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.

Figure 1

Differential pulse voltammograms of copper complexes 1^R1,R2 (solid lines) and 2^R1,R2 (dotted lines). Conditions: CH₂Cl₂ with 0.1 M Bu₄NClO₄, 1 mM Cu. mV vs Fc^∘/+

Figure 2

UV-Vis-NIR spectra of 1^R1,R2 and 2^R1,R2 (dashed), [1^R1,R2]⁺ and [2^R1,R2]⁺ (dotted), and [1^R1,R2]²⁺ and [2^R1,R2]²⁺ (solid) in CH₂Cl₂

Figure 3

(a) UV-Vis-NIR spectra of [1^OMe2]⁺ (dashed line), [1^SiPr2]⁺ (dotted line), and [1^OMe,SiPr]⁺ (solid line). (b) UV-Vis-NIR spectra of [1^OMe2]²⁺ (dashed line), [1^SiPr2]²⁺ (dotted line), and [1^OMe,SiPr]²⁺ (solid line)

Figure 4

Comparison of NIR absorptions for symmetric and non-symmetric [1^R1,R2]⁺ and [2^R1,R2]⁺.

Figure 5

Normalized sulfur K-edge XAS spectra of (dotted) 1^tBu,SMe and (solid) [1^tBu,SMe]⁺.

Figure 6

Plots of the electrochemical splitting (ΔE) versus 1/λ for 1^R1,R2 (squares) and 2^R1,R2 (circles), shown with linear fits.

Figure 7

(a) Scheme of Marcus-Hush coupling in symmetric mixed-valence complexes. (b) Scheme of Marcus-Hush coupling in non-symmetric mixed-valence complexes. The energy of the IVCT absorption corresponds to λ + ΔG^∘

Scheme 1

Active site and consensus mechanism of GO

Scheme 2

Structure of 1^SR2

Scheme 3

Synthetic scheme for Cu^II-phenolate complexes *duplicate