Imidazole-peptide foldamers: parabolic dependence of the folding process on the water content of the solvent

Oligomers of azole peptides have been isolated from a multitude of marine organisms. Up to now, these azole-containing dipeptide-analogue oligomers have only been found as cyclic n-mers (mostly tri- and tetramers) in nature. Herein, we show that imidazole-containing pseudopeptides form helixlike secondary structures in different solvents. The screw sense of the helix can be determined by attaching a single chiral imidazole unit to the N terminus of the oligomer. Investigation by means of circular dichroism (CD) spectroscopy showed that the folding process of the helix depends on the water content of the solvent in a parabolic way. In a pure organic medium, the helix is stabilized by hydrogen bonds between the hydrogen atoms of the amide groups and the nitrogen atoms of the azole ring. In aqueous solution, the formation of the helix is driven by dispersion interactions. The formation of the helix is more pronounced in aqueous solution than in organic solvents.