From the cogs and wheels of well-oiled machines to the spinning flagella of single-celled swimmers, rotation is one of the most versatile forms of mechanical motion at almost any size scale. The rotating wheels of cars and conveyor belts drive linear motion. At the subcellular scale, rotation powers not just mechanical processes but chemical ones. ATP synthase, the enzyme that assembles molecules of adenosine triphosphate to fuel cellular processes, is based on a spinning central protein cylinder.
Researchers have long sought to mimic the molecular machinery of life to build their own miniature molecule assemblers and more, but they’ve been challenged by the small-scale physics. A submicron spinning rotor in water lacks the inertia to keep turning in one direction. Instead, it’s pulled to a stop by viscous drag and batted around by the Brownian storm of random molecular movements. On top of the difficulty of just building the tiny machines, researchers need to engineer the physical mechanism of their operation, and it’s not clear what the best one would be.
There have been some successes. Bernard Feringa, honored with a share of the 2016 Nobel Prize in Chemistry, designed the first one-way synthetic molecular rotor, powered by alternating pulses of heat and light. (See Physics Today, December 2016, page 18.) David Leigh and colleagues at the University of Manchester have synthesized several rotors that, like biomolecules, are fueled chemically.1 But so far, even state-of-the-art rotors have been impractically slow, taking minutes or hours to complete a single rotation.
Now two overlapping groups, both including Hendrik Dietz of the Technical University of Munich and his graduate student Anna-Katharina Pumm, have sped things up. They’ve designed two different rotors, both made with DNA origami, that rotate several times a second: not quite as fast as their biological counterparts, but in the same ballpark. Although superficially similar—both rotor blades are bundles of DNA some 500 nm long—they operate in completely different ways. One, described in Nature Physics, works like a turbine that’s pushed by the flow of the surrounding fluid.2 The other, described in Nature, spins autonomously like a wireless electric motor, powered by an AC electric field applied to the whole system.3