title: Electrochemical and spectroscopic effects of mixed substituents in bis(phenolate)-copper(II) galactose oxidase model complexes.
abstract: Nonsymmetric substitution of salen (1(R(1),R(2))) and reduced salen (2(R(1),R(2))) Cu(II)-phenoxyl complexes with a combination of -(t)Bu, -S(i)Pr, and -OMe substituents leads to dramatic differences in their redox and spectroscopic properties, providing insight into the influence of the cysteine-modified tyrosine cofactor in the enzyme galactose oxidase (GO). Using a modified Marcus-Hush analysis, the oxidized copper complexes are characterized as Class II mixed-valent due to the electronic differentiation between the two substituted phenolates. Sulfur K-edge X-ray absorption spectroscopy (XAS) assesses the degree of radical delocalization onto the single sulfur atom of nonsymmetric [1((t)Bu,SMe)](+) at 7%, consistent with other spectroscopic and electrochemical results that suggest preferential oxidation of the -SMe bearing phenolate. Estimates of the thermodynamic free-energy difference between the two localized states (ΔG(o)) and reorganizational energies (λ(R(1)R(2))) of [1(R(1),R(2))](+) and [2(R(1),R(2))](+) lead to accurate predictions of the spectroscopically observed IVCT transition energies. Application of the modified Marcus-Hush analysis to GO using parameters determined for [2(R(1),R(2))](+) predicts a ν(max) of ∼13600 cm(-1), well within the energy range of the broad Vis-NIR band displayed by the enzyme.
authors.affiliation: Department of Chemistry, Stanford University, Stanford, California 94305, USA.
authors.forename: Russell C
authors.initials: RC
authors.lastname: Pratt
authors.forename: Christopher T
authors.initials: CT
authors.lastname: Lyons
authors.forename: Erik C
authors.initials: EC
authors.lastname: Wasinger
authors.forename: T Daniel P
authors.initials: TD
authors.lastname: Stack
mesh_terms: D056831:Coordination Complexes / Q000737:chemistry; D003300:Copper / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy; D005029:Ethylenediamines / Q000737:chemistry*; D005692:Galactose Oxidase / Q000737:chemistry*; D010084:Oxidation-Reduction; D010636:Phenols / Q000737:chemistry*; D013053:Spectrophotometry; D013455:Sulfur / Q000737:chemistry; D013816:Thermodynamics; D056928:X-Ray Absorption Spectroscopy
chemical_list: D056831:Coordination Complexes; D005029:Ethylenediamines; D010636:Phenols; D013455:Sulfur; D003300:Copper; C011452:disalicylaldehyde ethylenediamine; D005692:Galactose Oxidase