| 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: Co(EDTA)-- + St(i Qp in equilibrium Co(EDTA)-- ---St(I) Co(EDTA)-- ---St(I) k2 leads to Co(EDTA)2-- ---St(II) (Qp = 149 M--1, k2 = 0.169 sec--1; 25.1 degrees, pH 7.0 mu 0.5 M (phosphate)). Activation parameters based on k2 (deltaH not equal to = 1.8 kcal/mol, deltaS not equal to = --56 cal/mol-deg) indicate that the electron transfer process is substantially nondiabatic, in marked contrast with results obtained for Co(phen) 3 3+ as the oxidant. Linear kobsd 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, mu 0.5 M (carbonate); DELTaH not equal to 11.3 kcal/mol, deltaS not equal to = -16 cal/mol-deg) and at pH 7 in the presence of excess EDTA (k = 21.2 M--1 sec--1; 25.1 degree, pH 7.0, mu 0.5 M (phosphate), EDTA] tot = 5 X 10(--4) M; deltaH not equal to = 5.9 kcal/mol, delta S not equal to = --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. |
