| Abstract: | The electron transfer reactions of horse heart cytochrome c with a series of amino acid-pentacyanoferrate(II) complexes have been studied by the stopped-flow technique, at 25°C, μ = 0.100, pH 7 (phosphate buffer). A second-order behavior was observed in the case of the Fe(CN)5 (histidine)3? complex, with k = 2.8 x 105 M?1 sec?1. For the Fe(CN)5 (alanine)4? and Fe(CN)5(L-glutamate)5? complexes, only a minor deviation of the second-order behavior, close to the experimental error (k = 3.2 × 105 and 1.6 x 105 M?1 sec?1, respectively) was noted at high concentrations of the reactants (e.g., 6 × 10?4 M). The results are in accord with recent work on the Fe(CN)64?/cytochrome c system demonstrating weak association of the reactants. The calculated self-exchange rate constants including electrostatic interactions for the imidazole,L -histidine, 4-aminopyridine, glycinate, β-alaninate, andL-glutamate pentacyanoferrate(II) complexes were 3.3 × 105, 3.3 × 105, 2.8 × 106,4.1 × 102,5.5 × 102, and 6.0 M?1 sec?1, respectively. Marcus theory calculations for the cytochrome c reactions were interpreted in terms of two nonequivalent binding sites for the complexes, with the metalloprotein self-exchange rate constants varying from 104 M?1 sec?1 (histidine, imidazole, and 4-aminopyridine complexes) to 106 M?1 sec ?1 (glycinate, β-alaninate, and L-glutamate complexes). |
