Self-assembling peptides for biomedical applications: Ir and raman spectroscopies for the study of secondary structure

Self-assembling peptides are a category of peptides which undergo spontaneous assembling into ordered nanostructures. These designed peptides have attracted huge interest in the field of nanotechnology for its potential application in areas such as biomedical nanotechnology, cell culturing, molecular electronics, and more. In the emerging field of tissue engineering, the development of synthetic materials promoting cell growth has led to the study of regularly alternating polar/non-polar amphiphilic oligopeptides, such as EAK16 (AEAEAKAK)2, also called LEGO peptides, which have been considered particularly promising. Self-assembling LEGO peptides have a preferential β-sheet structure, are resistant to proteolytic cleavage, and are able to form 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 the complementary ionic bonds between acidic and basic amino acids. This stability can be enhanced by the pH regulation and the presence of monovalent ions. This chapter will be focused on some approaches useful for elucidating the influence of the sequence modifications and the interactions with a surface on the self-assembly capability of differently synthesised peptides. Eight different oligopeptides (from 16 to 19 residues), derived from EAK16, have been analysed by IR and Raman spectroscopies that are particularly useful for obtaining qualitative and quantitative information on the secondary structure of these peptides. Several modifications in the primary structure of peptides have been considered, namely acidic and/or basic substitutions (Glu → Asp; Lys → Orn), changes in the length of the aliphatic side chain of the spacer residues (Ala → Abu or Ala → Tyr), the addition of RGD (known to promote osteoblast adhesion), or scrambling of the sequence. As these peptides are widely used as biocompatible coatings of metallic implants, the peptide folding after adsorption on different surfaces, in particular on titanium oxide, will be also discussed. Finally, it will be shown that not all the oligopeptides examined can self-assemble into a homogeneous multilayer on metallic surfaces; however, most of the peptides take a prevailing β-sheet structure which guarantees the best peptide-surface interaction. In particular, the interactions of polar, ionic and aromatic residues with surfaces will be discussed more in detail.