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. The major emphasis of our research is to apply variable energy photoelectron spectroscopy (PES) combined with X-ray absorption edge spectroscopy (XAS) to define electronic structure contributions to electron transfer (ET) in biological systems. PES provides a direct method to study inorganic redox processes and the contribution of electronic relaxation to redox properties (E[unreadable], HAB, lambda). The shake-up satellite structure present in core and valence PES data can be used in combination with a valence bond configuration interaction (VBCI) model to experimentally quantify electronic relaxation (i.e. the change in electronic structure of metal complexes upon oxidation) and its contributions to reduction potentials and kinetics of ET. Variable-energy PES experiments provide a mechanism to maximize the metal while minimizing the ligand contributions to the valence band region through cross section effects (delayed maximum and Cooper minimum) and resonance enhancement. Our synchrotron-based studies on simple model systems [FeX4]2-/1 (X=Cl-, -SR) have afforded experimental results that define electronic structure and its changes upon redox. Complemented by density functional theory (DFT) calculations, these studies provide insight into electronic relaxation and allow extension to the metalloprotein sites. Using the methodology developed in the studies on [FeX4]2-/1, we are now examining model heme complexes. The data on the heme models combined with DFT calculations should also allow extention to and define the superexchange pathways for ET in the cytochromes. We will also extend these PES/DFT studies to [Fe4S4(SR)4] systems modeling the 4Fe ferredoxins and HiPIPs (high potential iron proteins), which will provide insight into electronic relaxation and its contributions to the kinetics and pathways for ET in the ferredoxins and HiPIPS.