On the elucidation of the pH dependence of the oxidation-reduction potential of cytochrome c at alkaline pH.

Kinetic measurements in the pH range 7.0 to 9.4 were made by the temperature jump technique, and comparative values of Keg were determined by a spectrophotometric titration method based on absorption difference at 550 rnp between ferriand ferrocytochrome C. Also, stopped flow experiments at 550 rnp were performed at pH 9.5. In all of these experiments, excess sodium ferrocyanide (8 X 10u3 M) was mixed with varying concentrations (5 X 10d6 to 1.6 X 10w4~) of ferricytochrome c at a constant ionic strength of 0.2. The quotient k(ox)k/(red) is essentially pa-independent in the pH region investigated; k(red) was found to be 2.4 X lo4 M-’ set+, and k(ox), 0.87 X 10’ M-’ se@. In the pH region investigated, the over-all equilibrium constant for the reaction, Keg, increased by a factor of 4 with increasing pH. The stopped flow experiments indicated that a slow process with a half-time of several seconds is responsible for the difference between k(ox)/k(red) and K,, values at alkaline pH values. The experiments indicate that pH has no effect on the electron transfer process itself. The pH dependence of the oxidation-reduction potential of cytochrome c at alkaline pH is evidently due to the effect of pH on a slow isomerization of ferricytochrome c, with increasing pH decreasing the concentration of cytochrome c which participates in the electron transfer process.