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

• Published in: Proceedings of the National Academy of Sciences - PNAS 2011-11, Vol.108 (46), p.18600-18605
• Main Authors: Verma, Pratik, Pratt, Russell C, Storr, Tim, Wasinger, Erik C, Stack, T. Daniel P
• Format: Article
• Language: English
• Subjects: absorption; Absorption spectroscopy; Active sites; Amino acids; Analogs; Atoms; Catalytic Domain; Charge transfer; Chemical compounds; Chemistry - methods; Copper; Copper - chemistry; crystal structure; Electrochemistry; Electrochemistry - methods; Electron Spin Resonance Spectroscopy; energy; Enzymes; Free radicals; galactose; Galactose Oxidase - chemistry; Models, Chemical; Orbitals; Oxidases; Oxidation; Oxidation-Reduction; Oxygen - chemistry; Phenols - chemistry; Physical Sciences; Spectroscopy; Spectroscopy, Near-Infrared - methods; Sulfur; Sulfur - chemistry; Temperature; tyrosine; Ultraviolet Rays; X-radiation
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1109931108

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 [1SR2]+ are class II mixed-valent CuII-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 1SR2 are lower than the alkyl-substituted analogs by up to ca. 150 mV and decrease following the steric trend: -StBu > -Si 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 [1SR2]+. 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.