HIGH RESOLUTION MN X-RAY FLUORESCENCE SPECTROSCOPY OF THE OXYGEN-EVOLVING COMPLE

这个子项目是众多研究子项目之一

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. Green plants and algae generate almost all of the oxygen present in the atmosphere by the oxidation of water. This light-requiring reaction is catalyzed by a four-manganese atom cluster associated with Photosystem II (PS II). In our ongoing investigations of the oxidation-state changes, the structure and the associated cofactors Ca and Cl, we have made extensive use of x-ray absorption and EPR spectroscopy. As the membrane-associated complex containing the Mn4 cluster is advanced through its intermediate states (S-states) by light absorption, x-ray absorption spectroscopy indicates that Mn undergoes an increase in oxidation state between S0 and S1 and between S1 and S2 but not between S2 and S3. This latter conclusion, which had been questioned by others, is now confirmed by a new and more definitive x-ray spectroscopy involving Mn K¿ fluorescence. We made use of an unique high-resolution x-ray emission spectrometer to carry out this experiment. The present proposal builds on the past experiments and addresses some new applications of x-ray emission spectroscopy to the Mn complex in PS II as follows: a) K¿ emission and site selective spectroscopy that will selectively probe different Mn sites or different oxidation states in the Mn complex, b) High resolution Mn Ka detection of EXAFS that will allow us to collect EXAFS beyond the Fe K-edge thus improving resolution and c) Interatomic Mn K¿2,5 spectra that will be a probe for the ligand atoms of Mn, especially to ascertain whether chloride is a ligand of Mn in PS II.