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Orchestrating Directional Molecular Motions: Kinetically Controlled Supramolecular Pathways of a Helical Host on Rodlike Guests

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posted on 13.06.2017, 00:00 by Xiang Wang, Barbara Wicher, Yann Ferrand, Ivan Huc
An aromatic oligoamide sequence was designed to fold and self-assemble into a double helical host having a cylindrical cavity complementary to linear oligocarbamate guests. Formation of helical pseudorotaxane complexes, foldaxanes, between the host and guests having binding stations of different affinities was evidenced by NMR and X-ray crystallography. Rodlike guests possessing two or three binding stations, long alkyl or oligoethylene glycol spacers or bulky barriers in-between the binding stations, and a single bulky stopper at one end were synthesized. Kinetic investigations of the threading and translation of the double helix along multistation rods were monitored by 1H NMR. Results show that multiple events may occur upon sliding of the host from the nonbulky end of the rod to reach the thermodynamically most stable state before unfolding-mediated dissociation has time to take place, including binding on intermediate stations and rapid sliding along nonbinding spacers. Conversely, installing a kinetic barrier that blocks sliding allows for the deliberate integration of a helix dissociation reassociation step in the supramolecular trajectory. Typical sliding processes can be monitored over the course of hours whereas steps involving unwinding–rewinding of the helix proceeded over the course of days. These results further demonstrate the interest in foldaxanes to design complex sequences of supramolecular events within networks of equilibria through the adjustment of the kinetics of the individual steps involved.