Ferricytochrome c oxidation of cobaltocytochrome c. Comparison of experiments with electron-transfer theories

Electron transfer from cobaltocytochrome c to ferricytochrome c has been studied by stopped-flow kinetics. The second-order rate constant at pH 7.0, 0.1 ionic strenght, 0.2 M phosphate, and 25 degrees C is 8.3 x 103 M-1 s-1. The activation parameters obtained from measurements made between 20 and 50 degrees C are deltaHnot equal to = 2.3 kcal mol-1 and deltaSnot equal to = -33 eu. The rate constant is not significantly dependent on ionic strength; it is also relatively independent of pH between the pK values for conformation transitions. The rate diminishes at pH greater than 12. The self-exchange reaction of cobalt cytochrome c was investigated with pulsed Fourier transform 1H NMR. The rate is too slow on the 1H NMR scale; it is estimated to be less than 133 M-1 s-1. These results together with the self-exchange rates of iron cytochrome c [Gupta, R.K., Koenig, S. H., and Redfield, A. G. (1972), J. Magn. Reson. 7, 66] were analyzed by theories of Jortner and Hopfield. The theories predict the self-exchange of Cocyt c to be too slow for 1H NMR determination. The rate constant calculated by the nonadiabatic multiphonon electron-tunneling theory for the Fecyt c-Fecyt c+ and Cocyt c-Fecyt c+ electron transfers are in good agreement with experiments.