Hybrid Sequences that Express both Aromatic Amide and α‐Peptidic Folding Features

Foldamers combining aliphatic and aromatic main‐chain units often produce atypical structures that cannot easily be accessed from purely aromatic or aliphatic sequences. We report solid‐state evidence that sequences comprising α‐amino acids and quinoline‐based monomers adopt conformations that combine the folding propensities of both components. Foldamers 2 and 3 having an XQQ repeat motif (X=α‐amino acid, Q=quinoline) were synthesized. Crystals of 2 (X=Phe, Q with an anionic side chain) obtained from water revealed an aromatic helix where amide groups belonging to the α‐amino acids created a hydrogen‐bond array typical of peptidic helices. Crystals of 3 (X=Ser, Q with a lipophilic side chain) obtained from organic solvents revealed a helix‐turn‐helix structure in which α‐amino acid side chains interfere with main‐chain hydrogen bonding. High sequence‐dependency of the conformation is typical of peptides but is shown here to include aromatic folding features.