Solution conformations of helix-forming beta-amino acid homooligomers

The conformational properties of beta-peptides comprised of enantiomericall y pure trans-2-aminocyclohexanecarboxylic acid (ACHC) or trans-2-aminocyclo pentanecarboxylic acid (ACPC) units were studied by NMR spectroscopy in org anic solvents. In pyridine-d(5) 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 calcul ated from the NMR-derived constraints were very similar to the conformation s found previously for 1 and 2 in the solid state. ACHC tetramer 3 displaye d a different sort of helical conformation, characterized by a series of 14 -membered hydrogen-bonded rings ("14-helix"), in methanol-d(3) solution. Th is solution conformation is similar to that previously found in the crystal structure of 3.