International audienceDespite 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

Surface‐enhanced Raman spectra of two cyclic cationic peptides with therapeutic interest, somatostatin‐14 and its synthetic analog octreotide, were simultaneously analyzed as a function of concentration and colloidal size. It appeared that the large size (~95 nm) citrate‐reduced gold nanoparticles are the most adequate substrates for providing an optimal Raman signal enhancement within the 10−6–10−7 m peptide concentration range. It could also be evidenced that these large size particles can give rise to such a remarkable result within the first month following their elaboration. In fact, the recorded Raman spectra after this period mainly originate from the molecular species in the coverage of gold particles and not from the adsorbed peptides. To bring clarification to the temporal evolution of gold colloids, a systematic analysis was performed during 3 months on different particle sizes in the 15–150 nm range. The data obtained by a combined use of different techniques (Ultraviolet‐visible absorption, z‐potential, transmission electron microscopy, and Raman scattering) are consistent with a permanent evolution of the colloidal coverage, assignable to the gradual transformation of citrate anions and their substitution by oxidized products. It can be concluded that only small and medium size nanoparticles (≤ 40 nm) are able to regenerate their initial features after the first month, while large size colloids are subjected to a continuous degradation of their plasmonic and electrostatic properties. Copyright © 2016 John Wiley & Sons, Ltd.

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