Direct electrochemical responses of cytochrome c on a gold electrode modified by thiol-containingmonolayer of Schiff base (SB) or its metal complexes (M-SB, M = Fe, Mn, Cr) have been investigated todetermine the role of the monolayer in promoting heterogeneous electron transfer. The monolayer of the SBprepared in situ by the sequential association of L-cysteine followed by conjugation with salicylaldehyde on afreshly cleaned gold electrode formed a stable thin film of cytochrome c. The thin films of cytochrome c on SB orM-SB modified electrode showed quasi-reversible cyclic voltammetric signals, and the observed midpointpotential agreed with that reported earlier. The surface coverage (Г) of the active cytochrome c in the thin filmwas found to vary with the nature of the metal ion in M-SB, and the value of Г increased in the order: SB < Mn-SB< Cr-SB < Fe-SB in the monolayer, suggesting that the metal ion coordination may be important for thestability of the monolayer of M-SB and the formation of the thin film of the protein. The electron transfer rates(ks) were found to be faster with the SB or M-SB monolayers compared to many other small promoters reportedearlier. The ks values were however almost independent of the metal binding to the SB, indicating that theelectron transfer across the monolayer of the SB complex may not be the rate determining step for the hetero-geneous electron transfer from the gold electrode to cytochrome c.
The monolayers of Schiff base (SB) and its metal complexes self-assembled through thiol linkages to the gold electrode was shown to efficiently promote direct electrochemistry of cytochrome c immobilized as thin films on the promoter at the electrode surface.