FT-Raman and SERS characterization of self-assembling oligopeptides.

The interest in functionalised biomimetic materials used in prosthetic applications as bone substitutes has led to studies on regular alternating polar/non-polar oligopeptides such as EAK-16 (AEAEAKAK)2, first synthesised by Zhang et al. These peptides have a preferential beta-sheet structure, are resistant to proteolytic cleavage and able to self-assemble into an insoluble macroscopic membrane under physiological conditions. Their ability to create such stable structures derive from the hydrophobic interactions between the aliphatic groups of non-ionic residues and complementary ionic bonds between acidic and basic amino acids: this stability can be enhanced by the regulation of pH and the presence of monovalent ions. In this context, we studied different oligopeptides derived from EAK-16, but modified in their sequence by amino acid substitution or by adding at the N-terminus of the sequence the RGD motif. The latter is present in the integrins located in the bone extracellular matrix and is able to control osteoblast adhesion. Raman spectroscopy was used to investigate the influence of the modifications in the sequences on the self-assembly capability of the peptides. In particular, useful qualitative and quantitative information on the secondary structure and the hydrogen bonding strength were provided by the different amide stretching modes. A SERS study in solution was performed in order to understand the interaction way of the EAK-16 derivatives and a titanium oxide rough surface: similar interactions are supposed to take place when oligopeptides interact with the surface of prosthetic devices. Peptides resulted to interact with silver colloids through their ionic moieties, in particular the carboxylate ions of Glutamate and Aspartate residues (as revealed by the enhancement of the COO symmetric stretching band) as well as by the amino groups of Lysine residues. The insertion of Tyrosine as spacer did not alter the peptide secondary structure but gave a very interesting SERS spectrum, indicating the establishment of the metal-surface interaction through the ionic moieties of the peptide, as well as the aromatic ring of Tyrosine residues.