SERS characterization of self-assembling oligopeptides for tissue engineering

Eight alternating polar/non-polar peptides derived from the self-assembling peptide EAK-16 (Ac- AEAEAKAKAEAEAKAK-NH2) were examined in comparison with the EAK-16 parent form (peptide 1), both as-synthesized and after gamma-radiation treatment in oxidative environment (formation of ∙OH radical), to mimic the inflammation process occurring during the first phases of implantation. The peptides were studied for their possible use as biomimetic materials [1] due to their self-assembling properties and to the presence, in two of them, of the RGD sequence, an active modulator of cell adhesion. Previous investigations on titanium surfaces [2] demonstrated that some oligopeptides, i.e. EAK16 (peptide 1), peptide 3 (obtained by K→Orn substitution), peptide 5 (A→Abu substitution), peptide 7 (insertion at the N-terminus of the RGD sequence), and peptide 8 (RGD insertion and “scrambling” of the sequence) were not severely affected by the treatment and retained their prevalent structure. The other peptides, i.e. peptide 2 (obtained by E→D substitution), peptide 4 (E→D and K→Orn substitutions) and peptide 6 (A→Y substitution), underwent significant conformational changes (i.e. increase in the á-helix content). Interestingly, these same last 3 peptides were severely affected also by the radical attack induced by irradiation [3], changing their secondary structure and lateral chains interactions. To better investigate the interaction between peptides and metallic surfaces, we carried out SERS experiments on silver nanoparticles. As regards the as-synthesized peptides, all of them interacted with the Ag colloid by the COO- lateral groups of aspartic or glutamic acid residues. Other observed interaction sites were: - NH2/NH3+ of basic aminoacidic residues (lysine and/or ornithine) in peptides 3, 4 and 7; - amide group in peptides 1, 2, 4 and 8; - the aromatic ring of Tyr side chains in peptide 6. As regards the peptides after irradiation, they still interact through the before-mentioned groups; however, those undergoing conformational changes showed differences in band intensities and positions (see Fig.1) References [1] A. Tinti, M. Di Foggia, P. Taddei, A. Torreggiani, M. Dettin, C. Fagnano, J. Raman Spetrosc. 39, 250 (2008) [2] M. Di Foggia, P. Taddei, A. Torreggiani, M. Dettin, A. Tinti, J. Raman Spetrosc. 42, 276 (2011) [3] M. Di Foggia, P. Taddei, A. Torreggiani, M. Dettin, A. Tinti, J. Raman Spetrosc. 44, 1446 (2013)