The molecular motor activated by light. Image credit: University of Groningen/RUGThe molecular motor activated by light. Image credit: University of Groningen/RUG

By using microwaves to unravel the exact structure of a tiny molecular motor, a team of scientists is paving the way to investigate nano-machines in action.

The nano-machine they studied consists of a single molecule, made up of 27 carbon and 20 hydrogen atoms (C27H20). Similar to a macroscopic motor, it has a stator, a rotor and an axle to connect them. It was synthesized by the team of the University of Groningen’s Ben Feringa, one of the recipients of the 2016 Nobel Prize in Chemistry for the design and synthesis of molecular machines.

The researchers’ analysis reveals just how the individual parts of the motor are constructed and arranged with respect to each other. The work holds promise for numerous applications.

Sérgio Domingos of the Deutsches Elektronen-Synchrotron (DESY) and the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) is first author of the team’s paper, published in the journal Angewandte Chemie International Edition. Domingos notes that the nano-machine operates through consecutive photochemical and thermal steps when activated by light, completing a half turn. A second trigger can then force the motor into completing a full turn and return to its starting position.

"Such an activation by light is ideal as it provides a non-invasive and highly localized means to remotely activate the motor," Domingos said. "It could be used, for instance, as an efficient motor function that can be integrated with a drug, establishing control over its action and release it at a precisely targeted spot in the body: the light-activated drugs of the future.”

The motor molecule’s atomic make-up had previously been investigated with X-rays, which necessitated a preliminary step of growing the molecules into crystals. In the new study, the researchers were able to look at free-floating, isolated molecules. Research team leader and DESY Leading Scientist Melanie Schnell explained that “this way we can see the molecule as it is, free from any external influences like solvents or bindings.”

By using a resonant microwave field to orient the free-floating molecules in the same direction and then recording their relaxation when the field is switched off, the rotational constants of the molecules were revealed — giving accurate information about structural arrangement. In order to float the molecules in the microwave chamber, they had to be heated to 180 degrees Celsius before being cooled down rapidly to negative 271 degrees Celsius. "Heating made some of the motors fall apart, breaking at the axle," said Domingos. "This way we could see the rotor and the stator independently of each other, confirming their structures. This also provides us with some hint about the mechanism via which it falls apart.”

The microwave technique allows the researchers to study the dynamics of the motor molecule like never before. "Now that we can see the molecule like it really is, we want to catch it in action," adds Domingos. The researchers are currently planning microwave spectroscopy studies for further investigation of how the molecular motor works.