Rate constants and free-energy plot for the reduction of horse-heart cytochrome c(III) by eight organic radicals

Rate constants k(12)/M-1 s(-1) (22 degrees C) for the reduction of the oxid ised form of horse-heart cytochrome c by eight organic radicals (OR), e.g., methyl viologen MV.+, determined by pulse radiolysis and/or stopped-flow m ethods, correlate well in a plot of log kit vs reduction potential E degree s(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 wit h dithionite, In both procedures it was necessary to consider formation of double-reduced parent forms. Rate constants k(12) are in the range 10(9) to 10(4) and E degrees(1) values from -0.446 to +0.194V. The Marcus free-ener gy plot of log(10) k(12) - 0.5 log(10) f vs E degrees(1)/0.059 gives a slop e of 0.49, in excellent agreement with the theoretical value of 0.50, with an intercept at -E degrees(1)/0.059 = 0 of 6.55 in good agreement with a ca lculated value of 6.51 from known parameters. The latter includes the reduc tion potential E degrees(2) (0.263V) and self-exchange rate constant k(22) (0.36 x 10(3) M-1 s(-1)) for the cytochrome c(III)/(II) couple, and assumes a common self-exchange rate constant k(11) of 1.0 x 10(6) M-1 s(-1) for th e ORs.