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An optically driven microstructure for torque measurement in rotary molecular motors

What is it about?

‘Light-mills’ are optically driven microstructures that can exchange orbital angular momentum with light [1,2] and thus rotate around a central axis with a controlled applied torque. Light-mills can be selectively coupled to a target nanometric flagellar motor in a live E. coli cell, functioning as an external, tunable torque clamp. This allowed us to slow, stall and even reverse the flagellar motor while simultaneously measuring both its rotational speed and the torque it generates.

 Why is it important?

The torque-speed curve is the most relevant phenomenological feature of any motor [3]. Our method therefore represents a powerful new tool for the study of rotary motors. Although many studies have explored the employment of light momentum for torque generation [4], only a few convincing applications in cellular and molecular biology have been demonstrated, and none of those has been successfully applied to bacterial flagellar motors, to our knowledge

 Perspectives

Optical tweezers have proven incredibly useful for characterising force and displacement in many molecular motors. However, torque generation and rotational motion also play crucial roles in many of these machines. Moreover, the class of known rotary motors is rapidly expanding [5] and still lacks a comprehensive toolbox for torque spectroscopy. Our light-mills, being able to apply a constant and reliable optical torque, have the potential to become a valuable tool for studying a broad range of molecular motors.

Video

In this video, we show the moment when the light-mill is attached to a 1.6 µm diameter polystyrene bead using an optical tweezer, which is employed both to drive and to displace it. We gently approach the rotating bead from above. Since the bead is rotating, we know it is coupled to a flagellar motor. When we switch off the optical trap, the light-mill reverses its direction of rotation. The natural rotation of the motor is counterclockwise, whereas the light-mill is optically driven clockwise. Therefore, two competing torques act on the micro-structure.

Giacomo Donini
PhD, Department of physics, Sapienza, Rome.
Email: giacomo.donini@uniroma1.it            
Link to the  paper:  
"An optically driven microstructure for torque measurement in rotary molecular motors." Donini, Giacomo, et al. Microsystems & Nanoengineering 12.1 (2026): 48.
https://doi.org/10.1038/s41378-026-01185-5