This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Oxygen, that supports all aerobic life, is abundant in the atmosphere because of its constant regeneration by photosynthetic water oxidation by green plants, algae and cyanobacteria. This light-requiring reaction is catalyzed by a Mn4Ca cluster associated with photosystem II (PS II). Given the role of PS II in maintaining life on the biosphere and the future visions of a renewable energy economy, it is vital to elucidate the structure and mechanism of the Mn4Ca catalyst, which is sometimes referred to as the ?heart? of the water oxidation process. Although, the electronic and geometric structure of the Mn4Ca water-splitting catalyst has been extensively investigated, the precise structure and mechanism of this catalyst has so far eluded all attempts of determination by x-ray diffraction, EXAFS and other spectroscopic techniques. The development of new methods of high resolution absorption and emission x-ray spectroscopy, including RIXS, Kb crossover emission, and time-resolved XES are critical for providing important new information, that will help in elucidating not only the structure of the complex, but also the mechanism of the photosynthetic water oxidation process. 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) RIXS spectroscopy that will address the electronic structure of the Mn complex, b) Interatomic Mn Kb2,5 spectra that will be a probe for the ligand atoms of Mn, especially to ascertain the state of the oxo/hydroxo bridge ligands of Mn in PS II, which are proposed to be involved in the O-O formation, and c) Develop energy-dispersive XES methodology to investigate the Mn4Ca cluster in a time-resolved manner.