Controllable molecular shuttles

A molecular shuttle whose speed can be precisely controlled has been developed by researchers in Germany. 

Christoph Schalley from the Kekul? Institute for Organic Chemistry and Biochemistry, Bonn, and co-workers, studied molecular motion using rotaxanes. Rotaxanes consist of a molecular wheel, which shuttles backwards and forwards along an axle. Schalley has developed a rotaxane where the speed of the shuttling motion can be precisely controlled. 

This ability to tune the shuttling speed lies in the phenol group incorporated into the rotaxane axle. At low pH, the wheel travels rapidly along the axle. When the pH is raised the phenol group loses its proton to become negatively charged, and then gains a coordinating counterion. As the wheel passes along the axle it has to momentarily disrupt the interaction between the axle and the coordinated ion, slowing the speed of travel. 

Another level of control can be added by lowering the solvent polarity. This strengthens the interaction between the axle and the counterion, the wheel experiences greater resistance as it displaces the ion, and so travels more slowly. 

As for the next big challenges facing the development of molecular machinery, Schalley sees significant future milestones being the development of molecular motors where the direction of rotation can be controlled and switchable molecules for miniaturised computers. 

James Crow