Interfacial Structure of Positively Charged Polypeptide Condensates

The interface of biomolecular condensates plays a crucial role in droplet stability, growth, and biochemical function, yet their properties remain poorly understood. Here, we develop an inhomogeneous mean-field theory to investigate the interfacial structure of condensates formed by polypeptides containing positively charged, polar, and aromatic residues in aqueous solutions. Our study focuses on the synergistic interplay between electrostatic interactions among charged residues and specific nonelectrostatic interactions among aromatics. We systematically explore how charge fraction α, aromatic fraction β, specific interaction strength ε_s, and salt concentration ρ_s regulate interfacial organization in aqueous solutions. At finite α, the condensate interface typically exhibits an electric double-layer structure with a positively charged periphery surrounded by a diffuse counterion cloud. In the absence of added salt, the polypeptide concentration profile develops a depletion layer on the supernatant side and an overshoot on the coacervate side of the interface. Increasing ρ_s narrows the interfacial width and weakens the local charge separation in condensates stabilized by specific attractions. The electrostatic potential displays an asymmetric diffuse profile across the interface. Overall, our predictions are qualitatively consistent with experimental observations, and the insights provided here may aid the rational design of biomolecular condensates with tailored interfacial properties.