Kinetics of the oxidation of rhus vernicifera stellacyanin by the Co(EDTA)-ion

Abstract

A kinetic study of the oxidation of the copper(I) form of the blue copper protein stellacyanin (St(I)) by Co(EDTA)^- has been performed. Observed rate constants approach a saturation limit with increasing [Co(EDTA)^-] at pH 7, consistent with a mechanism involving rapid pre-equilibrium oxidant-protein complex formation followed by rate-limiting intramolecular Cu(I) to Co(III) electron transfer:

. Activation parameters based on k₂ (ΔH^≠ = 1.8 kcal/mol, ΔS^≠ = -56 cal/mol-deg) indicate that the electron transfer process is substantially nonadiabatic, in marked contrast with results obtained for Co(phen)₃³⁺ as the oxidant. Linear k_obsd vs. [Co(EDTA)^-] plots are reported for the Co(EDTA)^- oxidation of cuprous stellacyanin at pH 10 (k = 8.9 M^-1 sec^-1; 25.0°, pH 10, μ 0.5 M (carbonate); ΔH^≠ = 11.3 kcal/mol, ΔS^≠ = -16 cal/mol-deg) and at pH 7 in the presence of excess EDTA (k = 21.2 M^-1 sec^-1; 25.1°, pH 7.0, μ 0.5M (phosphate), [EDTA]_tot = 5 X 10^-4 M; ΔH^≠ = 5.9 kcal/mol, ΔS^≠ = -33 cal/mol-deg). It is concluded that Co(EDTA)^- adopts an electron transfer mechanism similar to that preferred by Co(phen)₃³⁺ under conditions where the oxidant is prevented from binding strongly to reduced stellacyanin.