We report the results of an EPR potentiometric titration of cytochrome c 3 from Desulfovibrio vulgaris Miyazaki: the EPR spectral features of the four hemes are identified. The four midpoint redox potentials, which are deduced from the integrated intensity variations as a function of the redox potential, are within the range −230 to −360 mV with two nearly equal intermediate values, in agreement with previous electrochemical measurements. A structural change of the environment of the heme with the most negative potential is observed during the first step of the reduction. The correspondence between the redox sites as characterized by the EPR potentiometric titration, and the hemes in the tridimensional structure, is discussed.
