Investigation of the electronic structure of bioinorganic systems using ligand K-edge X-ray absorption spectroscopy: Application to copper(A), metal tetrathiolates and iron-sulfur clusters

Ligand K-edge X-ray absorption spectroscopy (XAS) has been investigate the electronic structure of inorganic model complexes and metalloproteins. S K-edge XAS studies were performed to explore contributions of Cu-ligand bonding and electron transfer in the CuA site of cytochrome c oxidase. In this study, previously developed methodology was used to determine Cu-S covalency. The data reveal significant Cu-S covalency, permitting comparison of competing N- and S-based ET pathways. In order to investigate other systems, S K-edge XAS has been applied to a series of S 4 [M(SR)4]2- complexes (M = Ni(II), Co(II), Fe(II) and Mn(II)) (R = 2-Ph-C6H4) to determine the covalency of their thiolate-metal bonds. This study extended a Cl K-edge analysis of Td [MCl4]2- complexes to metal tetrathiolates where S4 symmetry is applied and there is anisotropic covalency in the thiolate valence orbitals. An expression for quantitating covalency in S K-edge data of systems with more than one electron or hole in the valence d-orbitals was established. The methodology from the [M(SR)4]2- study was extended to 1Fe, 2Fe-2S and 4Fe-4S systems. The 1Fe study provided insight about iron-thiolate bonding which was applied to a series of rubredoxin proteins. It was determined that hydrogen bonding effects in the protein caused a reduction in thiolate covalency. This provided a basis for investigating the iron-thiolate bond in the 2Fe-2S systems. In the 2Fe-2S study, peak intensities in the S pre-edge region were assigned based on studies of complexes containing only terminal thiolate or bridging sulfide. There is one lower-energy peak in the 2Fe-2S pre-edge that is due to the sulfide transition, and a higher-energy peak due to the thiolate transition, consistent with the effective nuclear charge difference between S2- and SR-. A reference was then established for relating sulfide pre-edge intensity to covalency. This 2Fe-2S study was applied to the binuclear Rieske protein and contributions to covalency in each iron site were determined. The 2Fe-2S analysis was extended to a series of 4Fe-4S complexes to investigate electronic structural differences between dimers and tetramers and provide a basis for understanding the different ligand contributions to the electronic structures of these clusters.