Effect of the Electrostatic Interaction on the Redox Reaction of Positively Charged CytochromecAdsorbed 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 (MEL) 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, Gamma(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.