X-RAY SPECTROSCOPY OF PHOTOSYSTEM II SINGLE CRYSTALS

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。 光系统II(PS II)是在绿色植物和蓝藻中发现的一种膜结合蛋白复合体，它催化光合作用中的水氧化和放氧。反应中心的一个特殊的叶绿素分子的单电子光氧化与PS II中的放氧复合体(OEC)通过标记为SO-S4的五个中间态对水的四电子氧化作用相耦合。一个由四个锰原子组成的蛋白质结合络合物被认为是水氧化化学的中心，并具有电荷积累的功能。虽然这种锰络合物已经成为许多生化和光谱研究的对象，但该络合物的确切结构和水氧化机理仍不清楚。虽然X射线光谱分析方法可能是用于获得有关OEC中锰的氧化状态和周围环境的详细信息的技术，但大多数X射线光谱分析都是在随机取向的PS II膜上进行的；因此，没有观察到EXAFS特征与角度的关系。利用同步辐射的平面极化性质的能力使定向PS II膜的二色性研究成为可能，主要是根据已被证明存在于OEC中的双-氧代和单-氧代桥联的Mn2基序中的锰-锰向量的取向相关性。在这个方案中，我们描述了用单晶模型化合物和光系统II进行的实验。我们希望本研究将为理解水的氧化和放氧的机制提供重要的见解。

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. Photosystem II (PS II) is a membrane-bound protein complex found in green plants and cyanobacteria, which catalyzes photosynthetic water oxidation and oxygen evolution. Single-electron photo-oxidations of a specialized chlorophyll molecule in the reaction center are coupled to the four-electron oxidation of water by the oxygen-evolving complex (OEC) in PS II through five intermediate states labeled SO-S4. A protein-bound complex of four Mn atoms is thought to be the site of water oxidation chemistry and to function in charge accumulation. Although this Mn complex has been the subject of numerous biochemical and spectroscopic studies, the exact structure of the complex and the mechanism of water oxidation remain unknown. Although X-ray spectroscopic methods have been perhaps the most informative technique used to derive detailed information about the oxidation states of and the environment surrounding Mn in the OEC most of the X-ray spectroscopic studies were performed on randomly oriented PS II membranes; thus, no angle dependence of EXAFS features was observed. The ability to exploit the plane-polarized nature of synchrotron radiation has made possible the study of dichroism in oriented PS II membranes, mostly, in terms of the orientation dependence of the Mn-Mn vectors in the di-¿-oxo and mono-¿-oxo bridged Mn2 motifs that have been shown to be present in the OEC. In this proposal we describe experiments with single crystals of Mn model compounds and photosystem II. We expect the present study will provide important insights to the structure of the Mn complex and its electronic properties that are critical for understanding the mechanism of water oxidation and oxygen evolution.