Sulfur K-Edge XAS and DFT Calculations on Nitrile Hydratase: Geometric and Electronic Structure of the Non-heme Iron Active Site.

The geometric and electronic structure of the active site of the non-heme iron enzyme nitrile hydratase (NHase) is studied using sulfur K-edge XAS and DFT calculations. Using thiolate (RS-)-, sulfenate (RSO-)-, and sulfenate (RSO2-)-ligated model complexes to provide benchmark spectral parameters, the results show that the S K-edge XAS is sensitive to the oxidation state of S-containing ligands and that the spectrum of the RSO species changes upon protonation as the S-O bond is elongated (by ∼0.1 Å). These signature features are used to identify the three cysteine residues coordinated to the low-spin Fe... in the active site of NHase as CysS-, CysSOH, and CysSO2- both in the NO-bound inactive form and in the photolyzed active form. These results are correlated to geometry-optimized DFT calculations. The pre-edge region of the X-ray absorption spectrum is sensitive to the Z of the Fe and reveals that the Fe in [FeNO]6 NHase species has a Zeff very similar to that of its photolyzed Fe... counterpart. DFT calculations reveal that this results from the strong π back-bonding into the π* antibonding orbital of NO, which shifts significant charge from the formally t26 low-spin metal to the coordinated NO. [ABSTRACT FROM AUTHOR]