Rate Constants and Free-Energy Plot for the Reduction of Horse-Heart Cytochromec(III) by Eight Organic Radicals

Abstract

Rate constants k12/M-1 s-1 (22 °C) for the reduction of the oxidised form of horse-heart cytochrome c by eight organic radicals (OR), e.g., methyl viologen MV•+, determined by pulse radiolysis and/or stopped-flow methods, correlate well in a plot of log k12 vs reduction potential E°1 for the OR. In the pulse radiolysis experiments the ORs were generated in situ by rapid reduction of the parent form by the formate radical CO2•-, and in the stopped-flow experiments prior reduction of the parent was with dithionite. In both procedures it was necessary to consider formation of double-reduced parent forms. Rate constants k12 are in the range 109 to 104 and E°1 values from −0.446 to +0.194V. The Marcus free-energy plot of log10 k12 − 0.5 log10 f vs E°1/0.059 gives a slope of 0.49, in excellent agreement with the theoretical value of 0.50, with an intercept at −E°1/0.059 = 0 of 6.55 in good agreement with a calculated value of 6.51 from known parameters. The latter includes the reduction potential E°2 (0.263V) and self-exchange rate constant k22 (0.36 × 103 M-1 s-1) for the cytochrome c(III)/(II) couple, and assumes a common self-exchange rate constant k11 of 1.0 × 106 M-1 s-1 for the ORs.