TIME RESOLVED X-RAY SPECTROSCOPY OF PHOTOSYSTEM II

该子项目是利用该技术的众多研究子项目之一 资源由 NIH/NCRR 资助的中心拨款提供。子项目及 研究者 (PI) 可能已从 NIH 的另一个来源获得主要资金， 因此可以在其他 CRISP 条目中表示。列出的机构是 对于中心来说，它不一定是研究者的机构。 光系统 II (PS II) 是一种在绿色植物和蓝细菌中发现的膜结合蛋白复合物，可催化光合水氧化和氧气释放。 PSII 反应中心的特殊叶绿素分子的单电子光氧化与水氧化复合物 (WOC) 对水的四电子氧化耦合。该复合物由四个 Mn 和一个 Ca 组成，并通过多个氧 (Mn4OXCa) 桥接，并通过标记为 S0-S4 的五个中间状态前进，其中每个转变在大约 100 至 1400 毫秒内完成。尽管WOC已成为众多生化和光谱研究的主题，并且其静态结构已变得更加清晰，但催化转换过程中复合物内的动态变化和水氧化机制仍然很大程度上未知。尽管 UV/VIS 和 X 射线光谱方法已经提供了催化循环中 S3 和 S0 状态之间中间态存在的一些光谱证据，但尚未对每个 S 状态转变期间的动力学进行详细的 X 射线光谱表征。在本提案中，我们描述了使用步进扫描方法在 Mn K 边缘区域 (XANES) 上采集 PS II 上时间分辨光谱的实验，以及在 K X 射线发射线 (XES) 区域中使用色散光学检测方法一次性收集整个光谱信息的实验。我们期望本研究将为 Mn4OXCa 配合物在催化循环过程中电荷密度分布和锰-配体相互作用的变化提供重要的见解，从而提供关于水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.