title: Sulfanyl stabilization of copper-bonded phenoxyls in model complexes and galactose oxidase.
abstract: 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.
authors: Department of Chemistry, Stanford University, Stanford, CA 94305, USA.
authors: Pratik
authors: P
authors: Verma
authors: Russell C
authors: RC
authors: Pratt
authors: Tim
authors: T
authors: Storr
authors: Erik C
authors: EC
authors: Wasinger
authors: T Daniel P
authors: TD
authors: Stack
mesh_terms: D020134:Catalytic Domain; D002621:Chemistry / Q000379:methods*; D003300:Copper / Q000737:chemistry*; D004563:Electrochemistry / Q000379:methods; D004578:Electron Spin Resonance Spectroscopy; D005692:Galactose Oxidase / Q000737:chemistry*; D008956:Models, Chemical; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry; D010636:Phenols / Q000737:chemistry*; D019265:Spectroscopy, Near-Infrared / Q000379:methods; D013455:Sulfur / Q000737:chemistry*; D013696:Temperature; D014466:Ultraviolet Rays
chemical_list: D010636:Phenols; C042329:phenoxy radical; D013455:Sulfur; D003300:Copper; D005692:Galactose Oxidase; D010100:Oxygen