TIME RESOLVED X-RAY SPECTROSCOPY OF PHOTOSYSTEM II

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。PSII反应中心的一种特殊的叶绿素分子的单电子光氧化作用与水氧化复合体(WOC)对水的四电子氧化作用相耦合。该络合物由四个Mn和一个由多个氧(Mn4OXCa)桥联的Ca组成，经过五个标记为S0-S4的中间态，每个过渡态大约在100到1400ms内完成。尽管WOC已成为众多生化和光谱研究的对象，其静态结构已变得更加清晰，但在催化周转过程中络合物内部的动态变化和水氧化机理仍很不清楚。虽然UV/Vis和X射线光谱学方法已经提供了一些光谱证据，证明了催化循环中S3和S0态之间存在中间态，但还没有对每个S态转变过程中的动力学进行详细的X射线光谱表征。在这个方案中，我们描述了利用阶跃扫描方法在PSⅡ上的MN K边缘区域(XANES)和K X射线发射线(XES)区域获得时间分辨光谱的实验，使用色散光学探测方法一次收集全部光谱信息。我们期望本研究将对Mn4OXCa络合物在催化循环过程中电荷密度分布和Mn-配体相互作用的变化提供重要的见解，为水氧化和析氧的可能机制提供结论。

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.