Sulfanyl stabilization of copper-bonded phenoxyls in model complexes and galactose oxidase

Integrating sulfanyl substituents into copper-bonded phenoxyls significantly alters their optical and redox properties and provides insight into the influence of cysteine modification of the tyrosine cofactor in the enzyme galactose oxidase. The model complexes [1(SR2)](+) are class II mixed-valent Cu(II)-phenoxyl-phenolate species that exhibit intervalence charge transfer bands and intense visible sulfur-aryl π → π* transitions in the energy range, which provides a greater spectroscopic fidelity to oxidized galactose oxidase than non-sulfur-bearing analogs. The potentials for phenolate-based oxidations of the sulfanyl-substituted 1(SR2) are lower than the alkyl-substituted analogs by up to ca. 150 mV and decrease following the steric trend: -S(t)Bu > -S(i) Pr > -SMe. Density functional theory calculations suggest that reducing the steric demands of the sulfanyl substituent accommodates an in-plane conformation of the alkylsulfanyl group with the aromatic ring, which stabilizes the phenoxyl hole by ca. 8 kcal mol(-1) (1 kcal = 4.18 kJ; 350 mV) through delocalization onto the sulfur atom. Sulfur K-edge X-ray absorption spectroscopy clearly indicates a contribution of ca. 8-13% to the hole from the sulfur atoms in [1(SR2)](+). The electrochemical results for the model complexes corroborate the ca. 350 mV (density functional theory) contribution of hole delocalization on to the cysteine-tyrosine cross-link to the stability of the phenoxyl radical in the enzyme, while highlighting the importance of the in-plane conformation observed in all crystal structures of the enzyme.