The oxidation of ferrocytochrome c by Br −2, (SCN) −2, N 3 and OH radicals studied by pulsed-electron and γ-ray radiolysis

Abstract

The reactions of ferrocytochrome c with Br − 2, (SCN) − 2, N 3 and OH radicals were followed by measuring the change in the optical spectra of cytochrome c on γ-irradiation as well as the rate of change of absorbance upon pulse irradiation. Ferrocytochrome c is oxidized to ferricytochrome c by Br − 2, (SCN) − 2 or N 3 radical with an efficiency of about 100% through a second-order process in which no intermediates were observed. The rate constants in neutral solutions at I = 0.073 are 9.7 · 10 8 M −1 · s −1, 7.9 · 10 8 M −1 · s −1, 1.3 · 10 9 M −1 · s −1 for the oxidation by Br − 2, (SCN) − 2 and N 3 radicals, respectively. The rate constants do not vary appreciably in alkaline solutions (pH 8.9). The ionic strength dependence was observed for the rate constants of the oxidation by Br − 2 and (SCN) − 2. Those rate constants estimated on the assumption that the radicals react only with the amino acid residues with the characteristic steric correction factors were less than one-tenth of the observed ones. These results suggest that the partially exposed region of the heme is the probable site of electron transfer from ferrocytochrome c to the radical. Hydroxyl radicals also oxidize ferrocytochrome c with a high rate constant ( k > 1 · 10 10 M −1 · s −1), but with a very small efficiency (5%).