“We also discovered that these fibrils undergo a liquid-crystalline transition from the untwisted, regular nematic to cholesteric phases, depending on the volume of the liquid-crystalline droplet,” says team leader Raffaele Mezzenga. “The chirality of the fibrils also inverses from left to right handedness as they go to the cholesteric phase. This behaviour is quite different from that of all other classes of biological filamentous chiral colloids in which the chirality evolves in the opposite way, that is, from right-to-left, or not inverted at all (right-to-right, for example).”

Chirality or handedness is ubiquitous in nature and plays a critical role in biology, medicine, physics and materials science. It refers to a property of structures that exist in two versions – “enantiomers” - that are mirror images of each other but cannot be superimposed. Natural chirality is highly selective and shows distinct preferences. “For example, only D-sugars are included in the formation of DNA, and only L-amino acids in the formation of proteins,” says Mezzenga. “This molecular chirality transfers in a way to control both structure and biological function.”

Experiments on such systems may lead to a better understanding of the mechanisms behind chirality transfer and therefore directly impact on the design of new materials that mimic biological structures, he adds.

Cholesteric phases in amyloids

Amyloid fibrils form into twisted ribbon-like structures through the self-assembly of beta-sheet aggregates. Pathological amyloids are often found in patients with neurodegenerative diseases such as Parkinson’s or Alzheimer’s while functional amyloids are crucial for physical and biological function in living organisms. Researchers are now discovering that they may be used as versatile platforms for making new functional biomaterials too.

Until now, no one had ever seen any chiral colloidal liquid-crystalline phases, also known as the cholesteric phase, in these fibrils. “This was rather puzzling,” says Mezzenga, “because the fibrils have a well-defined chirality at the single fibril level – as do other filamentous biological systems such as DNA, collagen or nanocellulose. Here such chiral nematic phases are regularly observed.

“We spent a lot of time hunting for these cholesteric phases in amyloids, trying out a number of approaches,” he explains; “What finally worked in the end was to break down the amyloids into smaller pieces and then search for the cholesteric droplets at compositions that are predicted by thermodynamics calculations.

Extremely rich phase diagram

“Apart from the unconventional chiral switch pathways (left to right handedness as opposed to the other way around), these systems boast an extremely rich phase diagram in which we can identify at least three types of liquid-crystalline droplets. The cholesteric phase is only one of these three classes. Such rich phase behaviour is unprecedented within a single system.”

The researchers used energy functional theory to try and help explain their results. “We expanded theoretical treatments developed for non-chiral nematic droplets, to the case where the colloidal rods are chiral and thus can undergo collective twisting behaviour,” Mezzenga tells nanotechweb.org. “The extended theory we developed allows us to explain many of our observed experimental findings.”

As well as furthering our fundamental understanding of chiral transfer and helping to develop new materials that mimic biological structures, the new work could have practical applications. “Cholesteric droplets selectively interact and reflect light that is circularly polarized as opposed to normal polarizers that only interact with linearly polarized light,” explains Mezzenga. “This very appealing characteristic might be exploited in cholesteric liquid-crystal displays (ChLCDs) that could be operated with virtually no power. Another immediate possibility is to make photonic devices in which combinations of colours are produced by the combined effect of photonic bandgap and cholesteric liquid-crystal reflection.”

Surface anchoring effects?

This is a very interesting article, comments Rik Wensink of the Laboratoire de Physique des Solides at CNRS and Université Paris-Sud, who was not involved in this work. “The observations hint at a subtle role played by surface anchoring effects in stabilizing cholesteric order in these systems, which is not yet fully understood.”

So, where next? “There are many areas that we are currently exploring,” says Mezzenga. “For example, we are still trying to understand other features of these amyloid cholesterics that are still not completely clear and trying to determine how different parameters affect their overall liquid-crystalline order.”

The research is detailed in Nature Nanotechnology doi:10.1038/s41565-018-0071-9