Electrochemical and Spectroscopic Effects of Mixed Substituents in Bis(phenolate)–Copper(II) Galactose Oxidase Model Complexes

• Published in: Journal of the American Chemical Society 2012-05, Vol.134 (17), p.7367-7377
• Main Authors: Pratt, Russell C, Lyons, Christopher T, Wasinger, Erik C, Stack, T. Daniel P
• Format: Article
• Language: English
• Subjects: Coordination Complexes - chemistry; Copper - chemistry; Electron Spin Resonance Spectroscopy; Ethylenediamines - chemistry; Galactose Oxidase - chemistry; Oxidation-Reduction; Phenols - chemistry; Spectrophotometry; Sulfur - chemistry; Thermodynamics; X-Ray Absorption Spectroscopy
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja211247f

Nonsymmetric substitution of salen (1 R1,R2 ) and reduced salen (2 R1,R2 ) CuII–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 (λR1R2 ) of [1 R1,R2 ]+ and [2 R1,R2 ]+ 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 R1,R2 ]+ predicts a νmax of ∼13600 cm–1, well within the energy range of the broad Vis–NIR band displayed by the enzyme.