Electronic Structure of the Ferryl Intermediate in the α‑Ketoglutarate Dependent Non-Heme Iron Halogenase SyrB2: Contributions to H Atom Abstraction Reactivity
2016-03-29T00:00:00Z (GMT) by
Low temperature magnetic circular dichroism (LT MCD) spectroscopy in combination with quantum-chemical calculations are used to define the electronic structure associated with the geometric structure of the FeIVO intermediate in SyrB2 that was previously determined by nuclear resonance vibrational spectroscopy. These studies elucidate key frontier molecular orbitals (FMOs) and their contribution to H atom abstraction reactivity. The VT MCD spectra of the enzymatic S = 2 FeIVO intermediate with Br– ligation contain information-rich features that largely parallel the corresponding spectra of the S = 2 model complex (TMG3tren)FeIVO (Srnec, M.; Wong, S. D.; England, J.; Que, L. Jr.; Solomon, E. I. Proc. Natl. Acad. Sci. USA 2012, 109, 14326–14331). However, quantitative differences are observed that correlate with π-anisotropy and oxo donor strength that perturb FMOs and affect reactivity. Due to π-anisotropy, the FeIVO active site exhibits enhanced reactivity in the direction of the substrate cavity that proceeds through a π-channel that is controlled by perpendicular orientation of the substrate C–H bond relative to the halide–FeIVO plane. Also, the increased intrinsic reactivity of the SyrB2 intermediate relative to the ferryl model complex is correlated to a higher oxyl character of the FeIVO at the transition states resulting from the weaker ligand field of the halogenase.