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 RuII complexes including RuIIL(py-R1-11)x (where L3- 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: R1, H (-1); R2, CH2-OH (-1); R3, (CH2)2-SO3- (-2); R4, CH2-COO- (-2); R5, CH2-NH3+ (0); R6, (CH2)2-NH(2+)-cyclohexyl (0); R7, n-pentyl (-1); R8, n-tridecyl (-1); R9, benzyl (-1); R10, (CH2)2-NH-phenyl (-1); R11, (CH2)2-NH-(4-n-hexylphenyl) (-1). Cyclovoltammetry and the MLCT band of RuL(py-R)x complexes prove that all of the RuII 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 k ranging from (30.5 +/- 1.2).10(5) M-1 s-1 (R = R4, x = -2) to (1.9 +/- 0.1).10(5) M-1s-1 (R = R6, x = 0). For the reduction by RuIIL (py-R1)-, delta H = 9.2 +/- 1.2 kJ mol-1 and delta S = -97 +/- 4 J mol-1 K-1 are obtained. The study of the cross-reaction between Co(phen)3(3+) and RuIIL(py-R)x led to k22, the rate constant for self-exchange in the RuII/RuIII redox couple, which was found to be 440 (R1), 455 (R3), and 550 (R5) M-1 s-1 (298 K). The effect of the hydrophobic groups R7-R11 is small and rate reducing for long chains (R8 and R11). 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 RuII reductants dock at the exposed heme edge of cytochrome c and see there a local charge of +3.3 +/- 0.6.