THE EFFECT OF ELECTROSTATIC AND HYDROPHOBIC INTERACTIONS ON THE RATE OF CYTOCHROME-C REDUCTION BY RUTHENIUM(II) COMPLEXES

Stopped-flow spectrophotometry was used to study the reduction of cytochrome c by a series of Ru-II complexes including Ru-II(py-R(1-11))(x) (where L(3-) is the anion of N-2-hydroxyethylethylenediaminetriacetic acid; py-R is 4-R-substituted pyridine; x = 0, -1, -2) at 298 K under pseudo-first order conditions at pH 7 (phosphate buffer) and I = 0.1 M (NaCl). The variation in R(x) was as follows: R(1), H (-1); R2, CH2-OH (-1); R(3), (CH2)(2)-SO3- (-2); R(4), CH2-COO- (-2); R(5), CH2-NH3+ (O); R(6), (CH2)(2)-NH2+-cyclohexyl (O); R(7), n-pentyl (-1); R(8), n-tridecyl(-1); R(9), benzyl(-1); R(10), (CH2)(2)-NH-phenyl (-1); R(11) (CH2)(2)-NH-(4-n-hexylphenyl) (-1). Cyclovoltammetry and the MLCT band of RuL(py-R)(x) complexes prove that all of the Ru-II complexes have the same redox potential of 0.13 +/- 0.01 V (versus NHE). The electron transfer follows a second order rate law with rate constant It ranging from (30.5 +/- 1.2).10(5) M(-1) s(-1) (R = R(4), x = -2) to (1.9 +/- 0.1).10(5) M(-1)s(-1) (R = R(6), x = 0). For the reduction by Ru(II)L(py-R(1))(-), Delta H-double dagger = 9.2 +/- 1.2 kJ mol(-1) and Delta S-double dagger = -97 +/- 4 J mol(-1) K-1 are obtained. The study of the cross-reaction between Co(phen)(3)(3+) and Ru-II(py-R)(x) led to k(22), the rate constant for self-exchange in the Ru-II/Ru-III redox couple, which was found to be 440 (R(1)), 455 (R(3)), and 550 (R(5)) M(-1) s(-1) (298 K). The effect of the hydrophobic groups R(7)-R(11) is small and rate reducing for long chains (R(8) and R(11)). The change in the charge x from -1 to -2 is rate enhancing, the change from -1 to 0 is rate reducing. The electrostatic effects on the rate of electron transfer are compatible with the interpretation that the Run reductants dock at the exposed heme edge of cytochrome c and see there a local charge of +3.3 +/- 0.6.