Like the engines that power almost everything in daily life, the microscopic world contains “motors,” organic mechanisms that power tasks like transcribing RNA to DNA, synthesizing ATP and transferring cargo from one place to another in biological cells. Because of their size, these molecular motors don’t work like their macro-sized counterparts, and the processes that energize them have been largely inscrutable — until now.

A scientist from the University of Maine teamed with researchers from Northwestern University and the California Institute of Technology to make, understand and model an electrically driven artificial molecular motor, a discovery that could lead the way to developing a host of complex synthetic molecular machines.

Macroscopic motors — complex ones, like car engines, or simpler ones, like those in blenders — all function in fundamentally the same way: an outside force causes the parts of the motor to move in a particular way, with a well-defined direction. In a car, for example, combusting gas pushes a piston that rotates the crankshaft, which turns the gears that move the vehicle.

Smaller motors follow this model less reliably. At microscopic scales, factors that are normally negligible like thermal noise — the tiny disturbances that influence all particles, from electrons to atoms to molecules and beyond — become more significant. Molecules are moving randomly all the time because of a phenomenon called Brownian motion. For this reason, targeting specific, desired movements is extremely difficult. 

Since the 1990s, scientists have been looking to chemistry, rather than mechanical engineering, to understand molecular motors by taking advantage of the fact that chemical stimulation can change the stability of molecules in such a way that it shepherds their components in a specific sequence and direction.

Fraser Stoddart, professor of chemistry at Northwestern University and 2016 Nobel Laureate, led a team that aimed to make a molecular electric motor. They focused on a certain type of molecule with interlocking rings known as catenanes held together by powerful mechanical bonds, so the components could move freely relative to each other without falling apart. 

In initial tests, a single small ring molecule was interlocked onto a larger ring molecule on which a “pumping cassette,” a group of two elements which each impeded the rotation of the smaller ring flanking a switchable binding site that can be turned on and off depending on the charge of the small ring. However, the simple molecule did not display any significant directional motion even when provided with electrical energy to switch the binding site off and on. 

R. Dean Astumian, a theorist and professor of physics and astronomy at the University of Maine, collaborated with Stoddart’s experimental team to design a new catenane motor: one large ring interlocked with two smaller rings with two binding sites, instead of one. With this design, the researchers predicted that the small rings could move on and off the binding sites with directionality imposed by the repulsion between the two small rings and by the chemical groups in the pumping cassette that impede the movement of the small charged rings, either electrostatically or based on size. These barriers help prevent the rings from moving backward, and the desired forward motion is caused by the random Brownian motion.

“With the initial system, Brownian motion was causing the ring to move back and forth, but with no directional preference even when powered by electricity,” Astumian says. “The elaboration of the motor to include a second small ring for three rings total introduced electrostatic interactions that selected out certain concerted motions of the two rings that, when powered by electricity, resulted in directional rotation.”

The group synthesized the three-ring catenane and demonstrated that the electric motor operated through the repeated removal and addition of electrons to the rings (a process known as oxidation and reduction), which changed the electric charge on the rings. Each step resulted in a change of the orientation of the system with respect to the two barriers, which differentially allowed the small rings to pass over one or the other depending on the charge of the ring. A full cycle of oxidation and reduction between the two smaller rings of the molecule leads to the rings completing a 180 degree circumrotation. The motor, in effect, turned — and reliably, nearly 85% of the time.

In order to better understand the details of the energetics by which directed rotation occurs, William Goddard and his colleague Wei-Guang Liu at the California Institute of Technology carried out detailed Quantum Mechanical calculation of the energies of the various states of the motor molecule to confirm the understanding of the mechanism that had been developed. 

The study and a corresponding news brief were published Jan. 11, 2023, in Nature.

The findings both further the understanding of biological molecular motors, and may help to develop synthetic molecular machines that can possibly synthesize molecules that scientists have not yet been able to make with current methods. Other potential applications range from controllable drug delivery to fabricating wearables that amplify the effect of human muscles.

Astumian says that the field of developing synthetic molecular machines is “still in its infancy,” but “the future is bright” if experimental chemists, theoretical chemical physicists and computational chemists continue to collaborate on research like this.

“All of these applications are obviously rather far off at present, but it was only a few decades between the Wright brothers first flight at Kitty Hawk and the first commercial airline, so who knows,” Astumian says.