In sharp contrast with helical polypeptides carrying basic side chains, Api(8), a basic oligopeptide containing the non-natural achiral amino acid 4-aminopiperidine-4-carboxylic acid (Api), adopts a helical conformation only in acidic media. Alkaline titration of a protonated Api(8) oligomer appended with a leucine derivative at its N-terminus showed that disruption of its helical conformation occurs in a pH range of 7-10. NMR studies indicated that the piperidine groups in Api(8), when nonprotonated, possibly interact with the proximal amide protons in the peptide backbone and hamper the formation of the H-bonding network responsible for the helical conformation. The helical structure is induced not only by protonation but also by acylation of the piperidine groups.
CYCLOPEPTIDE, PHARMACEUTICAL OR COSMETIC COMPOSITION COMPRISING THE SAME AND METHOD FOR PREPARING THE SAME