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

Biomimetic materials “mimic” natural biological systems and tissues with the aim to elicit appropriate cellular response for tissue regeneration to direct the formation of new tissues within the material at the local implant site. The interest in functionalised biomimetic materials 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. Their ability to create such stable structures derived from the hydrophobic interaction between the -CH3 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. IR and Raman vibrational spectroscopies provided useful information on the secondary structure of the peptides by the help of different amide stretching modes. A quantitative evaluation of the secondary structure of the oligopeptides was obtained by the fitting of the amide I Raman band. We studied 8 different oligopeptides (16-19 amino acids residues) derived from EAK-16 but modified in their sequence by substitution of acid, basic or neutral amino acids or by the addition at the N-terminus of the RGD sequence, able to control osteoblasts adhesion. The peptides were examined after synthesis, after solubilisation in saline phosphate buffer (at pH=7.4) and after adsorption on porous oxidised titanium disks. As regards the just-synthesised peptides, 6 of them resulted to have a prevalent beta-sheet structure, while peptides containing the RGD head showed a mixed structure. After treatment in saline phosphate buffer (similar to phisiological conditions) all the peptides showed a prevaling beta-sheet structure. 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.