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 exc itation at 875 nm, in resonance with the red absorption band, contain numerous bands between 1100 and 1600 cm(-1), which are assigned, on th e basis of model compound (2,4-dimethyl- and 2-thiomethyl-4-methyl-sub stituted phenoxide anion and phenoxyl radical) UVRR spectra, to contri butions from the Y495 tyrosinate ligand, as well as from the tyrosyl r adical which is proposed to reside on the cysteinyl-substituted Y272 r esidue, which is also coordinated to the Cu2+. The RR spectrum of the azide adduct obtained at 647.1 nm is found to contain contributions fr om the tyrosyl radical but not from the tyrosinate, consistent with re cent evidence for displacement of the tyrosinate ligand upon azide bin ding. The red absorption band is proposed to arise from a tyrosinate - -> tyrosyl interligand charge transfer transition, mediated by the d(x z) orbital on Cu2+.