XAS OF MANGANESE IN THE PHOTOSYNTHETIC OXYGEN-EVOLVING COMPLEX

这个子项目是许多研究子项目中利用 资源由NIH/NCRR资助的中心拨款提供。子项目和 调查员(PI)可能从NIH的另一个来源获得了主要资金， 并因此可以在其他清晰的条目中表示。列出的机构是 该中心不一定是调查人员的机构。 大气中支持地球上有氧生命的大部分氧气是由植物和蓝藻通过光诱导将水氧化成氧气而产生的。这是生物圈中规模如此之大的最重要的化学过程之一。催化这一反应的光合作用装置的放氧复合体(OEC)包含一个由四个锰原子组成的簇。光系统II(PS II)中的水氧化是一个逐步的过程，反应中心吸收的4个连续的光子中的每一个都通过S态中间体S0-S4推动OEC的前进。在达到假设的S4状态时，复合体释放O2并返回到S0。与这一过程相关的关键问题是，当有机电化学反应通过S状态循环时，锰络合物中的氧化态和结构变化，以及锰络合物从两个水分子中去除四个电子以生成O2分子的机理。人们普遍认为，锰团簇是水氧化的活性中心，但实现这一点的化学机制还不够清楚。除锰外，氯离子和钙离子也是生物活性所必需的辅因子，但其确切的结构和功能作用尚不清楚。锰络合物和施主络合物的部分氧化中间体，即锰-OEC的S态(S0-S3)的结构和化学一直是人们非常感兴趣和研究的主题。锰的X射线光谱研究提供了关于络合物的微观结构和OEC的S0、S1、S2和S3态的锰原子的氧化态的直接信息。很少有其他光谱技术能提供这样的特异性来研究OEC中的锰的结构，这是目前唯一适合于监测S态锰的方法。

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Most of the oxygen in the atmosphere which supports aerobic life on earth is generated by plants and cyanobacteria by the photoinduced oxidation of water to dioxygen. This is one of the most important chemical processes occurring on such a large scale in the biosphere. The oxygen-evolving complex (OEC), of the photosynthetic apparatus that catalyzes this reaction contains a cluster of four Mn atoms. Water oxidation in photosystemII (PS II) is a stepwise process wherein each of 4 sequential photons absorbed by the reaction center powers the advance of the OEC through the S-state intermediates S0-S4. Upon reaching the hypothesized S4 state, the complex releases O2 and returns to S0. The critical questions related to this process are the oxidation state and structural changes in the Mn complex as the OEC proceeds through the S-state cycle, and the mechanism by which four electrons are removed from two water molecules by the Mn complex to produce an O2 molecule. It is generally accepted that the Mn cluster is the active site for water oxidation, but the chemical mechanism by which this is achieved is inadequately understood. In addition to Mn, Cl- and Ca2+ are essential cofactors that are required for activity, but their exact structural and functional role is not yet clear. The structure and chemistry of the Mn complex and the partially oxidized intermediates of the donor complex, the S-states of the Mn-OEC(S0-S3),have been the subject of intense interest and study. Mn X-ray spectroscopy studies provide direct information on the microstructure of the complex and on the oxidation states of the Mn atoms in the S0, S1, S2 and S3 states of the OEC. Few other spectroscopic techniques provide such specificity for studying the structure of Mn in the OEC, and at present this is the only method suitable to monitor Mn in the S-states.