Non-classical Helices with cis Carbon-Carbon Double Bonds in the Backbone: Structural Features of α,γ-Hybrid Peptide Foldamers.

The impact of geometrically constrained cis α,β-unsaturated γ-amino acids on the folding of α,γ-hybrid peptides was investigated. Structure analysis in single crystals and in solution revealed that the cis carbon-carbon double bonds can be accommodated into the 12-helix without deviation from the overall helical conformation. The helical structures are stabilized by 4→1 hydrogen bonding in a similar manner to the 12-helices of β-peptides and the 310 helices of α-peptides. These results show that functional cis carbon-carbon double bonds can be accommodated into the backbone of helical peptides.