metal oxidation states in biological water splitting
Metal oxidation states in biological water splitting
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DOI：
10.1039/c4sc03720k
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发表时间：
2015-01-01
期刊：
影响因子：
8.4
通讯作者：
Pantazis, Dimitrios A.
中科院分区：
文献类型：
--
作者：
Krewald, Vera;Retegan, Marius;Pantazis, Dimitrios A.
生物水分解的一个中心问题是锰离子的氧化态，锰离子是光系统II的放氧复合物。理解在五个Si状态（i = 0-4）的催化循环期间发生的氧化事件的性质和顺序对于自然系统和人工水氧化催化剂都具有根本重要性。尽管广泛采用所谓的“高价方案”-其中，例如，S-2状态下的Mn氧化态被指定为III、IV、IV、IV-竞争性的“低价方案”，其相差总共两个金属未配对电子（即S-2状态下的III、III、III、IV），但最近的几项生物催化剂研究都支持这种方案。正确的氧化态分配的问题在这里解决了一个详细的计算比较两个计划使用一个共同的结构平台和理论方法。基于晶体学约束的模型被构建为所有可能的氧化态分配在四个（半）稳定的S态的氧释放复合物，采样各种质子化水平和模式，以确保全面的覆盖面。模型进行评估，其几何，能量，电子和光谱特性对现有的实验EXAFS，XFEL-XRD，EPR，ENDOR和锰K前边缘XANES数据。还报道了2.5KMn-55 ENDOR新的S-2态数据。我们的研究结果最终表明，整个S态现象只能容纳在高价方案通过采用一个单一的主题和质子化模式，顺利地从S-0（III，III，III，IV）到S-3（IV，IV，IV，IV），满足所有的实验约束和再现所有的观测。相比之下，不可能为所有S态构建基于低价方案的一致循环。相反，这里开发的低价模型可能会提供新的见解过度还原的S状态和参与组装的催化活性水氧化簇的状态。
A central question in biological water splitting concerns the oxidation states of the manganese ions that comprise the oxygen-evolving complex of photosystem II. Understanding the nature and order of oxidation events that occur during the catalytic cycle of five Si states (i = 0-4) is of fundamental importance both for the natural system and for artificial water oxidation catalysts. Despite the widespread adoption of the so-called "high-valent scheme"-where, for example, the Mn oxidation states in the S-2 state are assigned as III, IV, IV, IV-the competing "low-valent scheme" that differs by a total of two metal unpaired electrons (i.e. III, III, III, IV in the S-2 state) is favored by several recent studies for the biological catalyst. The question of the correct oxidation state assignment is addressed here by a detailed computational comparison of the two schemes using a common structural platform and theoretical approach. Models based on crystallographic constraints were constructed for all conceivable oxidation state assignments in the four (semi) stable S states of the oxygen evolving complex, sampling various protonation levels and patterns to ensure comprehensive coverage. The models are evaluated with respect to their geometric, energetic, electronic, and spectroscopic properties against available experimental EXAFS, XFEL-XRD, EPR, ENDOR and Mn K pre-edge XANES data. New 2.5 K Mn-55 ENDOR data of the S-2 state are also reported. Our results conclusively show that the entire S state phenomenology can only be accommodated within the high-valent scheme by adopting a single motif and protonation pattern that progresses smoothly from S-0 (III, III, III, IV) to S-3 (IV, IV, IV, IV), satisfying all experimental constraints and reproducing all observables. By contrast, it was impossible to construct a consistent cycle based on the low-valent scheme for all S states. Instead, the low-valent models developed here may provide new insight into the over-reduced S states and the states involved in the assembly of the catalytically active water oxidizing cluster.