Influence of Sequential
Thiolate Oxidation on a Nitrile
Hydratase Mimic Probed by Multiedge X-ray Absorption Spectroscopy
- Publication date
- Publisher
Abstract
Nitrile hydratases (NHases) are Fe(III)- and Co(III)-containing
hydrolytic enzymes that convert nitriles into amides. The metal-center
is contained within an N<sub>2</sub>S<sub>3</sub> coordination motif
with two post-translationally modified cysteinates contained in a <i>cis</i> arrangement, which have been converted into a sulfinate
(R-SO<sub>2</sub><sup>–</sup>) and a sulfenate (R-SO<sup>–</sup>) 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)RuPPh<sub>3</sub>, (bmmp-O<sub>2</sub>-TASN)RuPPh<sub>3</sub>, and (bmmp-O<sub>3</sub>-TASN)RuPPh<sub>3</sub>). 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)RuPPh<sub>3</sub> to the sulfenato species (bmmp-O<sub>2</sub>-TASN)RuPPh<sub>3</sub> and then to the mixed sulfenato/sulfinato
speices (bmmp-O<sub>3</sub>-TASN)RuPPh<sub>3</sub> the complexes become
progressively more ionic, and hence the Ru<sup>II</sup> 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
