Mechanism of the reduction and oxidation reaction of cytochrome c at a modified gold electrode

The reduction and oxidation of cytochrome c at a gold electrode modified with an adsorbed layer of 4,4'-bipyridyl has been investigated by using rotating-disk electrodes. The current voltage curves for both the oxidation and reduction reactions show that the system is nearly reversible, but the rotation speed dependences of the limiting currents in either direction indicate that there are additional potential independent rate-limiting processes before and after the electron transfer. From the dependence of the limiting currents on the concentraiton of reactant and product, we deduce that there is considerable adsorption of both reactant and product. This adsorption step appears to be essential for rapid electron transfer between the electrode and the protein and the adsorption and desorption rates are rapid, as expected from the near reversibility of the overall electrode process. The adsorption of both reactant and product was also measured by using a ring-disk electrode with modulation of disk current. From these results a free-energy profile for the overall electrode reaction is deduced. This free-energy profile is symmertrical and the three transition states are of about equal energy at the standard electrode potential of cytochrome c. The relationship between the binding of cytochrome c to the modified electrode and its interaction with its physiological redox partners is discussed.