A single synthetic small molecule that generates force against a load

Journal name:
Nature Nanotechnology
Year published:
Published online


Some biomolecules are able to generate directional forces by rectifying random thermal motions. This allows these molecular machines to perform mechanical tasks such as intracellular cargo transport or muscle contraction1 in plants and animals. Although some artificial molecular machines have been synthesized2, 3, 4 and used collectively to perform mechanical tasks5, 6, 7, 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]rotaxane8—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 cycles. 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 pN. Using fluctuation theorems, we were able to relate measurements of the work done at the level of individual rotaxane molecules to the free-energy change as previously determined from ensemble measurements. The results show that individual rotaxanes can generate directional forces of similar magnitude to those generated by natural molecular machines.

At a glance


  1. Chemical structure of the rotaxane-based molecule shuttle.
    Figure 1: Chemical structure of the rotaxane-based molecule shuttle.

    The rotaxane consists of a benzylic amide molecular ring (blue) mechanically locked onto an axle (black) by bulky diphenylethyl ester groups situated at either end. The axle contains a fumaramide group (green) and a succinic amide-ester group (orange), and the ring can bind to either of these sites through up to four intercomponent hydrogen bonds. The affinity of the ring for the fumaramide site is much higher than for the succinic amide-ester site, so the fumaramide:succinic amide-ester occupancy ratio is ~95:5. Next to the fumaramide binding site, a disulphide group (red) was introduced to enable the grafting of the molecule onto gold substrates. A 4,600 Mn PEO tether (blue) is attached to the ring so that it can be linked to an AFM probe, which allows the motion of the ring along the axle to be tracked.

  2. Single-molecule force spectroscopy of the rotaxane.
    Figure 2: Single-molecule force spectroscopy of the rotaxane.

    a, The rotaxane is grafted onto gold and the PEO tether is caught by the AFM tip and stretched by moving the tip away from the surface. The black arrow shows the direction of the cantilever displacement. b, Force–extension curve (red trace, approach curve; blue trace, retraction curve) of an individual rotaxane–PEO molecule in TCE. The arrow indicates the peak characteristic of the rupture of the hydrogen bonds between the fumaramide site and the ring. c, Force–extension curve of an individual PEO polymer chain in TCE for comparison.

  3. Experimental AFM pulling curves.
    Figure 3: Experimental AFM pulling curves.

    a, High-resolution force–extension curve for the rotaxane–PEO molecule in TCE. b, Histograms of the rupture forces for the hydrogen bonds in TCE (average ± s.d., n = 318) (left) and in DMF (average ± s.d., n = 249) (right) at a loading rate of 500 pN s−1. c, Force–extension curve (data as in a) with worm-like chain model fits to the data (thin solid lines) with an increase in length (ΔLc) of the molecule after rupture of the hydrogen bonds of 3.9 nm. d, Interpretation of the sequence of events taking place when pulling on the rotaxane–PEO. Black arrows show the direction of cantilever displacement. Blue arrows show the direction of the force exerted on the molecular ring. (I) Progressive stretching of the PEO tether. (II) Once the force exerted on the tether exceeds the force of the hydrogen bonds between the ring and the fumaramide site, the hydrogen bonds break. (III) After rupture, the ring is free to move along the thread, the tension in the PEO backbone is partly released, and the force decreases until the displacement of the cantilever increases again the tension in the PEO tether. (IV) Further cantilever displacement continues the stretching of PEO until the force exceeds the interaction strength of the chain with the tip, which leads to detachment.

  4. Pulling-relaxing cycles for the rotaxane-PEO molecule.
    Figure 4: Pulling–relaxing cycles for the rotaxane–PEO molecule.

    a, Pulling (blue) and relaxing (red) curves for a single rotaxane–PEO molecule in TCE. The relaxing trace is offset vertically for clarity. b, Schematic of the relaxing experiment showing our interpretation of events. Black arrows show the direction of cantilever displacement. Blue arrows show the direction of the force exerted on the ring. (I) Progressive release of the tension in the PEO tether. (II) The force suddenly increases as a result of the ring shuttling in the opposite direction (red arrow) to the force exerted on it (blue arrow). (III) The ring has rebound to the fumaramide site. c, Relaxing curve (data as in a) with the area under the trace representing the work done by the molecule as the ring shuttles back to its preferred binding site. d, Example of five successive pulling–relaxing curves. The stochastic nature of the rupture and rebinding process is characterized by a distribution of work trajectories.


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  1. Department of Chemistry, University of Liège, B6a Sart-Tilman, 4000 Liège, Belgium

    • Perrine Lussis,
    • Tiziana Svaldo-Lanero &
    • Anne-Sophie Duwez
  2. School of Chemistry, University of Edinburgh, The King's Buildings, West Mains Road, Edinburgh EH9 3JJ, UK

    • Andrea Bertocco &
    • David A. Leigh
  3. Institute of Condensed Matter and Nanosciences (IMCN), Place Louis Pasteur 1, Université catholique de Louvain, Belgium

    • Charles-André Fustin


P.L. and T.S-L. performed the AFM experiments and analysed the data. A.B. carried out the rotaxane synthesis and characterization studies. C-A.F. participated in rotaxane synthesis. A-S.D., C-A.F. and D.A.L. designed the experiments and prepared the manuscript.

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