Bacteria, zooplankton, sperm cells, ciliates and flagellate protozoa swim along helical trajectories – they follow the threads of a screw . Although biologists still debate over the advantages of such a behavior, one reason for evolution selecting this specific mode of propulsion might be that helical motion shows better persistence in directionality than rectilinear motion.
Helical motility is so effective, in fact, that swimming cells also had to develop mechanisms of reorientation so that they could follow light or chemical cues, with patterns varying from tumbling and oscillations, to sharp reversals of direction. In many cases, chiral inversion supports these reorientation mechanisms, such as in the ‘run-and-tumble’ of E-coli where helix inversion disassembles the flagellum ; in the helix-based chemotaxis of sperm cells ; and the photo-tactic behavior of unicellular green algae that swim along helical trajectories towards the light, and invert the handedness of their helical trajectory to swim away from the light when it becomes too intense . Dynamic inversion of chirality is essential to the complex and purposeful motility of these systems.
Chemists, in aiming to unravel the interplay between chemistry and tactic motion, have designed and studied active molecular systems with motile patterns, yet those reported so far rarely display helical or other chiral features [5,6,7].
Our collaborators had investigated the steady-state, helical trajectories of chiral droplets before. In this collaborative work, we have now discovered that dynamic motility patterns are also encoded in chirality. The chiral droplets we studied are helix-based, with a spiral defect line that patterns the surface. The handedness of these spiral droplets inverts under illumination, provided that artificial molecular motors are used as photo-responsive chiral dopants. The video below shows how helix inversion happens in a droplet, albeit in different conditions as in the study.
Upon helix inversion, not only does the handedness of the trajectory invert, but also and simultaneously the droplets operate a deterministic reorientation (see the video here). We proved experimentally that both the helical motion and the ability to re-orientate can be described by the number of spiral turns on the droplet, that is associated with non-binary chirality.
We believe that this work where molecular motors steer proto-cellular constructs can contribute to a mechanistic understanding of bacterial behavior, based on the rules of physical chemistry. Moreover, there is growing evidence that the chirality of cells is the key to define their motility and consequently also their functionality. In this context, we think that the interplay between dynamic handedness and directionality of movement that we have found in active droplets could bring complexity and eventually also purpose to the motion of membrane-less synthetic compartments and artificial cells.
Find the paper here.
Blog post written by Federico Lancia, Alexander Ryabchun and Nathalie Katsonis.
 Wheeler, J. R. Use of chiral cell shape to ensure highly directional swimming in trypanosomes. PLoS Comput. Biol. 13, e1005353 (2017).
 Taute, K. M., Gude, S., Tans, S. J. & Shimizu, T. S. High-throughput 3D tracking of bacteria on a standard phase contrast microscope. Nat. Commun. 6, 8776 (2015).
 Sperm navigation along helical paths in 3D chemoattractant landscapes. Nat. Commun. 6, 7985 (2015)
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 Kumar, B.V.V.S.P., Patil, A.J. & Mann, S. Enzyme-powered motility in buoyant organoclay/DNA protocells. Nat. Chem. 10, 1154–1163 (2018).
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