Synthesis and Characterization of <i>trans</i>-2-Aminocyclohexanecarboxylic Acid Oligomers: An Unnatural Helical Secondary Structure and Implications for β-Peptide Tertiary Structure

Abstract

The preparation, 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 β-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 α-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 β-amino acid oligomers (“β-peptides”). In addition, the crystal packing pattern observed for the hexamer offers a blueprint for the design of β-peptides that might adopt a helical bundle tertiary structure.