TIME RESOLVED X-RAY SPECTROSCOPY OF PHOTOSYSTEM II

该子项目是利用 由NIH/NCRR资助的中心赠款提供的资源。子项目和 研究者（PI）可能从另一个NIH来源获得主要资金， 因此可以在其他CRISP条目中表示。列出的机构是 中心，不一定是研究者的机构。 光系统II（PS II）是存在于绿色植物和蓝藻中的一种膜结合蛋白复合物，它催化光合作用中的水氧化和放氧。在PSII的反应中心的一个专门的叶绿素分子的单电子光氧化耦合到水的四电子氧化的水氧化复合物（WOC）。这种复合物由四个Mn和一个Ca组成，由许多氧（Mn 4 OXCa）桥接，并通过五个标记为S 0-S4的中间状态，其中每个跃迁在约100至1400 ms内完成。在催化转换过程中络合物内的动态变化和水氧化的机理在很大程度上仍然未知。虽然一些光谱证据的存在的中间状态，S3和S 0状态之间的催化循环，已提供了UV/维斯和X-射线光谱方法，没有详细的X-射线光谱表征的动力学在每个S-状态的过渡尚未进行。在这个建议中，我们描述的实验，用于采集PS II的时间分辨光谱在Mn K边缘区域（XANES），使用一个步骤扫描的方法和在K X射线发射线（XES）的区域，使用色散光学检测方法收集整个光谱信息在一个镜头。我们期望本研究将提供重要的见解的电荷密度分布的变化和锰配体的相互作用在Mn 4 OXCa配合物在其催化循环，提供有关水的氧化和析氧的可能机制的结论。

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 of PSII are coupled to the four-electron oxidation of water by the water-oxidizing complex (WOC). This complex is composed of four Mn and one Ca bridged by a number of oxygens (Mn4OXCa), and advances through five intermediate states labeled S0-S4 where each transition is completed in about 100 to 1400 ms. Although the WOC has been the subject of numerous biochemical and spectroscopic studies and the static structure has become more clearly resolved, the dynamic changes within the complex during the catalytic turnover and the mechanism of water oxidation remain largely unknown. Although some spectroscopic evidence for the existence of intermediate states, between the S3 and S0 states in the catalytic cycle, have been provided by UV/VIS and x-ray spectroscopic methods, no detailed x-ray spectroscopic characterization of the dynamics during each S-state transition has been undertaken yet. In this proposal we describe experiments for acquisition of time resolved spectra on PS II in the Mn K edge region (XANES) using a step scan approach and in the region of the K x-ray emission lines (XES), using a dispersive optics detection approach to collect the entire spectral information in one shot. We expect the present study will provide important insights into the changes of the charge density distribution and the Mn-ligand interactions within the Mn4OXCa complex during its catalytic cycle, providing conclusions about the possible mechanism of water oxidation and oxygen evolution.