α-Aminoxy Oligopeptides: Synthesis, Secondary Structure, and Cytotoxicity of a New Class of Anticancer Foldamers.

alpha-Aminoxy peptides are peptidomimetic foldamers with high proteolytic and conformational stability. To gain an improved synthetic access to alpha-aminoxy oligopeptides we used a straightforward combination of solution- and solid-phase-supported methods and obtained oligomers that showed a remarkable anticancer activity against a panel of cancer cell lines. We solved the first X-ray crystal structure of an alpha-aminoxy peptide with multiple turns around the helical axis. The crystal structure revealed a right-handed 2(8)-helical conformation with precisely two residues per turn and a helical pitch of 5.8 angstrom. By 2D ROESY experiments, molecular dynamics simulations, and CD spectroscopy we were able to identify the 2(8)-helix as the predominant conformation in organic solvents. In aqueous solution, the alpha-aminoxy peptides exist in the 2(8)-helical conformation at acidic pH, but exhibit remarkable changes in the secondary structure with increasing pH. The most cytotoxic alpha-aminoxy peptides have an increased propensity to take up a 2(8)-helical conformation in the presence of a model membrane. This indicates a correlation between the 2(8)-helical conformation and the membranolytic activity observed in mode of action studies, thereby providing novel insights in the folding properties and the biological activity of alpha-aminoxy peptides.