Relationship of two electroactive forms of horse heart cytochrome c at gold and glassy carbon electrodes in water and methanol

Electrochemical reduction of horse heart ferricytochrome c has been examined in detail at gold and glassy carbon electrodes in water and methanol solutions. Two well-defined reduction pathways are found to exist which are mutually dependent on the electrode material, pH, solvent and the presence of promoters. The observation of two processes in co-existence means that an extension of the reduction mechanism is required to that reported in the literature. The protein exists in different conformational states in aqueous bulk solution in the pH range 7 to 10. It is proposed that under conditions where cytochrome c is specifically adsorbed onto an electrode, the thermodynamically reversible pathway for the biologically active form is blocked. However, under these circumstances, cytochrome c is not electroinactive. Rather, the reduction of an adsorbed conformer of cytochrome c gives rise to a well-defined but irreversible reduction process at -0.42 V vs. SCE under conditions of cyclic voltammetry. In the presence of 4,4′-bipyridyl or iodide, the biologically active form is detected near its thermodynamic potential of 0.01 V vs. SCE at a gold electrode. Adjustment of the proportion of protein and promoter permits both responses to be detected in the same solution and their ratio is pH-dependent. The behaviour in methanol at a gold electrode is similar to that in water solution at pH 10. At glassy carbon electrodes, both responses are seen in the absence of added promoter. It is concluded that promoters, which are themselves adsorbed, facilitate electron transfer to the biologically active form by inhibiting adsorption of cytochrome c; that is, an alternative electron transfer pathway is made available. The new data rationalize different reports on electrochemistry of cytochrome c.