A molecular reel: shuttling of a rotor by tumbling of a macrocycle

A molecular shuttle is a prototype of molecular machines capable of shuttling a part back and forth in the molecule. To control the shuttling of a macrocycle, we studied the reeling of an axis molecule into a macrocyclic host molecule located at the end of the axis. [2]Rotaxane and [3]rotaxane are composed of alpha-cyclodextrin (alpha-CD) rotors, a decamethylene chain, and a stilbene unit as axes and an altro-alpha-CD stopper containing one altropyranose unit and five glucopyranose units. The alpha-CD rotor of [2]rotaxane includes the decamethylene chain in DMSO-d(6) and moves to include the stilbene group in D(2)O. It should be noted that the altro-alpha-CD stopper group reorients to include the decamethylene chain in D(2)O. The mechanism for tumbling was elucidated with 2D ROESY NMR measurements and kinetic studies. These studies showed that an altropyranose unit of the altro-alpha-CD stopper tumbles to form a self-inclusion complex in aqueous solution. The altro-alpha-CD stopper of [2]rotaxane reels an axis molecule into its cavity and then pushes the alpha-CD rotor onto the stilbene group, resulting in conversion to pseudo[2]rotaxane in D(2)O. In contrast, the rotors of [3]rotaxane did not show shuttling because there was insufficient space not only for the rotors to shuttle but also for the altro-alpha-CD stopper to include an axis after tumbling. The decamethylene chain and the stilbene group are already included in the rotors of [3]rotaxane. The tumbling of the altro-alpha-CD stopper was found to play a critical role in controlling the shuttling of rotors. We successfully controlled the shuttling of a rotor by reeling the axis molecule into a host molecule at the end of an axis.