Spectroscopic investigation on the structural modifications induced by radical stress on oligopeptides for tissue engineering

EAK16 (AEAEAKAKAEAEAKAK) and seven alternating polar/hydrophobic oligopeptides (derived from EAK16 by primary structure modifications) were tested for their resistance to free radical attack, in view of their possible use as biomimetic coatings of biomedical devices; in fact, oxidative radical stress, mainly due to hydroxyl radicals, is a common physiological condition occurring under acute or chronic inflammatory response. OH radicals, mimicking an endogenous radical stress, were generated by gamma-radiolysis of water solutions. IR and Raman spectroscopies were used to investigate the damages induced by free radicals on the oligopeptides, in particular on their folding and inter-chain interactions. Some oligopeptides, i.e. EAK16 (pept1), pept3 (obtained by K→O substitution), pept5 (A→Abu substitution), pept7 (insertion at the N-terminus of the RGD sequence), and pept8 (RGD insertion and "scrambling" of the sequence) were not severely affected by the treatment and retained their prevalent structure (i.e. α-helix for pept8 and β-sheet for the others). The other peptides, i.e. pept2 (obtained by E→D substitution), pept4 (E→D and K→O substitutions) and pept6 (A→Y substitution), showed significant conformational changes (i.e. increase in the α-helix content) upon irradiation. In pept2 and pept4, modifications in hydrophobic and ionic interactions were observed; the latter were mainly ascribed to the decarboxylation of acidic amino acids. In pept6, the main target of the OH attack was Tyr, as deduced by the strong variations observed in Tyr bands both in IR and Raman spectra. This study may contribute to gain insight into the different biological performances of the EAK16-derived peptides and to identify the most adequate materials usable for bone regeneration.