On the Electron Transfer Reaction between Ferricytochrome c and Ferrohexacyanide in the pH Range 5 to 7

Abstract

Abstract The electron transfer kinetics between horse heart ferricytochrome c and ferrohexacyanide in the pH range 5 to 7 is essentially the same as the corresponding kinetics for the pH range 7 to 10. The equilibrium constant, computed from the ratio of rate constants is slightly under 300 for the range investigated. Spectrophotometrically (5 to 20 min after mixing), a somewhat different result emerged. The value of the equilibrium constant (with ferrohexacyanide in the numerator) increased about 2 times for a pH decrease from 7.0 to 5.0. A small minimum is indicated at pH 6.5. There is some similarity between this equilibrium behavior and the one found for the range above pH 7. If one accepts the previously established apparent pKh of 6.0 for ferricytochrome c, one arrives at a protonic dissociation for ferrocytochrome c of about 5.4. However, both of them should be considered upper limits. Lower limits are 5.0 for ferricytochrome c and 4 (or less) for ferrocytochrome c. The enthalpy change of the fast electron transfer process is in the range of 14 Cal per mole and shows practically no pH dependence. The differences between the kinetic and the spectrophotometric equilibrium constants are probably due to the fact that slow structural rearrangements are coupled to the electron transfer. Such steps also may be the reason for the fact that the measured rate constants are practically independent of pH. Various control experiments were conducted, to establish the constancy of the spectral characteristics of reduced and oxidized cytochrome c between pH 5 and 7 and in the wavelength range 520 to 555 nm.