Sulfur K-edge XAS and DFT studies on metal-sulfur covalent bonds in bioinorganic active sites [electronic resource]
Keywords: Chemistry.

S K-edge X-ray Absorption Spectroscopy (XAS) developed by this group has been shown as a powerful tool to directly measure the amount of sulfur character mixed into the sulfur-metal bonds, which are highly related with the chemical property and reactivity of the complex. Such techniques were then coupled with Density Functional Theory (DFT) calculations to study a series of bioinorganic systems. First, the technique was used to study the [Fe4S4] clusters in the DNA repair glycosylases EndoIII and MutY to evaluate the effects of DNA binding and solvation on Fe-S bond covalencies. Increased covalencies in both iron-thiolate and iron-sulfide bonds would stabilize the oxidized state of the [Fe4S4] clusters. The results are compared to those on previously studied [Fe4S4] model complexes, ferredoxin (Fd), and to new data on high potential iron-sulfur protein (HiPIP). A limited decrease in covalency is observed upon removal of solvent water from EndoIII and MutY, opposite to the significant increase observed for Fd where the [Fe4S4] cluster is solvent exposed. Importantly, in EndoIII and MutY, a large increase in covalency is observed upon DNA binding which is due to the effect of its negative charge on the iron-sulfur bonds. In EndoIII, this change in covalency can be quantified and makes a significant contribution to the observed decrease in reduction potential found experimentally in DNA repair proteins, enabling their HiPIP like redox behavior. Second, S K-edge XAS and DFT have been used to determine the electronic structures of two complexes [MoIVO(bdt)2]2- and [MoVIO2(bdt)2]2- (bdt = benzene-1,2-dithiolate(2-)) that relate to the reduced and oxidized forms of sulfite oxidase (SO). These are compared with those of previously studied DMSO reductase (DMSOr) models. DFT calculations supported by the data are extended to evaluate the reaction coordinate for oxo transfer to a phosphite ester substrate. Three possible transition states are found with the one at lowest energy, stabilized by a P-S interaction, in good agreement with experimental kinetics data. Comparison of both oxo transfer reactions shows that in DMSOr, where the oxo is transferred from the substrate to the metal ion, the oxo transfer induces electron transfer, while in SO, where the oxo transfer is from the metal site to the substrate, the electron transfer initiates oxo transfer. This difference in reactivity is related to the difference in frontier molecular orbitals (FMO) of the metal-oxo and substrate-oxo bonds. Finally, these experimentally related calculations are extended to oxo transfer by sulfite oxidase. The presence of only one dithiolene at the enzyme active site selectively activates the equatorial oxo for transfer, and allows facile structural reorganization during turnover. Finally, S K-edge XAS and DFT are applied on [Fe(NO)(N3PyS)]BF4 complex, the first structural and electronic model of NO-bound cysteine dioxygenase, to understand the electronic structure of this {FeNO}7 S=1/2 species, and its contribution to the reversible release of NO upon photoirradiation.