Manganese Oxyl Radical Intermediates and O−O Bond Formation in Photosynthetic Oxygen Evolution and a Proposed Role for the Calcium Cofactor in Photosystem II
Manganese Oxyl Radical Intermediates and O−O Bond Formation in Photosynthetic Oxygen Evolution and a Proposed Role for the Calcium Cofactor in Photosystem II
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DOI：
10.1021/ja982290d
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发表时间：
1999
期刊：
影响因子：
--
通讯作者：
P. Siegbahn;R. Crabtree
中科院分区：
文献类型：
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作者：
P. Siegbahn;R. Crabtree
提出了自旋态的考虑，以严格限制光系统II的出氧中心在水氧化过程中可能的O−O键形成步骤。在简单模型配合物的量子化学研究基础上，提出了一系列Kok S态的中间体；这些也与主要的生物物理数据一致。活性位点簇中只有一个Mn原子被认为具有氧化还原活性并介导O−O键的形成。一个关键的概念是在S2状态下形成非活性的MnO氧，然后在S3水平上转化为活性的Mn−O•氧形式，在氧上具有自由基特征，此时氧与外球水分子之间的耦合可以形成O−O键。S3处的MnOOH中间体会失去一个氢原子生成O2。Ca辅因子的作用是在S2中引起5- 6配位变化，这是在S3中形成活性氧所必需的。氯离子辅因子被赋予电荷中性的作用。
Spin state considerations are proposed to sharply limit the possible O−O bond-forming steps in water oxidation by the oxygen evolving center of Photosystem II. A series of intermediates are proposed for the Kok S states on the basis of quantum chemical studies on simple model complexes; these are also consistent with the main biophysical data. Only one Mn atom in the active site cluster is thought to be redox-active and mediate O−O bond formation. A key concept is the formation of an unreactive MnO oxo at the S2 state, followed by its conversion to a reactive Mn−O• oxyl form at the S3 level, with radical character on the oxyl oxygen, at which point O−O bond formation can occur by a coupling between the oxyl and an outer-sphere water molecule. An MnOOH intermediate at S3 is proposed to lose a hydrogen atom to give O2. The role of the Ca cofactor is to bring about a 5- to 6-coordination change at S2, necessary for formation of a reactive oxo in S3. The chloride cofactor is assigned the role of charge neutra...