Promotion of sheet formation in alpha-peptide strands by a beta-peptide reverse turn

[GRAPHICS] We show that a tetrapeptide with a heterogeneous backbone, i.e., with two d ifferent classes of amino acid residues, adopts a hairpin conformation in w hich each type of residue plays a different structural role. The alpha resi dues at the ends form hydrogen bonds characteristic of antiparallel beta-sh eet secondary structure, while the central di-beta-peptide segment forms a reverse turn. The configuration of the turn residues is critical to sheet f ormation.