Kinetic studies on 1 : 1 electron-transfer reactions involving blue copper proteins. Part 5. Reactions of parsley plastocyanin and Pseudomonas aeruginosa azurin with the negatively charged oxidants [(NC)5FeCNCo(CN)5]5– and [Fe(CN)6]3–

Abstract

The oxidations of plastocyanin, PCu(I), and azurin, ACu(I), with [(NC)₅FeCNCo(CN)₅]^5–(0.46 V) and [Fe(CN)₆]^3–(0.41 V) have been studied by the stopped-flow method at pH 7.0, I= 0.10 M (NaCl) and 0.22 M (phosphate), with the oxidant in large (>10-fold) excess (concentrations up to 4 × 10^–4 M). All four reactions conform to a rate law first order in oxidant and reductant concentrations, with no evidence for rate constants tending to a limiting value as a function of increasing concentration of oxidant. In the case of the ACu(I)+[Fe(CN)₆]^3– reaction (K < 100 M^–1) this is contrary to an earlier report. Enthalpies of activation, ΔH(kcal mol^–1), are however negative (–2.9 to –6.0) and support a two-stage process involving association (K) prior to electron transfer (k_et) and/or changes involving the degree of solvation of the protein. Replacement of NaCl (0.094 M) by LiCl and KCl produces no effect on rate constants for the PCu(I)+[Fe(CN)₆]^3– reaction. Re-examination of previous data for the [Fe(CN)₆]^4–+ PCu(II) reaction suggests that here also limiting kinetics are probably not effective. Other metalloprotein–complex reactions which display limiting kinetics are considered and rationalized in terms of electrostatics as the dominant factor. For the two reactions of [(NC)₅FeCNCo(CN)₅]^5–, similar effects of pH are observed to those previously reported with [Fe(CN)₆]³ as oxidant. Thus on decreasing the pH from 7 to <5 the redox reactivity of PCu(I) decrease sharply whereas rate constants for the ACu(I) reaction increase.