Helix Formation in Preorganized β/γ-Peptide Foldamers: Hydrogen-Bond Analogy to the α-Helix without α-Amino Acid Residues

We report the first high-resolution structural data for the beta/gamma-peptide 13-helix (i,i+3 C=O center dot center dot center dot H-N H-bonds), a secondary structure that is formed by oligomers with a 1:1 alternation of beta- and gamma-amino acid residues. Our characterization includes both crystallophaphic and 2D NMR data. Previous studies suggested that beta/gamma-peptides constructed from conformationally flexible residues adopt a different helical secondary structure in solution. Our design features preorganized beta- and gamma-residues, which strongly promote 13-helical folding by the 1:1 beta/gamma backbone.