Thiolate S-oxygenation controls nitric oxide (NO) photolability of a synthetic iron nitrile hydratase (Fe-NHase) model derived from mixed carboxamide/thiolate ligand.

In order to determine the origin of the NO photolability of the active site of Fe-containing nitrile hydratase (Fe-NHase), a model complex of the NO-bound active site (dark form) has been isolated and structurally characterized. The model, NEt(4)[(Cl(2)PhPepS)Fe(NO)(DMAP)] (2), is derived from a tetradentate ligand comprising carboxamido N and thiolato S donor centers much like the donors present in the active site of Fe-NHase. This {Fe-NO}(6) nitrosyl effectively mimics the NO-bound active site in terms of structural and spectroscopic parameters. However, this model lacks the key property of NO photolability. Interestingly, S-oxygenation of the model complex results in formation of Na[(Cl(2)PhPep{SO(2)}(2))Fe(NO)(DMAP)] (3), in which the -S donors are oxygenated to -SO(2) moieties, and this species exhibits NO photolability. These results indicate that S-oxygenation could be the key reason for the observed NO photolability of the active site of the dark form of Fe-NHase.