Redox reactions of cytochrome c on self-assembled monolayers of mercaptobenzoic acid and mercaptohydrocinnamic acid adsorbed on Au (111) electrodes

The redox reactions of cytochrome c (cyt c) adsorbed on self-assembled monolayers (SAMs) of 4-mercaptobenzoic acid (4-MBA) and 4-mercaptohydrocinnamic acid (4-MHCA) are studied using cyclic voltammetry and ER spectroscopy. A monolayer of cyt c in a stable, electroactive state is formed on close-packed monolayers of both aromatic thiols. The redox potentials of cyt c are 35 mV for 4-MBA and 20 mV vs. Ag/AgCl for 4-MHCA. The electron transfer (ET) rate constants, k_et, of cyt c at pH 7 are determined from the frequency dependence of the ER response to be 1800 s^-1 for 4-MBA system and 3900 s^-1 for 4-MHCA. The k_et of adsorbed cyt c tends to be larger on the SAMs of both aromatic thiols than on the SAM of 6-mercaptohexanoic acid having a similar thickness, suggesting that the difference in the SAM structure is reflected to the k_et of cyt c.