The Ability of an α-Aminoisobutyric Acid Residue to Promote Helical Folding in Oligopeptides

In order to investigate the ability of an Aib residue to promote helical folding in oligopeptides, oligo(Leu)s containing an Aib residue were prepared by stepwise elongation and fragment condensation methods. The peptides prepared were the following: Boc–Aib–Leun–OBzl (n=3–6 and 9), Boc–Leun–Aib–OBzl (n=3–6 and 9), Boc–Leu3–Aib–Leu3–OBzl, Boc–Leu4–Aib–Leu4–OBzl, Boc–Leu8–Aib–Leu4–OBzl, Boc–Leu4–Aib–Leu8–OBzl, and Boc–Leu8–Aib–Leu8–OBzl. The IR absorption conformational analyses of Boc–Aib–Leun–OBzl (n=3–6) in dichloromethane have shown the occurrence of incipient helical structures (α- or 310-helixes) formed by one, two, three, and so forth i→i–4 or i→i–3 hydrogen-bonding patterns. All the peptides except Boc–Aib–Leu9–OBzl and Boc–Leun–Aib–OBzl (n=6 and 9) have also shown helical structures (α- or 310-helixes), indicating the great ability of an Aib residue to promote helical folding in peptides. This is in remarkable contrast with the fact that homologous oligo(Leu) counterparts have β-sheet structures. ...