Pentacyanoiron(III) pyridine derivatives as topological probes for electron transfer cytochrome C

The kinetics of oxidation of ferrocytochrome C by a series of substitute pentacyano(pyridine) iron(III) complexes is studied in an aqueous solution at 23°C, pH 7.2, μ = 0.10 mol/l (KCl). Second-order rate constants for the complexes of pyridine, 4-methylpyridine, isonicotinamide, 4-phenylpyridine, and 4-tert-butylpyridine are, respectively, (6.0 ± 0.7) × 10⁷, (1.7 ± 0.3) × 10⁷, (5.7 ± 1.5) × 10⁷, (8.2 ± 0.8) × 10⁷, and (2.8 ± 0.3) × 10⁷ l mol^-1 s^-1. For the same series of oxidants, the specific rates of the reaction with the aqueous ferrocyanide ion are, respectively, (1.7 ± 0.1) × 10⁵, (5.5 ± 0.2) × 10⁴, (2.8 ± 0.1) × 10⁵, (5.5 ± 0.1) × 10⁴, and (1.9 ± 0.1) × 10⁴ l mol^-1 s^-1. The results a interpreted in terms of the steric and hydrophobic influences of the 4-pyridine substituents on the stability of the precursor complexes to electron transfer.