Self-assembling oligopeptides on oxidised titanium surfaces: IR and Raman chacterization.

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 proteolitic 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 7 different oligopeptides derived from EAK-16, but modifìed in their sequence by amino acid substitution or by adding at the N-terminus of the sequence the RGD motif, present in the integrins located in the bone extracellular matrix and able to control osteoblast adhesion. IR and Raman vibrational spectroscopies were used to investigate the influence of the sequence modifications on the self-assembly capability of the peptides and to monitor the influence of the surface on peptide conformation. In particular, useful qualitative and quantitative information on the secondary structure of the peptides were provided by the different amide stretching modes. The peptides were examined after solubilisation in saline phosphate buffer (pH = 7.4) and adsorption on porous oxidised titanium disks from the same buffered solution: the two conditions only differ for the presence of the surface. We chose a titanium surface as it is widely used as biocompatible metallic implant material. After solubilisation in saline phosphate buffer and lyophilization, all the peptides showed a prevaling beta-sheet structure. On oxidised titanium disks we observed that not all the oligopeptides could self-assemble into a homogeneous multilayer: peptides in which the polar amino acids were changed, aggregated in crystals even if beta-sheet was always the prevailing structure as observed after the solubilisation treatment. Peptides interact with the surface through their ionic and polar moieties, in particular the carboxylate ions of Glu and Asp residues (as revealed by the enhancement of the symmetric stretching band of COO- in the Raman and IR spectra) and the carbonylic oxygen of the amide group (as revealed by the enhancement of amide I and II bands). The insertion of the RGD sequence or the variation of the aliphatic side chain did not alter the peptide ability to form a homogeneous layer on TiO2 and therefore these peptides could be good candidates in the preparation of biomimetic devices.