Oxidation - Reduction Potentials of the Cytochrome

There have been many investigations of the oxidation-reduction potential of the cytochrome c system. Some of the results are concordant, though none at the time this work was undertaken included a complete potentiometric titration of the system over a range of pH values. Potentials have been reported for the system at varying pH values, usually within a narrow range bordering on the physiological. These results are not concordant, the most divergent being those reported by Green (10). The results of previous workers are summarized in Table I. We report here complete potentiometric titrations, both oxidative and reductive, of the cytochrome c system at different pH and E’,, values for the system throughout the pH range, 0.4 to 10.0. Since undertaking this work, a report of Paul (16) has appeared which covers the range of pH from 4 to 8.0 but with results and conclusions somewhat different from ours.