Electrochemical and spectroscopic effects of mixed substituents in bis(phenolate)-copper(II) galactose oxidase model complexes.

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.