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Lamouroux, Arthur; Sebaoun, Laure; Wicher, Barbara; Kauffmann, Brice; Ferrand, Yann; Maurizot, Victor; Huc, Ivan (2017): Controlling Dipole Orientation through Curvature: Aromatic Foldamer Bent beta-Sheets and Helix-Sheet-Helix Architectures. In: Journal of the American Chemical Society, Vol. 139, No. 41: pp. 14668-14675
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The helix, turn, and beta-strand motifs of biopolymer folded structures have been found to prevail also in non-natural backbones. In contrast, foldamers with aryl rings in their main chains possess distinct conformations that may give access to folded objects beyond the reach of peptidic and nucleotidic backbones. In search of such original architectures, we have explored the effect of bending aromatic amide beta-sheets using building blocks that impart curvature. Cyclic and multiturn noncyclic sequences were synthesized, and their structures were characterized in solution and in the solid state. Stable bent sheet conformations were shown to prevail in chlorinated solvents. In these structures, folding overcomes iritramolecular electrostatic repulsions and forces local dipoles in each layer of the stacked strands to align in a parallel fashion: Sequences having helical segments flanking a central bent aromatic beta-sheet were then synthesized and shown to form well-defined helix turn helix architectures in which helical and sheet subcomponents conserve their respective integrity. These objects have a unique basket shape;they possess a cavity the depth and width of which reflects the curvature of the beta-sheet segment. They can be compared to previously described helical closed-shell receptors in which a window has been open, thus providing a means to control guest binding and release pathways and kinetics. As a proof of concept, guest binding to one of the helix sheet-helixstructures is indeed found to be fast on the NMR time scale while it is senerally slow in the case of helical capsules.