Petrie Simon, Stranger Rob, Pace Ron J
Research School of Chemistry, College of Physical & Mathematical Sciences, College of Science, Australian National University, Canberra, ACT 0200, Australia.
Phys Chem Chem Phys. 2017 Oct 18;19(40):27682-27693. doi: 10.1039/c7cp04797e.
The oxygen evolving complex, (OEC) in Photosystem II contains a MnCa cluster and catalyses oxidation of water to molecular oxygen and protons, the most energetically demanding reaction in nature. The catalytic mechanism remains unresolved and the precise Mn oxidation levels through which the cluster cycles during functional turnover are controversial. Two proposals for these redox levels exist; the 'high' and 'low' oxidation state paradigms, which differ systematically by two oxidation equivalents throughout the redox accumulating catalytic S state cycle (states S…S). Presently the 'high' paradigm is more favored. For S the assumed mean redox levels of Mn are 3.5 (high) and 3.0 (low) respectively. Mn K region X-ray spectroscopy has been extensively used to examine the OEC Mn oxidation levels, with Kβ emission spectroscopy increasingly the method of choice. Here we review the results from application of this and closely related techniques to PS II, building on our earlier examination of these and other data on the OEC oxidation states (Pace et al., Dalton Trans., 2012, 41, 11145). We compare the most recent results with a range of earlier Mn Kβ experiments on the photosystem and related model Mn systems. New analyses of these data are given, highlighting certain key spectral considerations which appear not to have been sufficiently appreciated earlier. These show that the recent and earlier PS II Kβ results have a natural internal consistency, leading to the strong conclusion that the low paradigm oxidation state assignment for the functional OEC is favoured.
光系统II中的析氧复合体（OEC）含有一个锰钙簇，可催化水氧化为分子氧和质子，这是自然界中能量需求最高的反应。其催化机制仍未解决，并且该簇在功能转换过程中循环的精确锰氧化态存在争议。关于这些氧化还原态有两种提议，即“高”和“低”氧化态范式，在整个氧化还原积累催化S态循环（S₀…S₄态）中，它们系统地相差两个氧化当量。目前，“高”范式更受青睐。对于S₀态，假设锰的平均氧化态分别为3.5（高）和3.0（低）。锰K区域X射线光谱已被广泛用于研究OEC的锰氧化态，其中Kβ发射光谱越来越成为首选方法。在此，我们在之前对OEC氧化态的这些及其他数据的研究基础上（Pace等人，《道尔顿汇刊》，2012年，41卷，11145页），回顾将此技术及密切相关技术应用于光系统II的结果。我们将最新结果与一系列早期关于光系统及相关模型锰体系的锰Kβ实验进行比较。给出了对这些数据的新分析，突出了某些之前似乎未得到充分重视的关键光谱考虑因素。这些表明，近期和早期的光系统II Kβ结果具有自然的内在一致性，从而有力地得出结论：功能OEC的低范式氧化态归属更受青睐。
