Interactions between Oligopeptides and titanium surface detected by Vibrational spectroscopy

The interest in functionalised biomimetic materials used in prosthetic applications as bone substituted has led to studies on regular alternating polar/non-polar oligopeptides such as EAK-16 (AEAEAKAKAEAEAKAK), first synthesised by Zhang et al. [1]. These peptides have a preferential beta-sheet structure, are resistant to proteolitic cleavage and able to self-assemble into an insoluble macroscopic membrane under physiologial conditions. Their ability to create such stable structures derived from the hydrophobic interaction 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 by the presence of monovalent ions. In this contest, we studied eight different oligopeptides derived from EAK-16, but modified in their sequence by amino acid substitution or by the addition, at the N-terminus of the sequence, of the RGD motif, present in the integrins located in the bone extracellular matrix and able to control osteoblasts adhesion. IR and Raman vibrational spectroscopies were used to investigate the influence of the modifications in the sequences on the self-assembly capability of the peptides and to monitor the influence of the interactions with metallic surface on peptides. In particular, useful qualitative and quantitative information on the secondary structure of the peptides were provided by using different amide stretching modes. The peptides were examined after solubilisation in saline phosphate buffer (pH = 7.4) and after adsorption on porous oxidised titanium disks from the same buffered solution: the two conditions differ only for the presence of the metallic surface. We chose a titanium surface as it is widely used as biocompatible metallic implant material. After treatment in saline phosphate buffer (similar to phisiological conditions), all the peptides showed a prevaling beta-sheet structure [2]. On oxidised titanium disks we observed that not all the oligopeptides could self-assemble in 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 surface with their ionic and polar moiety, in particular the carboxylate ions of Glu and Asp residues (enhancement of the symmetric stretching band of COO- in the Raman spectra) and with the carbonylic oxygen of the amide group. The insertion of the RGD sequence or the variation of the aliphatic side chain did not altered the peptide ability to form omogeneous layer on TiO2 and therefore these peptides should be good candidates in the preparation of biomimetic devices.