Molecular arrangement in polyelectrolyte complex coacervates

Polyelectrolyte complex coacervation is an associative liquid-liquid phase separation process in systems of oppositely charged polyelectrolytes driven by concomitant entropic processes of water reorganization and counterion release. Typically, droplets of the coacervate phase form suspended in a continuous supernatant phase. As the droplets contain a large volume fraction of water, and all their molecular constituents are individually water-soluble, the interfacial tension is very low and can be made extremely low by adding salt to approach a critical salt concentration. Therefore, complex coacervation has been known and used practically for decades as a method of encapsulation or surface adhesion, relying on these interfacial properties, albeit with readily available but poorly characterized constituents. In the last decade, this process has become the object of intense focus in the fundamental polymer physics community, and simultaneously, of the molecular biology community, in several respects. The polymer physics community has focused on the formation, structure and dynamics of polyelectrolyte complexes. The biology community has recognized that polyelectrolyte complexes give rise to many previously unrecognized compartments and condensates within cells. There is also an enduring curiosity about the possible role of complex coacervation in the formation of protocells with the capacity to evolve compartmentalized RNA synthesis in prebiotic environments.

Our own group has produced rigorous new data on phase diagrams and on the effects of chain charge density on phase separation. Recently published, or as-yet unpublished, work from our group has (1) demonstrated positional correlation between polyanions and polycations in coacervate phases via small angle neutron scattering; (2) shown that multi-component mixtures of different charge densities form multi-compartment coacervate droplets, which could be relevant to biological compartmentalization; (3) observed that exposure of coacervate droplets deionized water leads to the formation of electrostatic crosslinks on the interface of coacervate droplets that not only suppress droplet fusion indefinitely but also allow the spatiotemporal compartmentalization of RNA on a time scale of days, leading to speculation that such non-fusing membrane-less coacervate droplets could potentially act as protocells.