Determinants of Phase-Separation Propensities, Material States, and Material Properties of Biomolecular Condensates

Abstract

Phase separation of various materials has been studied for one and a half centuries, laying foundational principles in materials science. In the last two decades, phase separation of proteins and nucleic acids has received enormous attention, due to its relevance to cellular functions. Despite this explosion of empirical data, many observations on the resulting biomolecular condensates lack a rigorous theoretical underpinning. The first goal of this Account is to put forward theoretical frameworks for the phase-separation propensities, material states, and material properties of biomolecular condensates. Using these frameworks, I rationalize mechanistic interpretations in our recent experimental and computational studies, and synthesize these studies with prior literature to draw novel, unifying conclusions. To evaluate phase-separation propensities, I establish a direct relationship between the threshold (or saturation) concentration (Cth) and the excess chemical potential in the dense phase (μIIex). μIIex is fundamentally dictated by the collective strength and valency of intermolecular interactions. The relationship between Cth and μIIex readily rationalizes the effects of amino-acid composition and solvent conditions like temperature, pH, and salt on phase-separation propensities. Regarding material states, I posit that liquid droplets represent the completion of phase separation, whereas other states─including amorphous dense liquids, reversible aggregates, and gels─arise from premature termination of spinodal decomposition. Specifically, amorphous dense liquids occur near the critical point and early during spinodal decomposition, kinetically trapped as overly weak interactions prevent further condensation. Gels and aggregates are different forms of dynamically arrested states, one driven by directional interactions that favor tip growth, extending outward into system-spanning networks, the other driven by monomer addition at interior sites to maximize valency, producing densely packed, amorphous particles. For material properties, I highlight the crucial roles of the stress relaxation time (τ1), which is determined by the mean lifetime of intermolecular bonds in a condensate. This relaxation time dictates how the condensate manifests viscoelasticity, including shear thickening and shear thinning, and accounts for the wide variation in zero-shear viscosity (ηz) among different condensates. Indeed, I propose a correlation between ηz and τ1, which is supported by available data. I also devise a theoretical model to explain the relative constancy of surface tension. Given this constancy, the inverse fusion speed can be used as an indicator of viscosity. Validating and expanding these conclusions will further strengthen the theoretical foundations of the biomolecular condensate field. Looking forward, several high-impact opportunities emerge. First, it may be possible to bridge the persistent gap between all-atom molecular dynamics (MD) simulations and in-vitro experiments, by extending the size and time limits of MD simulations via taking advantage of deep learning. Second, expanding the experimental toolkit, by borrowing techniques from materials research, will further push the study of biomolecular condensates from mesoscales to molecular scales. Finally, testing mechanistic hypotheses inside cells will connect the physics of phase separation to cellular functions.