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 are coupled to the four-electron oxidation of water by the oxygen-evolving complex (OEC) in PS II through five intermediate states labeled S0-S4. A protein-bound complex of four Mn atoms is thought to be the site of water oxidation chemistry and to function in charge accumulation. Although this Mn complex has been the subject of numerous biochemical and spectroscopic studies, the exact structure of the complex and the mechanism of water oxidation remain unknown. Although x-ray spectroscopic methods have been perhaps the most informative technique used to derive detailed information about the oxidation states of and the environment surrounding Mn in the OEC most of the x-ray spectroscopic studies were performed on randomly oriented PS II membranes;thus, no angle dependence of EXAFS features was observed. The ability to exploit the plane-polarized nature of synchrotron radiation has made possible the study of dichroism in oriented PS II membranes, mostly, in terms of the orientation dependence of the Mn?Mn vectors in the di-u-oxo and mono-u-oxo bridged Mn2 motifs that have been shown to be present in the OEC. In this proposal we describe experiments with single crystals of Mn model compounds and photosystem II. We expect the present study will provide important insights to the structure of the Mn complex and its electronic properties that are critical for understanding the mechanism of water oxidation and oxygen evolution.