Solution Conformations of Helix-Forming β-Amino Acid Homooligomers

The conformational properties of β-peptides comprised of enantiomerically pure trans-2-aminocyclohexanecarboxylic acid (ACHC) or trans-2-aminocyclopentanecarboxylic acid (ACPC) units were studied by NMR spectroscopy in organic solvents. In pyridine-d₅ solution, ACPC hexamer 1 and ACPC octamer 2 displayed well-defined helical structures characterized by a series of 12-membered hydrogen-bonded rings (“12-helix”). The solution structures calculated from the NMR-derived constraints were very similar to the conformations found previously for 1 and 2 in the solid state. ACHC tetramer 3 displayed a different sort of helical conformation, characterized by a series of 14-membered hydrogen-bonded rings (“14-helix”), in methanol-d₃ solution. This solution conformation is similar to that previously found in the crystal structure of 3.