Anchoring Sites of Fibrillogenic Peptide Hormone Somatostatin-14 on Plasmonic Nanoparticles

Despite numerous investigations devoted to the control of peptide and protein self-assemblies using nanostructured materials, the molecular details of the peptide-material interaction sites remain still under debate. Here, we suggest a solution by using jointly the middle- and low wavenumber regions of surface-enhanced Raman spectra. To achieve our goal, adequately prepared gold and silver colloids, of which the nanofabrication was controlled by means of zeta potential and extinction spectra, were used to enhance the Raman signal arising from a natural fibrillogenic peptide hormone, somatostatin-14, at very low (10(-6)-to-10(-8) M) concentrations. Transmission electron microscopy has revealed that the interacting partners are involved in a mutual aggregation process. In fact, while the presence of plasmonic colloids facilitates the aggregation of the peptide, leading to the formation of the so-called corona around nanoparticles, we could observe the appearance of relatively large size peptide-nanoparticle agglomerates in solution. However, the comparison between the data obtained from gold and silver nanoparticles is consistent with the fact that the nature of metal surface, as well as its ionic coverage, considerably affect the peptide binding and subsequent aggregation process. Furthermore, this work led us to conclude that the adsorption of somatostatin-14 on silver nanoparticles is rendered possible through an ionic pair interaction, whereas its anchoring on gold nanoparticles occurs by a direct binding, in which the metal atom and the nitrogen of the unique peptide tryptophan residue are involved.