Macromolecular association creates dispersity of components that determines liquid-liquid phase separation of oppositely charged polyions.

Developing insight into liquid-liquid phase separation in complex polyelectrolyte mixtures has acquired new urgency considering the role this process plays in the formation of biomolecular condensates, which are involved in diverse processes including, stress response, signal transduction, metabolism, gene regulation, and many human diseases. Condensates can be viewed as a biological membraneless structure situated, in scale and function, between macromolecules and cells. The physical properties of, and the physico-chemical environments within, condensates are very important determinants of their biological function. These properties and environments are determined by the phase separation process. Earlier experimental work in one of our laboratories characterized in detail a phase separation process in solutions of oppositely charged polypeptides. Considering new information being developed on macromolecular association in homogeneous solutions of oppositely charged polyions, we have revisited our earlier data looking into the influence of intermolecular association on the outcome of the phase transition. Such influence is clearly established in this Brief Report. We demonstrate that dispersity of species in homogeneous solution owing to macromolecular association affects phase separation by creating an open miscibility gap, leading to a dependence of the phase compositions on the initial polymer volume fraction. It is very likely that this plays a significant role in the formation of condensates in the biological context that may have been overlooked.