Protein Electrochemistry: Metal Complexes as a Promoters of the Electron Transfer from a Gold Electrode to Cytochrome c


Direct protein electrochemistry is an important method to determine the redox potential and electron transfer kinetics of metalloproteins. The bare metal electrode generally causes irreversible adsorption and denaturation of the protein on the electrode surface. There have been extensive studies on modification of the gold electrode using thiol functionalized promoters. We have investigated the effect of metal ion binding to a thiol-based Schiff base complex immobilized on the gold electrode for the electron transfer to cytochrome c. Selfassembled Monolayer (SAM) contains Schiff-Base ligand formed by cysteine and salicylaldehyde on the gold electrode was shown to promote quasi-reversible electrochemistry of cytochrome c. Various transition metal complexes of the Schiff-base ligand immobilized on the gold electrode were also found to promote electrochemistry of cytochrome c. The redox potential of the protein was determined to be 0.05±0.02 V (vs Ag/AgCl), which agrees with the reported value. In heterogeneous electron transfer rates for were determined by analyses of the cyclic voltammetric data using Laviron’s equation. The heterogenous electron rate constant (ks) calculated by Laviron’s equation suggest that there is small increase in the electron transfer rates when metal ion (Cr, Fe, Mn) is coordinated to the Schiff base on the electrode surface, suggesting that metal ion coordination possibly stabilizes the SAM of the Schiff base and favours electron transfer to cytochrome c.