Electrocatalytic four-electron reduction of oxygen at the cytochrome c3-adsorbed electrode

The electrocatalytic activity of cytochrome c3 for the reduction of molecular oxygen was characterized from the studies of the adsorption of cytochrome c3 and the co-adsorption of cytochrome cs with cytochrome c on the mercury electrode by the a.c. polarographic technique. The adsorption of cytochrome c3 on the mercury electrode is irreversible and is diffusion-controlled. The maximum amount of cytochrome c3 absorbed was 0.92 . 10(-11) mol . cm-2 at -0.90 V. The amount of cytochrome c3 in the mixed adsorbed layer with cytochrome c was determined from the differential capacitance measurement. It was shown that the fractional coverage of cytochrome c3 can be estimated from its bulk concentration and the diffusion coefficient (1.05 . 10(-6) cm2 . s-1). Cytochrome c3 catalyzes the electrochemical reduction of molecular oxygen from the two-electron pathways via hydrogen peroxide to the four-electron pathway at the mercury electrode in neutral phosphate buffer solution. The catalytic activity varies with the bulk concentration of cytochrome c3. The highest catalytic activity for the oxygen reduction (no hydrogen peroxide formation) is attained when one-half of the mercury electrode surface is covered by cytochrome c3. The addition of cytochrome c or bovine serum albumin to the cytochrome c3 solution inhibits the catalytic activity of cytochrome c3. The reversible polarographic behavior of cytochrome c3 through the mixed adsorbed layer of cytochrome c3 and cytochrome c was also investigated.