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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 CuII -phenoxyl-phenolate species that exhibit intervalence charge transfer bands and intense visible sulfur-arylπ  → 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-substituted1 SR2 are lower than the alkyl-substituted analogs by up toca. 150 mV and decrease following the steric trend: -St Bu &gt; -Si  Pr &gt; -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 byca. 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 ofca. 8–13% to the hole from the sulfur atoms in [1 SR2 ]+ . The electrochemical results for the model complexes corroborate theca. 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.

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