Yano Junko, Yachandra Vittal K
Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
Inorg Chem. 2008 Mar 17;47(6):1711-26. doi: 10.1021/ic7016837.
Light-driven oxidation of water to dioxygen in plants, algae, and cyanobacteria is catalyzed within photosystem II (PS II) by a Mn 4Ca cluster. Although the cluster has been studied by many different methods, its structure and mechanism have remained elusive. X-ray absorption and emission spectroscopy and extended X-ray absorption fine structure studies have been particularly useful in probing the electronic and geometric structures and the mechanism of the water oxidation reaction. Recent progress, reviewed here, includes polarized X-ray absorption spectroscopy measurements of PS II single crystals. Analysis of those results has constrained the Mn 4Ca cluster geometry to a set of three similar high-resolution structures. The structure of the cluster from the present study is unlike either the 3.0- or 3.5-A-resolution X-ray structures or other previously proposed models. The differences between the models derived from X-ray spectroscopy and crystallography are predominantly because of damage to the Mn 4Ca cluster by X-rays under conditions used for the structure determination by X-ray crystallography. X-ray spectroscopy studies are also used for studying the changes in the structure of the Mn 4Ca catalytic center as it cycles through the five intermediate states known as the S i states ( i = 0-4). The electronic structure of the Mn 4Ca cluster has been studied more recently using resonant inelastic X-ray scattering spectroscopy (RIXS), in addition to the earlier X-ray absorption and emission spectroscopy methods. These studies are revealing that the assignment of formal oxidation states is overly simplistic. A more accurate description should consider the charge density on the Mn atoms, which includes the covalency of the bonds and delocalization of the charge over the cluster. The geometric and electronic structures of the Mn 4Ca cluster in the S states derived from X-ray spectroscopy are leading to a detailed understanding of the mechanism of O-O bond formation during the photosynthetic water-splitting process.
在植物、藻类和蓝细菌中，光驱动的水氧化成氧气的过程是由光系统II（PS II）中的Mn₄Ca簇催化的。尽管已经通过许多不同方法对该簇进行了研究，但其结构和机制仍然难以捉摸。X射线吸收和发射光谱以及扩展X射线吸收精细结构研究在探测水氧化反应的电子和几何结构以及机制方面特别有用。本文综述的最新进展包括对PS II单晶的偏振X射线吸收光谱测量。对这些结果的分析将Mn₄Ca簇的几何结构限制为一组三个相似的高分辨率结构。本研究中该簇的结构与3.0埃或3.5埃分辨率的X射线结构以及其他先前提出的模型均不相同。X射线光谱学和晶体学得出的模型之间的差异主要是由于在X射线晶体学确定结构所使用的条件下，X射线对Mn₄Ca簇造成了损伤。X射线光谱学研究还用于研究Mn₄Ca催化中心在经历被称为S_i态（i = 0 - 4）的五个中间态循环时其结构的变化。除了早期使用的X射线吸收和发射光谱方法外，最近还利用共振非弹性X射线散射光谱（RIXS）对Mn₄Ca簇的电子结构进行了研究。这些研究表明，形式氧化态的分配过于简单化。更准确的描述应该考虑Mn原子上的电荷密度，这包括键的共价性以及电荷在簇上的离域。从X射线光谱学得出的S态下Mn₄Ca簇的几何和电子结构正在促成对光合水分解过程中O - O键形成机制的详细理解。
