A single synthetic small molecule that generates force against a load

Biological molecular machines are able to rectify random thermal motions to generate directional force and carry out tasks on both the molecular and macroscopic length scales1. Although some artificial nanomachines have been synthesized2 and used to collectively carry out mechanical tasks3, so far there have been no direct measurements of mechanical processes at the single-molecule level. Here we report measurements of the mechanical work performed by a synthetic molecule less than 5 nm long. We show that biased Brownian motion of the sub-molecular components in a hydrogen bonded [2]rotaxane4 -a molecular ring threaded onto a molecular axle- can be harnessed to generate significant directional forces. We used the cantilever of an atomic force microscope to apply a mechanical load to the ring during single-molecule pulling–relaxing cycle. The ring was pulled along the axle, away from the thermodynamically favoured binding site, and was then found to travel back to this site against an external load of 30 pN5. Using fluctuation theorems6, we were able to relate the measurements of the work done at the level of individual molecules to the free energy change measured previously by ensemble measurements. The results show that individual rotaxane molecules can generate directional forces of similar magnitude to those generated by natural biological machines, and extend the capabilities of AFM-based single molecule mechanics to the world of small molecules. It opens up the possibility of testing modern theories of non-equilibrium statistical mechanics, such as Jarzynski’s equality7 and the Crooks fluctuation theorem6, in single-molecule AFM measurements. 1. Schilva, M. (ed.) Molecular Motors (Wiley-VCH, Weinheim, 2003). 2. Kay, E. R., Liegh, D. A. & Zerbetto, F., Angew. Chem. Int. Ed. 46, 72-191 (2007). 3. Berna, J., Leigh, D. A., Lubomska, M., Mendoza, S. M., Pérez, E. M., Rudolf, P., Teobaldi, G. & Zerbetto, F., Nature Mater. 4, 704-710 (2005). 4. Kay, E. R. & Liegh, D. A., Top. Curr. Chem. 262, 133-177 (2005). 5. Lussis, P., Svaldo-Lanero T., Bertocco, A., Fustin C-A., Leigh, D. A., Duwez A-S., Nature Nanotech. 6, 553-557 (2011). 6. Crooks, G.E., Phys.Rev.E, 60, 2771-2726 (1999) 7. Jarzynski, C., Phys.Rev.Lett., 78, 2690-2693 (1997