Azetidine-derived amino acids versus proline derivatives. alternative trends in reverse turn induction

The influence of 2-alkyl-2-carboxyazetidines (Aze) on the 3D structure of model tetrapeptides R2CO-2-R1Aze-l-Ala-NHMe has been analyzed by molecular modeling, 1H NMR, and FT-IR studies. The conformational constraints introduced by the four-membered ring resulted in an effective way to stabilize gamma-turn-like conformations in these short peptides. The conformational preferences of these Aze-containing peptides have been compared to those of the corresponding peptide analogues containing Pro or alpha-MePro in the place of 2-alkyl-Aze residue. In the model studied, both Pro and Aze derivatives are able to induce reverse turns, but the nature of the turn is different as a function of the ring size. While the five-membered ring of Pro tends to induce beta-turns, as previously suggested by different authors, the four-membered ring of Aze residues forces the peptide to preferentially adopt gamma-turn conformations. In both cases, the presence of an alkyl group at the alpha-position of Pro or the azetidine-2-carboxylate ring enhances significantly the turn-inducing ability. These results might open the opportunity of using 2-alkyl-Aze residues as versatile tools in defining the role of gamma-turn structures within the bioactive conformation of selected peptides, and represent an alternative to Pro derivatives as turn inducers.