Chem. Sci. http://doi.org/rhn (2014)
Catenanes are mechanically interlocked molecules that are of interest in the development of synthetic molecular machines because of their large conformational flexibility and range of dynamic response. Probing the submolecular interactions and kinetics of catenanes is challenging because it is difficult to couple the molecules to single-molecule characterization techniques without significantly perturbing the system. David Leigh, Anne-Sophie Duwez, Charles-André Fustin and colleagues have now shown that single-molecule force spectroscopy can be used to study the intramolecular dynamics of the two macrocycles in a catenane molecule.
The researchers — who are based at the Université catholique de Louvain, University of Liège and the University of Manchester — first attached a polymer chain to each of the macrocycles in the catenane. One of these chains was adsorbed on a surface, whilst the other was picked up by the tip of an atomic force microscope. The microscope tip was then slowly pulled away from the surface and the restoring force of the system measured; this force is directly related to the mobility of the two macrocycles.
By carrying out measurements in different solvents, the team show that the technique can identify 'locked' and 'unlocked' catenane rings — in a nonpolar solvent the motion of the rings is constrained by hydrogen bonds between them, whereas in a polar solvent the rings can freely rotate and assume an entropically favoured coiled conformation.