TEMPERATURE AND VISCOSITY DEPENDENCE OF THE ELECTRON-TRANSFER REACTION BETWEEN PLASTOCYANIN AND CYTOCHROME-C LABELED WITH A RUTHENIUM(II) BIPYRIDINE COMPLEX

The temperature and viscosity dependence of the photo-induced electron -transfer reaction between plastocyanin and cytochrome c labeled at Ly s13 with Ru(4,4'-dicarboxybipyridine)(bipyridine)(2)(+2) have been inv estigated. In these studies, a short pulse of 450 nm light was used to excite the ruthenium complex which was oxidatively quenched by the ir on center of cytochrome c. The resulting Fe(II) cytochrome c was then rapidly reoxidized by plastocyanin. The reactions were investigated ov er a temperature range of 3.5 to 37 degrees C under low ionic strength conditions such that protein/protein complex formation was favored. T he enthalpy of activation was 7 kcal mol(-1) and the entropy of activa tion was -20 cal mol(-1) K-1. Increasing the viscosity by the addition of sucrose up to 70% resulted in a 4-fold decrease in the rate consta nt for electron transfer. The overall results suggest a rate-limiting step that involves either dissociation of the dominant protein/protein complex or surface diffusion of the associated proteins.