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α-Aminoxy peptides are peptidomimetic foldamers with high proteolytic and conformational stability. To gain an improved synthetic access to α-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 α-aminoxy peptide with multiple turns around the helical axis. The crystal structure revealed a right-handed 28-helical conformation with precisely two residues per turn and a helical pitch of 5.8 Å. By 2D ROESY experiments, molecular dynamics simulations, and CD spectroscopy we were able to identify the 28-helix as the predominant conformation in organic solvents. In aqueous solution, the α-aminoxy peptides exist in the 28-helical conformation at acidic pH, but exhibit remarkable changes in the secondary structure with increasing pH. The most cytotoxic α-aminoxy peptides have an increased propensity to take up a 28-helical conformation in the presence of a model membrane. This indicates a correlation between the 28-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 α-aminoxy peptides. [ABSTRACT FROM AUTHOR]