Oxidation of cuprous stellacyanin by aminopolycar☐ylatocobaltate(III) complexes

Rate parameters are reported for the oxidation of cuprous stellacyanin by Co(PDTA)⁻ (k(25.0°) = 17.9 M⁻¹sec⁻¹, Δ H^≠ = 8.5 kcal/mol. ΔS^≠ = −24 cal/mol-deg; pH 7.0, μ 0.5 M) and Co(CyDTA)⁻ (k(25.1°) = 17.0 M⁻¹ sec⁻¹, ΔH^≠ = 8.7 kcal/mol, ΔS^≠ = −24 cal/mol-deg; pH 7.0, μ 0.5 M). The first order Co(PDTA)⁻ and Co(CyDTA)⁻ dependences observed over wide concentration ranges contrast with the saturation behavior reported previously for Co(EDTA)⁻ as the oxidant. It is concluded that the -CH₃ and -(CH₂)₄-substituents of PDTA and CyDTA, respectively, prevent the alkylated derivatives of Co(EDTA)⁻ from hydrogen bonding with the reduced blue protein, causing precursor complex formation constants to fall far below that of 149 M⁻¹ (25.1°) observed for the EDTA complex. The similarity between ΔH^≠ and ΔS^≠ values for the oxidation of stellacyanin by Co(PDTA)⁻ and Co(CyDTA)⁻ indicates that the size of alkyl substituents linked to the carbon atoms of the EDTA ethylenediamine backbone has little influence on activation requirements for Cu(I) to Co(III) electron transfer. The electron transfer reactivity of aminopolycar☐ylatocobalt(III) complexes with cuprous stellacyanin therefore appears to be linked to the accessibility of one or more of the ligated acetate groups to outer-sphere contact with the type 1 Cu(I) center. Saturation in k_obsd vs. [oxidant] plots found for the reactions of Co(PDTA)⁻ and Co(CyDTA)⁻ with stellacyanin at pH 6 and at pH 7 in the presence of EDTA is attributed to the formation of “dead-end” oxidant-protein complexes.