Phase Separation of Positively Charged Polypeptide Solutions: Interplay between Electrostatics and Nonelectrostatic Specific Interactions

The liquid-liquid phase separation (LLPS) of biomacromolecules governs various biological functions, such as ribosome biogenesis and cellular stress response. The understanding of the underlying driving forces, however, remains elusive. Here, we develop a simple mean-field theory to systematically examine how electrostatics and nonelectrostatic specific interactions influence the LLPS of positively charged polypeptide solutions. Our theory combines the generalized Debye-Hückel correlation, first-order perturbation for chain connectivity, and the sticker-spacer model for specific interactions. In salt-free systems, phase separation at a moderate charge fraction requires strong electrostatic correlation. For salty solutions without specific interactions, the miscibility gap exhibits a closed-loop feature on the polypeptide-salt concentration surface. Increasing the specific interaction strength or salt concentration can promote phase separation in otherwise homogeneous solutions, in good agreement with previous experiments and our molecular dynamics simulations. Our study reveals the intricate interplay among counterion entropy, excluded volume, electrostatic correlation, and specific interactions in regulating the LLPS of polypeptide solutions.