Adsorption of oligopeptides on silver nanoparticles: surface-enhanced Raman scattering and theoretical studies

Abstract

The MRKDV peptide structurally associated with an immunomodulatory protein, as well as model peptides ADEDRDA and LGRGISL with common amino acid residues were studied using surface enhanced Raman scattering (SERS) supported by quantum chemical computations. Peptides display different net charges and hydrophobic characteristics, which are related to particular structural aspects of the peptide–metal interaction. Samples were photostable when probed with laser lines at 514, 633 and 785 nm. SERS samples were prepared by coating the solid peptides with metal colloids on a quartz slice. This innovation makes possible to obtain high spectral batch to batch reproducibility. MRKDV SERS spectrum is dominated by signals coming from the guanidiniummoiety of the arginine amino acid (R); guanidinium is the intrinsic probe which drives the orientation of the peptide on the metal surface. LGRGISL interacts with the metal surface through the guanidinium group and other amino acid residues; a single structural conformation of the peptide on the surface is proposed. ADEDRDA interacts with themetal surface through various amino acid residues, also including the guanidinium moiety; at least two different structural conformations seem to coexist on the surface. Theoretical calculations performed by using extended H¨ uckel theory and 6-31G∗ methods for amodel of arginine interacting with a silver cluster support the observed experimental result. Similar calculations involving theMRKDV peptide are also reported.