From the cogs and wheels of a well-oiled machine to the spinning flagella of single-celled swimmers, rotation is one of the most useful forms of mechanical motion at almost any size scale. Researchers have long sought to mimic the molecular machinery of life, with some of their successes honored in the 2016 Nobel Prize in Chemistry (see Physics Today, December 2016, page 18). But until now, most synthetic molecular rotors have been painfully slow, taking minutes or even hours to complete a single rotation.
Now two overlapping groups have sped things up, creating DNA origami molecular rotors that spin in a controlled direction several times a second. Both groups include DNA origami expert Hendrik Dietz (Technical University of Munich), Dietz’s graduate student Anna-Katharina Pumm, and theoretical physicist Ramin Golestanian (Max Planck Institute for Dynamics and Self-Organization). Despite their material similarities, the rotors work in completely different ways.
The first group, which also includes Cees Dekker (Delft University of Technology) and his postdoc Xin Shi, created the rotor shown above, which works like a turbine or a windmill. The rotor blade is a 450-nm-long bundle of six DNA strands with a small protrusion that allows it to dock to a nanopore in a silicon nitride membrane. As an ion current (induced by a salt gradient or electrochemical potential) flows over the curved blade, it’s pushed around in a single direction up to 20 times a second.
Curiously, the chiral symmetry-breaking curvature that enables the one-way rotation is not inherent to the blade’s structure. At equilibrium, the DNA bundle is a straight rod; it becomes curved only once the fluid starts to flow. The researchers tried and failed several times to create a curved blade on purpose; they succeeded only by accident after they stripped all the symmetry-breaking elements from the structure.
The second group, which includes Dietz’s lab neighbor Friedrich Simmel, built a molecular device that’s more akin to an electric motor. The rotor blade is a DNA bundle, as before, that docks to a static structure that’s also made of DNA origami. Applying an oscillating electric field to the whole system turns the blade up to four rotations per second.
The AC field doesn’t drive the directed motion itself; rather, it shakes the rotor back and forth between the two potentials shown in the figure. The rotor–stator interaction creates the little potential dips shown at 45° and 225°, and the applied field superposes the flip-flopping sine wave on top of them. The asymmetry of the combination means that when the rotor hops from one dip to the other, it almost always turns in the same direction.
The researchers hope to eventually use rotors like these to power not just mechanical processes but also chemical ones. The inspiration, once again, is biological: The enzyme that assembles the molecule adenosine triphosphate, the energy source for cellular processes, is a rotary molecular machine. (X. Shi et al., Nat. Phys., 2022, doi:10.1038/s41567-022-01683-z; A.-K. Pumm et al., Nature 607, 492, 2022.)
