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. 

Many biological molecules can perform useful work. The protein motors kinesin and dynein, for example, transport cargo around cells using the chemical energy stored in ATP, the chemical fuel of biological systems. Scientists have created their own molecular machines that perform useful work, such as moving liquid droplets uphill or rotating microscale objects. But these synthetic machines have all had to work collectively, perhaps in groups of billions or more. 

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. 

’This last part is a bit like a dog held on a loose leash coming across a fire hydrant, and then exerting a force on the holder of the leash,’ says Leigh. 

After measuring a few hundred push-and-relax cycles, the team built up a picture of the force generated by the rotaxane molecule: 30pN, which translates to mechanical work of about 6 kcal mol^-1. Kinesin and myosin produce forces in the range 5 to 60pN. 

Dean Astumian, an expert in molecular motors at the University of Maine, US, says the work complements previous work performed by Fraser Stoddart of Northwestern University, US, and others in 2005. In these experiments, Stoddart’s group did not initiate any push-and-relax cycles, but instead examined how the mechanical behaviour of a rotaxane changed when it was switched between different chemical states.  

’The next logical step would be to combine these two results into a single experiment,’ Astumian says, adding: ’Such an experiment would provide a remarkable insight into the principles of transduction between chemical and mechanical energy - an essential element of operation of many molecular machines.’ 

Jon Cartwright

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