TIME RESOLVED X-RAY SPECTROSCOPY OF PHOTOSYSTEM II

Stanford University, Stanford CA

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Abstract

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.