Molecules that have chiral symmetry or handedness cannot be superimposed onto their mirror image, and this has far reaching effects in many areas of science. For example, rod-like, non-chiral molecules can form a “nematic” liquid-crystal phase that has long-range order. However, if a few atoms in the molecule are rearranged to make it chiral, this nematic phase can transform into a “cholesteric” phase that has very different physical properties.

Dogic and co-workers began by isolating flagellar filaments from Salmonella typhimurium, which they then labelled using a fluorescent dye. Bacteria use flagellar filaments – macromolecular structures that are made up of a single protein called flagellin – to “swim” and find food. The US team then suspended the filaments in aqueous solution.

The shape of the filament can be precisely controlled because it depends on the amino acid sequence in the flagellin, as well as the temperature and pH of the solution. For instance, the filaments can be changed from achiral rods to highly twisted helices that look like springs. Using a polarisation microscope, Dogic and colleagues found that helical filaments undergo a phase transition to a novel liquid crystalline state in which the flagella become cone-shaped for concentrations above a certain level. In contrast, this phase transition is not seen in experiments with rod-shaped filaments.

The results could also be used to model polymers: “Nature is very good at making structures with well-defined symmetry,” says Dogic. “We realised that we could use our purified flagella as ‘ideal polymers’ to test long-standing predictions of how the packing of chiral helices is different from that of achiral rods. There are no synthetic polymers that would permit us to do such experiments.”