Electronic structure and hyperfine interactions in thioether-substituted tyrosyl radicals

Electron-magnetic resonance spectroscopy and computational studies of the cysteine cross-linked tyrosyl radical in apogalactose oxidase have led to conflicting ideas regarding spectral assignments and protein-environment effects. We report DFT calculations on model radicals that clarify these issues. Calculated Fermi contact interactions do not resolve the ambiguity in spectral assignments; better insight is provided by the anisotropy of the hyperfine interactions and the systematic effects of thioether substitution. DFT results on model systems do not account for salient properties of the apogalactose radical. This inadequacy suggests that the protein environment exerts significant effects on the electronic structure of the radical.