Molecular Dynamics and Electrochemical Investigations of a pH-Responsive Peptide Monolayer
The secondary structure of the synthetic HELELELELELC peptide sequence was studied by molecular dynamics simulations at two different pH values. At pH 2, the alpha helix conformation is stable over 200 ns, but at pH 7, a random coil is irreversibly obtained after 70 ns, due to the electrostatic repulsions between the negatively charged carboxylate groups of the glutamic acid residues. The sensitivity of the structure to the pH was used to fabricate pH-responsive monolayers of peptide by self-assembly. Electrochemical measurements were performed to assess the properties of the monolayer as a function of the pH, using redox couples either dissolved in solution or attached to the peptide by coordination at the amino-terminal histidine side chain. In this latter case, no discernible electron transfer to/from the redox center took place at pH 6.9 because of the extended conformation adopted by the peptide. By contrast, the rate of electron transfer became increasingly faster upon decrease of the pH, with rate constants of 78, 110, and 230 s(-1) determined at pH 3.0, 2.0, and 1.0.