Spectroscopic study of the structural changes induced by free radical stress on oligopeptides for bone regeneration

Regular oligopeptides with polar and nonpolar residues alternating as the EAK16 (AEAEAKAK)2 are interesting biomimetic materials for bone implants, since they are able to self-assemble, forming a macroscopic insoluble and highly biocompatibile membrane. Their ability to create stable structures derives from both hydrophobic interactions between the aliphatic groups of non-ionic residues and ionic interactions between charged amino acids side-chains. This biomimetic membrane (mainly organized into beta-sheet secondary structure), may be subjected to oxidative radical stress during the phases of colonization and growth of bone cells (osteoblasts) as result of inflammatory phenomena. This stress could lead to alterations in the membrane structure with a consequent decrease of its biomimetic properties. In the physiological environment, the hydroxyl radical (•OH) is considered one of the most reactive oxidative species and therefore one of the most harmfull. In this context, we have studied the structure alterations suffered by 7 oligopeptides derived by EAK16 (through selective substitution of amino acids) under conditions of high oxidative radical stress. The radical species have been produced by γ-radiolysis of aqueous solutions. Structural damages, especially in terms of variation of the secondary structure and interactions between side chains, caused by the attack of •OH radicals generated at different concentrations, were evaluated by means of IR, traditional Raman and Surface-Enhanced Raman (SERS) spectroscopies. The results have showed a different response of the oligopeptides to oxidative stress. In particular, the sequences in which aspartic acid replaces glutamic acid residue (i.e. with a shorter lateral chain) are very sensitive to radical stress exposure. In contrast, other oligopeptides showed no significant changes, thus revealing a higher resistance to degradation due to radical stress.