Synthesis and characterization of trans-2-aminocyclohexanecarboxylic acid oligomers:: An unnatural helical secondary structure and implications for β-peptide tertiary structure

The preperation, crystal structures, and circular dichroism(CD)spectra of two oligomers of optically active trans-2-aminocyclohexanecarboxylic acid are reported. In the solid state, both the tetramer and the hexamer of this beta-amino acid display a helical conformation that involves 14-membered-ring hydrogen bonds between a carbonyl oxygen and the amide proton of the second residue toward the N-terminus. (For comparison, the familiar cc-helix observed in conventional peptides is associated with a 13-membered-ring hydrogen, bond between a carbonyl oxygen and the amide proton of the fourth residue toward the C-terminus.) These. :crystallographic data, along with CD data obtained in methanol, suggest that the 14-helix constitutes a stable secondary structure for beta-amino acid oligomers ("beta-peptides"). In addition, the cryst al packing pattern observed for the hexamer offers a blueprint for the design of beta-peptides that might adopt a helical bundle tertiary structure.