Physicists built a molecular motor entirely from strands of DNA, and used it to store energy by filtering a DNA ‘spring’.
It’s not the first DNA nanoengineer, but it is “certainly the first to do measurable mechanical work,” says Hendrik Dietz, a biophysicist at the Technical University of Munich in Germany whose team announced the results on July 20 in temper nature1. This technique adds to a growing list of “DNA origami” tricks that are being used to build structures at the molecular level. The approach aims to find applications in areas such as chemical synthesis and drug delivery.
Living cells are filled with molecular machines, including rotary actuators; These perform a range of tasks, from shaking the flagella of bacteria to producing the ATP molecules that make up the cell’s energy stores. These drives often use ratchet mechanisms, similar to sprockets in a clockwork mechanism that allow rotation in one direction but not the other.
Like everything else in the cell, biological machines are constantly moved as a result of Brownian motion – the constant random movement of molecules and other molecules in the cytoplasm. Often when particles collide with each other, they can transfer a “kick” of energy to one another.
Dietz and his colleagues wanted to design a DNA motor that could be driven by Brownian motion in a similar way to the protein-based machines found in cells. In the DNA origami technique they used, loops of single-stranded DNA from a phage virus are mixed together in a solution with short strands of synthetic DNA; These are made to match the nuclear nucleolar sequences of specific sites in the viral genome. The short pieces are attached to the long threads and force them to be folded into the desired shape. Since this technology was first demonstrated in 20062Researchers have built DNA origins of increasing complexity.
Kicks and bumps
Dietz and his team built triangular pads of DNA, each with a rod sticking out from the middle. They fixed these structures to a glass surface and added long arms of DNA, which were attached to the platforms in a way that allowed them to rotate around the rod.
To create the ratchet effect, the researchers designed platforms with bumps that made this rotation more difficult. Only the kicks provided by the Brownian movement enabled the arms to overcome bumps and rotate, usually by half a turn.
Without any additional intervention, the rotation will swing back and forth randomly. So the team also dipped two electrodes in the solution and ran an electric current in alternating directions. The altered voltage altered the energy landscape experienced by the long DNA arms, and made rotation in one direction more convenient, through a mechanism known as a blinking Brownian ratchet.
This turns passive devices into actual motors. The micrographs showed that under these conditions, each arm—despite random vibration—continued to rotate in the same direction on average. (The orientation depends on the exact orientation of the trigonometric base with respect to the electrodes.)
like a wind clock
By itself, the nanoactuator does nothing more than overcome the resistance of the surrounding solution. “It’s like you’re swimming: You’re going forward and doing a lot of work, dissipating in the water,” Dietz says. But to prove it could also do potentially useful work, the researchers took another step: They attached another string of DNA to their rotor and made it spin like the solenoid spring used to turn gears in a mechanical watch. Such a mechanism could help nanomachines store energy or pull other mechanical components, Dietz says.
“It’s a fantastic achievement for the team, first and foremost for them to be able to design a system that turns into a complex and functional structure using DNA origami, and second, to be able to accurately describe its dynamics,” says David Lee, a chemist at the University of Manchester, UK. Using an entirely different approach, this year Lee and his team demonstrated an atomic-level rotor motor, which orbits around a single molecular bond.3.