The pulling force of a synthetic molecular machine

The widespread utilization of sub-molecular motion in key biological processes is inspiring chemists who synthesize molecular machines able to imitate the machinery of biological world. It has been proved possible to design synthetic molecular systems in which positional displacements of sub-molecular components occur upon the application of external stimuli.1-3 The architecture of synthetic systems is crucial to translate molecular level effects into a useful response exploitable in the macroscopic world. Pioneering works1-2 have shown that rotaxanes (molecules consisting of a ring threaded onto an axle capped with bulky end-stoppers) are a particularly promising kind of synthetic 'molecular shuttles'.

Although nanodevices based on molecular machines have been conceived, there is a huge gap between those exploratory studies and truly functional systems, able to use an external source of energy to induce a directional motion and perform useful tasks.4 One of the major challenges is the interfacing between the molecular machines and the outside world.

Here we to demonstrate the feasibility of transducing sub-molecular movements into mechanical work by combining the controlled translational motion of the ring in a rotaxane coupled to a polymer chain, and the ability of AFM-based single molecule force spectroscopy to be used as a mechanical device.5 For that purpose, a rotaxane with a long thread and two stations onto which the ring can bind through H-bonds was synthesized. We have attached a polymer chain to the ring and the resulting system was grafted onto substrates. We then fished the polymer chain with an AFM tip and realized single molecule pulling-relaxing cycles. The ring was moved away from the most stable station along the thread and was found to shuttle back to this station against the external force, thus delivering mechanical work against the AFM cantilever. We have estimated the work done by the ring and show that the value is in good agreement with predicted theoretical values.2

1 J.F. Stoddart et al., Special Issue on Molecular Machines, Acc. Chem. Res. 2001, 34, 409-522.
2 E R Kay, D A Leigh and F Zerbetto, Angew. Chem. Int. Ed. 2007, 46, 72.
3 D. A. Leigh et al., Nature 2000, 406, 608; Science 2001, 291, 2124; Nature 2003, 424, 174; Science 2004, 306, 153; Nature 2006, 440, 286; Nature 2007, 445, 523; Nature 2009, 458, 314.
4 W. R. Browne, B. L. Feringa, Nature Nanotech. 2006, 1, 25.
5 H. E. Gaub et al., Science 2002, 296, 1103.