SURFACE-ENHANCED RAMAN-SPECTROSCOPY OF PEPTIDES - PREFERENTIAL N-TERMINAL ADSORPTION ON COLLOIDAL SILVER

Surface-enhanced Raman spectroscopy (SERS) has been used to probe the surface interactions of dipeptides, tripeptides, and enkephalins adsorbed from aqueous solution onto colloidal silver. The SER spectra of homodipeptides of Gly, Phe, Tyr, and Trp differ considerably from the spectra of the corresponding amino acids. The zwitterionic dipeptides present in solution deprotonate upon adsorption and interact with the silver surface through the amine and not the carboxylate group. Tyrosyl residues may adsorb as tyrosinate. The homodipeptide SER spectra and vibrational assignments provide the basis for interpreting the spectra of heterodipeptides containing Gly, Leu, Phe, Tyr, and Trp. With the exception of Gly-Tyr, in all of the heterodipeptides we have examined, the N-terminal residue is adsorbed on the silver surface and only very small contributions from the C-terminal residue are observed in the SER spectra. The spectra of Gly-Tyr and Gly-Tyr-Gly provide evidence for the strong affinity of Tyr side chains for the surface. In Leu- and Met-enkephalin, the N-terminal tyrosyl and fourth residue phenylalanyl groups interact most strongly with the surface, while in Des-Tyr-Leu-enkephalin the N-terminal glycyl and third residue phenylalanyl groups adsorb. These results indicate that interactions of amine groups and aromatic side chains with the silver surface are favorable, and may dictate the orientation and conformation of adsorbed peptides. The relative importance of specific functional groups in controlling peptide adsorption is established and provides the basis for predicting the orientation and conformation of biomolecules adsorbed on metal surfaces.