Amyloid fibrils are rigid, elongated protein aggregates that form condensed phases in functional biological assemblies and pathological deposits. From a colloidal physics perspective, two separation pathways can lead to these condensed phases: liquid-liquid phase separation (LLPS), which results from a trade-off between entropy and enthalpy, and liquid-liquid crystalline phase separation (LLCPS), an essentially entropic process in which the condensed phase is a nematic liquid following precise symmetry rules. The interplay between these pathways in amyloid fibril dispersions is debated. Here we show that lysozyme and β-lactoglobulin amyloid fibrils dominantly undergo LLCPS but exhibit a pH-dependent transition to LLPS. Particularly, increasing pH lowers the critical concentration for formation of nematic condensates until, near isoelectric point, where condensates without a coherent nematic order form. This behavior is consistent with the classical picture of rigid rods retaining nematic order once formed, with enthalpic attraction overcoming entropic ordering at low charge densities. Our work establishes that LLCPS and LLPS in amyloid fibrils are separable and provides a framework for controlling fibril organization using simple solution conditions. The competition between LLCPS and LLPS may clarify the mesoscopic organization of amyloid fibrils in condensates in vivo and direct design principles of multiphase amyloid-based materials.
Radiom et al. (Wed,) studied this question.