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), ket, 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 ket 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, Γe, 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 ket 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 ket 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.
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