Influence of Sequential Thiolate Oxidation on a Nitrile Hydratase Mimic Probed by Multiedge X-ray Absorption Spectroscopy

Nitrile hydratases (NHases) are Fe­(III)- and Co­(III)-containing hydrolytic enzymes that convert nitriles into amides. The metal-center is contained within an N₂S₃ coordination motif with two post-translationally modified cysteinates contained in a <i>cis</i> arrangement, which have been converted into a sulfinate (R-SO₂^–) and a sulfenate (R-SO^–) group. Herein, we utilize Ru L-edge and ligand (N-, S-, and P-) K-edge X-ray absorption spectroscopies to probe the influence that these modifications have on the electronic structure of a series of sequentially oxidized thiolate-coordinated Ru­(II) complexes ((bmmp-TASN)­RuPPh₃, (bmmp-O₂-TASN)­RuPPh₃, and (bmmp-O₃-TASN)­RuPPh₃). Included is the use of N K-edge spectroscopy, which was used for the first time to extract N-metal covalency parameters. We find that upon oxygenation of the bis-thiolate compound (bmmp-TASN)­RuPPh₃ to the sulfenato species (bmmp-O₂-TASN)­RuPPh₃ and then to the mixed sulfenato/sulfinato speices (bmmp-O₃-TASN)­RuPPh₃ the complexes become progressively more ionic, and hence the Ru^II center becomes a harder Lewis acid. These findings are reinforced by hybrid DFT calculations (B­(38HF)­P86) using a large quadruple-ζ basis set. The biological implications of these findings in relation to the NHase catalytic cycle are discussed in terms of the creation of a harder Lewis acid, which aids in nitrile hydrolysis