Effect of the electrostatic interaction on the redox reaction of positively charged cytochrome c adsorbed on the negatively charged surfaces of acid-terminated alkanethiol monolayers on a Au(111) electrode

The electrochemical properties of cytochrome c (cyt c) adsorbed on mixed self-assembled monolayers (SAMs) of 2-mercaptoethanesulfonate (MES)/2-mercaptoethanol (MED are compared with those on single-component SAMs of MES, MEL, and mercaptopropionic acid (MPA), using cyclic voltammetry and potential-modulated UV-vis reflectance spectroscopy. The rate constant of electron transfer (ET), k_et, of cyt c adsorbed on the SAM of MPA decreases from 1450 ± 210 s^-1 at pH 7 to 890 ± 100 s^-1 at pH 9. In contrast, the value of k_et of cyt c on the SAM of MES is pH-independent at 100 ± 15 s^-1. Those facts suggest that a large negative charge density on the SAM surface slows down the ET between cyt c and the electrode. The surface charge density of the SAM affects also the amount of electroactive cyt c, Γ_e, which decreases from 10.0 ±1.0 to 5.3 ± 1.1 pmol cm^-2 with increasing pH from 7 to 9 on the SAM of MPA. Similarly, the k_et of cyt c adsorbed on the mixed SAMs of MES/MEL sharply decreases from 900 ± 300 s^-1 to 110 s^-1 as the surface mole fraction of MES increases beyond 0.5, suggesting the presence of a negative surface charge threshold beyond which the rate of ET of cyt c is dramatically lowered. The decrease in the k_et on the SAMs at high negative charge densities probably results from the confinement of adsorbed cyt c by the strong electrostatic force to an orientation that is not optimal for the ET reaction.