Crystallographic Characterization of the α/β-Peptide 14/15-Helix

We report the first high-resolution structural data for the 14/15-helix, a secondary structure that is formed by oligomers with a 1:1 alternation of α- and β-amino acid residues. Previously, we concluded from NMR data that short α/β-peptides containing cyclopentane-constrained β-residues display rapid interconversion between two helical folding patterns, the 11-helix (i,i+3 CO···H−N H-bonds) and the 14/15-helix (i,i+4 CO···H−N H-bonds). Subsequent work in other laboratories, however, has called this hypothesis into question. Partial support for our original hypothesis was obtained when we obtained the first crystal structure in this α/β-peptide series, which revealed an 11-helical conformation. The present report of a 14/15-helical conformation strengthens the original hypothesis.