Adsorption of the cysteine–tryptophan dipeptide at the Au(110)/liquid interface studied using reflection anisotropy spectroscopy

The adsorption of a cysteine–tryptophan dipeptide has been monitored at a Au(110)/electrolyte interface using reflection anisotropy spectroscopy. At −0.6 V the dipeptide adsorbed through the formation of Au–S bonds and a link between the NH2 group at the Au surface. As the applied potential was changed to + 0.6 V the NH2 group left the surface and the spectra suggest that two dipeptides form a disulphide bridge that binds to the Au surface. This event is accompanied by changes in the orientation of, and possibly interactions between, tryptophan moieties. Returning the applied potential to −0.6 V failed to re-establish the initial population of Au–S bonds and the changes induced in the region of the spectrum associated with the tryptophanʹs by the positive potentials were permanent on the time scale of an hour. Subtle changes associated with the tryptophan moieties indicate that the orientation of the tryptophans is very sensitive to the applied potential. Cycling of the potential showed a stable inter-conversion between Au–S bonds and Au–disulphide bonds at low scan rates, but at higher scan rates not all the broken disulphide bonds were able to reform. Irreversible changes were also observed in the tryptophan region of the spectrum during potential cycling.