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 c P), of temperature (in the range of 260.7-307.7 K), and of ionic stren gth (in the range of 2.5-20.0 mM) on the kinetics of photoinduced elec tron-transfer reaction (3)Zncyt/pc(II) --> Zncyt(+)/pc(I) within the e lectrostatic complex of zinc cytochrome c and cupriplastocyanin at pH 7.0. The unimolecular rate constant is k(F). The apparent activation p arameters 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) val ues obtained at the constant composition into the dependence of k(F) o n temperature at constant viscosity gave the proper activation paramet ers, which agree with those obtained in experiments with solutions hav ing a constant viscosity. This agreement validates the latter method, which is more efficient than the former, for determining activation pa rameters of processes that are modulated by viscosity. The smooth chan ge in k(F) is governed by the change in viscosity, not in other proper ties of the solvent, and it does not depend on the choice of the visco sigen. 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 transfe r and with electron transfer that is coupled to, or gated by, a preced ing 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 vi scosity. Kinetic effects and noneffects of ionic strength, viscosity, and thermodynamic driving force indicate, but do not prove, that the r eaction under consideration is gated. The kinetic effect of viscosity is analyzed in terms of two models. Because ln k(F) is a nonlinear fun ction of ln eta, protein friction has to be considered in the analysis of viscosity effects on kinetics.