Phenoxyl, (methylthio)phenoxyl, and (methylthio)cresyl radical models for the structures, vibrations, and spin properties of the cysteine-linked tyrosyl radical in galactose oxidase

A cysteine-linked tyrosyl radical is implicated as a redox-active subunit i n the stereospecific oxidation of D-alcohols to aldehydes by galactose oxid ase. This contribution reports hybrid Hartree-Fock/density functional B3LYP /6-31G(d) quantum chemical calculations to compare the structures and prope rties of phenoxyl, (methylthio)phenoxyl, and (methylthio)cresyl radicals-in creasingly accurate structural models for the biological radical. Calculate d isotropic hyperfine coupling constants (hfcc's) for (methylthio)cresyl ra dical most closely resemble hfcc's measured for the apoenzyme (with Cu2+ re moved), although the odd-alternant spin density pattern of phenoxyl radical is preserved in all three models. All three radicals are similarly accurat e models for the vibrations of the enzyme's radical, although the calculate d frequency for Wilson mode 19a, considered diagnostic for the cysteine-lin ked tyrosyl radical in galactose oxidase, appears closest to experimental r esults for the (methylthio)cresyl radical. Similarities between vibrational normal modes for the three radicals studied here are quantified by using a recently proposed comparative tool, vibrational projection analysis (Graft on, A; K.; Wheeler, R. A. J. Comput. Chem. 1998, 19, 1663; Comput. Phys. Co mmun 1998, 113, 78).