Vibrational study of autoassembling oligopeptides for biomedical applications.

Biomimetic material "mimic" natural biological systems and tissues with the aim to elicit appropriate cellular responses for tissue regeneration to direct the formation of new tissues within the material at the local implant site. In natural bone, the interaction between osteoblasts and type 1 collagen fìbres are due to the binding of integrin receptors to arginine-glycine-aspartic acid (RGD) sequences of anionic glycoproteins. The adsorption of synthetic RGD peptides on biomaterial surface may be employed to control the bioactivity of materials used for bone regeneration. The recent literature about RGD synthetic peptides is quite large, but the interest on functionalised biomimetic materials has led to studies on regular alternating polar/non-polar oligopeptides, such as EAK-16, first synthesised by Zhang et al. These peptides has a preferential beta-sheet structure, resistant to proteolytic cleavage, and are able to self-assemble into an insoluble macroscopic membrane. By vibrational IR and Raman spectroscopy we investigated the secondary structure of eight different oligopeptides (from 16 to 19 amino acid residues) derived from EAK-16. These samples were modifìed in their primary sequences in order to evaluate the ability of oligopeptides to form self-assembled layers. The peptides were examined as synthesised and after solubilisation in phosphate buffer and liophylisation. A quantitative evaluation of the secondary structure of the oligopeptides was obtained by fitting the Raman and IR amide I band. As regards the just-synthesised peptides, 6 of them with only 16 alternating polar/non-polar residues resulted to have a prevalent beta-sheet structure, as revealed by the position of the amide I band at 1670-1673 cm-1 in their Raman spectra and at 1694-1697 cm-1 and 1620-1626 cm-1 in the IR ones. Mainly alfa-helix or mixed content was found in the peptides containing the RGD head, as revealed by the presence of a Raman band at 1659 cm-1 and IR components at 1663 and 1640 cm-1. After treatment for 6 hours in phosphate buffer and liophylisation, also the two oligopeptides containing the RGD sequence changed their secondary structure to a prevalent beta-sheet one.