Self-assembly of the octapeptide lanreotide and lanreotide-based derivatives: the role of the aromatic residues.

We investigated the spectroscopic properties of the aromatic residues in a set of octapeptides with various self-assembly properties. These octapeptides are based on lanreotide, a cyclic peptide analogue of somatostatin-14 that spontaneously self-assembles into very long and monodisperse hollow nanotubes. A previous study on these lanreotide-based derivatives has shown that the disulfide bridge, the peptide hairpin conformation and the aromatic residues are involved in the self-assembly process and that modification of these properties either decreases the self-assembly propensity or modifies the molecular packing resulting in different self-assembled architectures. In this study we probed the local environment of the aromatic residues, naphthyl-alanine, tryptophan and tyrosine, by Raman and fluorescence spectroscopy, comparing nonassembled peptides at low concentrations with the self-assembled ones at high concentrations. As expected, the spectroscopic characteristics of the aromatic residues were found to be sensitive to the peptide-peptide interactions. Among the most remarkable features we could record a very unusual Raman spectrum for the tyrosine of lanreotide in relation to its propensity to form H-bonds within the assemblies. In Lanreotide nanotubes, and also in the supramolecular architectures formed by its derivatives, the tryptophan side chain is water-exposed. Finally, the low fluorescence polarization of the peptide aggregates suggests that fluorescence energy transfer occurs within the nanotubes.