Tripeptide Self-Assembled Monolayers as Biocompatible Surfaces for Cytochrome Electrochemistry.

Biocompatible tripeptide self-assembled monolayers (SAMs) are designed with a carboxylate group on the terminal amino acid (glutamate, aspartate, or amino adipate) to electrostatically attract the lysine groups around the heme crevice in horse heart cytochrome (cyt ), creating an electroactive protein/tripeptide/Au interfacial structure. Exposing the peptide/Au electrode to cyt resulted in an 11 ± 3 pmol/cm electroactive protein surface coverage. Topographical images of the interfacial structure are obtained down to single-protein resolution by atomic force microscopy. Uniform protein monolayer assemblies are formed on the Au electrode with no major surface roughness changes. The cyt /peptide/Au electrode systems were examined electrochemically to probe surface charge effects on the redox thermodynamics and kinetics of cyt . Neutralization of protein surface charge due to adsorption on anionic COOH-terminated SAMs was found to change the formal potential, as determined by cyclic voltammetry. The cyt /peptide/Au electrodes exhibit formal potentials shifted to more positive values, have a surface carboxylic acid p of 6 or higher, and produce effective cyt surface charges () of -6 to -14. The Marcus theory is utilized to determine the protein electron transfer rates, which are ∼5 times faster for cyt /tripeptide/Au compared to cyt /11-mercaptoundecanoic acid SAMs of similar chain lengths.