Effects of viscosity and temperature on the kinetics of the electron-transfer reaction between the triplet state of zinc cytochrome c and cupriplastocyanin

This is a study of the effects of viscosity (in the range of 0.8-790 cP), of temperature (in the range of 260.7-307.7 K), and of ionic strength (in the range of 2.5-20.0 mM) on the kinetics of photoinduced electron-transfer reaction (3)Zncyt/pc(II) --> Zncyt(+)/pc(I) within the electrostatic complex of zinc cytochrome c and cupriplastocyanin at pH 7.0. The unimolecular rate constant is k(F). The apparent activation parameters Delta H-not equal, Delta S-not equal, and Delta G(not equal) for this reaction were obtained in experiments with aqueous glycerol solutions having a constant composition. The interpolation of k(F) values obtained at the constant composition into the dependence of k(F) on temperature at constant viscosity gave the proper activation parameters, which agree with those obtained in experiments with solutions having a constant viscosity. This agreement validates the latter method, which is more efficient than the former, for determining activation parameters of processes that are modulated by viscosity. The smooth change in k(F) is governed by the change in viscosity, not in other properties of the solvent, and it does not depend on the choice of the viscosigen. Donor/acceptor electronic coupling (H-AB) and reorganizational energy (lambda), obtained by fitting of the temperature dependence of k(F) to the Marcus equation, are consistent with true electron transfer and with electron transfer that is coupled to, or gated by, a preceding structural rearrangement of the diprotein complex (3)Zncyt/pc(II). The fact that at very high viscosity k(F) approaches zero shows that the reaction is probably gated throughout the investigated range of viscosity. Kinetic effects and noneffects of ionic strength, viscosity, and thermodynamic driving force indicate, but do not prove, that the reaction under consideration is gated. The kinetic effect of viscosity is analyzed in terms of two models. Because ln k(F) is a nonlinear function of ln eta, protein friction has to be considered in the analysis of viscosity effects on kinetics.