Resonance Raman spectroscopy of galactose oxidase: A new interpretation based on model compound free radical spectra

Resonance Raman spectra of active galactose oxidase, obtained with excitation at 875 nm, in resonance with the red absorption band, contain numerous bands between 1100 and 1600 cm^-1, which are assigned, on the basis of model compound (2,4-dimethyl- and 2-thiomethyl-4-methyl-substituted phenoxide anion and phenoxyl radical) UVRR spectra, to contributions from the Y495 tyrosinate ligand, as well as from the tyrosyl radical which is proposed to reside on the cysteinyl-substituted Y272 residue, which is also coordinated to the Cu²⁺. The RR spectrum of the azide adduct obtained at 647.1 nm is found to contain contributions from the tyrosyl radical but not from the tyrosinate, consistent with recent evidence for displacement of the tyrosinate ligand upon azide binding. The red absorption band is proposed to arise from a tyrosinate → tyrosyl interligand charge transfer transition, mediated by the d_xz orbital on Cu²⁺.