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. From our previous studies it was determined that ligand K-edges could be used to quantitate covalency in open-shell metal ions. Initially, the Cl K-edge of a series metal tetrachlorides was performed. The analysis of metal sites with more than one hole in the d-manifold was performed using multiple theory and the irreducible tensor method. The analysis was then extended to a series of metal tetrathiolates and monomeric and dimeric Fe-S proteins and model complexes. It was found that the effect of H-bonding in the proteins in comparison to the models can be detected in the intensity of the S pre-edge feature and that the contributions of thiolate and sulfide can be resolved due to differences in effective nuclear charge of the ligands. Based on these results, the analysis will be extended to tetra and trinuclear Fe-S model complexes and proteins, heterometal cubanes and the FeMoco of nitrogenase. The change of the sulfide bonding by going from the dimeric Fe-S clusters to the tetrameric Fe-S cluster which affects the delocalization behavior in the mixed valent forms will be studied. The effect on H-bonding and its effect on the reduction potential of HIPIP vs. 4Fe ferredoxin will be evaluated. Also the effect of the serine to cysteine mutation in 2Fe ferredoxin of C. pasteurianum on delocalization in the mixed valent form will be examined by S K-edge XAS. Further the effect of a heterometal on the electronic structure of [Fe3S4] will be analyzed and the results will build the basis for analysis of FeMoco from nitrogenase. These studies will be used to understand covalency and electronic structure of bioinorganic systems and evaluate their contributions to function.
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