Not going back: Unidirectional movement by intramolecular one-way ratcheting of functionalized cyclodextrin

Abstract

The achievement of unidirectional molecular movement is a significant challenge due to competition by Brownian motion. Nature can overcome this problem by employing chemically fueled Brownian ratcheting mechanisms to power biomolecular motors, the understanding of which has, in turn, inspired chemists to design artificial molecular systems with similar functionality. Here, we demonstrate that a selectively functionalized cyclodextrin threaded onto an axle with three segments undergoes unidirectional movement. The cyclodextrin’s unique 3D structure enables both rim-selective functionalization and a regioselective deprotection reaction of temporary stoppers on the rotaxane axle. In this system, the cyclodextrin can actively open stoppering gates in one direction only. Its forward movement is further favored by a gate-closing reaction, which occurs faster when the cyclodextrin has crossed the gate, which is also caused by its cone shape. We have thus delineated a synergistic double-gated one-way ratchet thanks to the specific 3D structure of the cyclodextrin.