First direct measurement of force generated by an individual synthetic molecular machine

News articles by Jon Cartwright on the Chemistry World news site and by Michael Berger at Nanowerk describe a significant molecular machine milestone achieved by the research groups of David A. Leigh (winner of the 2007 Foresight Institute Feynman Prize in Nanotechnology for Theory) and Anne-Sophie Duwez. The research was reported in Nature Nanotechnology [abstract]. They demonstrated a single rotaxane molecule less than 5 nm long working against an external load of 30 pN. From “Manmade molecular machine goes to work“:

Researchers in the UK and Belgium have measured the work performed by a single manmade molecule. The result demonstrates that manmade molecules can generate similar forces to natural molecular machines, and could help chemists to design artificial molecular machines for meaningful tasks. …

David Leigh, of the University of Edinburgh, and Anne-Sophie Duwez of the University of Liege, and colleagues, have now managed to extract work from a single manmade molecule. The molecule is a rotaxane that comprises a molecular ring with a long axle running through it. The ring can move along the length of the axle, but it prefers to stay at one end where it can form several hydrogen bonds with the spindle.

The team anchored the axle to a gold surface, and attached a curly polymer chain to the ring. To the other end of this polymer chain they attached the tip of an atomic force microscope (AFM), a device that can measure minute forces via the deflection of a cantilever. As they pulled the AFM tip, the polymer first unfolded and then pulled the ring away from the hydrogen bonds to the other end of the axle. However, once the researchers stopped pulling the AFM tip, the ring could move with random thermal fluctuations, and gradually took itself back to its preferred site. …

From “Measuring the force of a single synthetic small molecule“:

By doing the pull-relax cycles hundreds of times, the researchers could build up a statistical analysis of the force generated by the synthetic molecular machines. They found that the ring of a single rotaxane molecule can move to its preferred binding site against a resisting force of 30 pN.

In other words, a single rotaxane molecule was measured to pull on the AFM tip with a force of 30 pN. That’s not dissimilar to the forces measured for single biological machines such as myosin, kinesin, ATPases and DNA- and RNA-polymerases which tend to be in the range 5-60 pN.

This is the first time that the force generated by an individual synthetic molecular machine has been directly measured. …

Now, Leigh and his team plan to examine the behavior of other small-molecule synthetic molecular machines like synthetic molecular walkers in the same way. …

This work is a great complement to earlier work looking at large assemblages of small synthetic molecular machines and at single molecule studies of large biomolecular machines. We can hope it will lead to further advances in understanding and designing synthetic molecular machines that do useful work, including eventually atomically precise manufacturing.

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