Control of liquid crystallinity of amyloid-forming systems

Many proteins are known to be capable of forming amyloid fibrils, long thin fibrils that, if they possess sufficiently large aspect ratios and volume fractions, have been shown to form liquid crystals in several protein systems. However, denatured proteins exhibit polymorphism as they aggregate, and external factors affect which type of structure forms. Here we demonstrate the ability to form liquid crystalline phases in three different protein systems, without the need for the prior formation of fibrils, by the use of mechanical shear during the aggregation step to modify the aggregation process. It is shown that the rate of stirring affects the aspect ratio of the amyloid fibrils formed in vitro, and hence the ability to form liquid crystalline phases, as predicted by theory. A shift is observed with increasing stirring rate from the large spherulitic structures (structures formed via suprafibrillar aggregation under highly acidic conditions) to the alignment of amyloid fibrils into a nematic liquid crystal phase in insulin, β-lactoglobulin (BLG) and hen egg white lysozyme (HEWL). Because the stirring modifies the fibril formation, above a threshold stirring rate, the aspect ratio of insulin and BLG fibrils drops below the point at which liquid crystal-like phases can form. However, in the case of HEWL, the decrease in the fibril aspect ratio at higher stirring rates is still sufficiently small to enable liquid crystalline phases to form. The overall phase behaviour can be rationalised within the Onsager model for liquid crystallinity.