Reverse Turn Foldamers: An Expanded β-Turn Motif Reinforced by Double Hydrogen Bonds.

Hybrid tetrapeptides sharing a backbone with a central α/β-dipeptide segment flanked by aromatic γ-amino acid residues fold into the same hairpin conformation with an expanded β-turn. This hairpin/β-turn motif is general for accommodating different α- and β-amino acid residues. Replacing glycine with other α-amino acid residues has an insignificant influence on or slightly decreases the stabilities of the folded conformations; substituting β-alanine with other β-amino acid residues enhances the stabilities of the folded structures.