Aromatic oligoamide β-sheet foldamers.

A rational approach for the construction of multi-stranded artificial β-sheets based not on hydrogen bonding, but rather on π-π aromatic stacking, is presented. Using 4,6-dinitro-1,3-phenylenediamine units, rigid turns were designed that allow face-to-face π-π interactions between appended linear aromatic segments to be strong enough for folding in an organic solvent, but weak enough to prevent aggregation and precipitation. Solution and solid-state studies on a series of turn units showed that the desired degree of rigidity, resulting from hindered bond rotation, could be fine-tuned by the inclusion of additional methyl substituents on the aromatic rings. The high degree of preorganization afforded by these qualities further allowed the facile preparation of macrocyclic sheet structures from their noncyclic precursors. These macrocycles were shown to have slow internal dynamics and defined conformational preferences. Using this background, three- and five-stranded artificial β-sheets were synthesized and their folded conformations extensively characterized in solution by NMR. The solid-state structures of the three- and five-stranded sheets were also elucidated in the solid state by X-ray crystallography and confirmed intramolecular π-π aromatic stacking.