Dipole Moment Effect on the Electrochemical Desorption of Self-Assembled Monolayers of 310-Helicogenic Peptides on Gold

A series of 12 thiolated oligopeptides of α-aminoisobutyric acid (Aib) were used to make self-assembled monolayers (SAMs) on carefully annealed gold plates. These peptides, which form 310-helices and have a strong dipole moment along the main peptide axis, were devised to give rise from zero to seven C=O⋅⋅⋅H−N intramolecular hydrogen bonds. The orientation of the natural dipole moment of the peptide was controlled by linking the thiolated moiety to either the nitrogen or carbon terminus. To assess the interactions between the chains forming the monolayer and determine the effect of the peptide orientation, the SAMs were characterized by polarization-modulation infrared-reflection absorption spectroscopy, in comparison with the FTIR behavior of the free peptides in solution. The stability of the peptide SAMs toward electroreduction was studied by cyclic voltammetry in 0.1 m NaOH. We show that electrodesorption is significantly affected by both peptide length and orientation in a way consistent with a strong effect exerted by the dipole moment of the peptide.