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

• Published in: Inorganic chemistry 2012-06, Vol.51 (11), p.6032-6045
• Main Authors: Shearer, Jason, Callan, Paige E, Masitas, César A, Grapperhaus, Craig A
• Format: Article
• Language: English
• Subjects: Biomimetic Materials - chemistry; Coordination Complexes - chemistry; Hydro-Lyases - chemistry; Models, Molecular; Oxidation-Reduction; Ruthenium - chemistry; Sulfhydryl Compounds - chemistry; X-Ray Absorption Spectroscopy
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic202453c

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 cis 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 RuII 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.