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 in the stereospecific oxidation of D-alcohols to aldehydes by galactose oxidase. This contribution reports hybrid Hartree-Fock/density functional B3LYP/6-31G(d) quantum chemical calculations to compare the structures and properties of phenoxyl, (methylthio)phenoxyl, and (methylthio)cresyl radicals-increasingly accurate structural models for the biological radical. Calculated isotropic hyperfine coupling constants (hfcc's) for (methylthio)cresyl radical most closely resemble hfcc's measured for the apoenzyme (with Cu2+ removed), although the odd-alternant spin density pattern of phenoxyl radical is preserved in all three models. All three radicals are similarly accurate models for the vibrations of the enzyme's radical, although the calculated frequency for Wilson mode 19a, considered diagnostic for the cysteine-linked tyrosyl radical in galactose oxidase, appears closest to experimental results 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 (Grafton, A; K.; Wheeler, R. A. J. Comput. Chem. 1998, 19, 1663; Comput. Phys. Commun 1998, 113, 78).