Molecular machines are ubiquitous in nature and are at the heart of nearly every biological process. They have evolved over billions of years to exploit energy from sunlight or complex chemical reactions in the body, and are made up of complicated assemblies of proteins responsible for a host of processes in living organisms, such as ion transport, ATP synthesis and cell division.

“Our synthetic molecular motor works in a manner reminiscent of motor proteins, in which motion is powered by the protein catalyzing the hydrolysis of ATP,” explains team leader David Leigh. “Our molecular motor also runs using a chemical fuel, Fmoc-Cl, and it derives its energy by catalytically decomposing the fuel to dibenzofulvene and CO2.”

New machine continues to function until fuel runs out

Previous such molecular motors needed to be continuously fed several reagents in different chemical steps, he adds. “Our new machine is quite different in that once we have added fuel to it, it continues to function until the fuel runs out, just like a motor car.”

The researchers made their motor using conventional chemical synthesis. The device comprises a small molecular ring (benzylic amide macrocycle) that continuously moves around a cyclic molecular track when powered by irreversible reactions of Fmoc-Cl. “Key to the design is that the rate of reaction of this fuel with reactive sites on the cyclic track is faster when the macrocycle is far from the site than when it is near to it, explains Leigh.

Information ratchet mechanism

“Indeed, the position of the ring on the track determines the (kinetic) rate at which a bulky pyridine group adds itself onto the ring. Once added, the bulky group blocks the ring and prevents it from moving backwards. This type of mechanism is called an ‘information ratchet’ and is based on the Maxwell demon thought experiment. But rest assured, since the molecular motor works using a fuel, it does not fall foul of the second law of thermodynamics”.

According to the team, this “first generation” or “chemically–fuelled molecular motor 1.0” is slow, inefficient and probably not suitable for applications in its present state. “However, this will change as we move to future versions of the device,” Leigh tells nanotechweb.org. “We could envisage applications such as transporting cargoes, like tiny amounts of substances and materials for analysis, transferring building blocks for molecular construction or pharmaceuticals along nanometre tracks. The motors might also be used as power packs for molecular muscles, nanoscale robotics and molecular factories, to name but a few.”

Fraser Stoddard of Northwestern University in the US, who was not involved in this work, says that he and his colleagues, who are also artificial molecular machinists, are “impressed”. “With artificial molecular machines on the rise, this elegant design and impressive launch of a chemically driven nanomotor could invoke wonder and receive acclaim from the broader scientific community. Although the speed and efficiency of the unidirectional movement of two mechanically interlocked rings with respect to each other leaves something to be desired, the demonstration in this Letter to Nature doi:10.1038/nature18013 represents an important proof-of-principle in a burgeoning field of scientific endeavour that is intellectually challenging and experimentally demanding in the extreme. Another key milestone on the long road to applications in molecular nanotechnology has been reached.”

Nicolas Giuseppone of the Université de Strasbourg in France, who was not part of the work, agrees: "This is extremely elegant work that indeed shows autonomous biased motion using a Feynman ratchet that is chemically fuelled. This last aspect is new and I find it remarkable that it is achieved by using the very simple idea of exploiting differential kinetic rates for making and breaking bonds."

Leigh and colleagues say they are now busy trying to make their motors more efficient and effective at transferring the energy from fuel into mechanical work.