Stabilization of Biomimetic Organic-Inorganic Polymeric Coacervates by Divalent Ions

Two aqueous liquid-liquid phase separating (LLPS) coacervates have attracted much research interest due to their relevance to the formation of various biomolecular condensates in the cell composed of mostly proteins and nucleic acids and the origin of life inside the membraneless protocells. Ripening and coalescence are often considered as the hallmark of the liquid behavior of dense coacervate droplets. However, some recent studies have shown that some complex coacervates exhibit high stability without Ostwald ripening. Proteins, peptides, and some small molecules can even adsorb on the droplet surfaces to achieve coacervate stability. In this work, we have investigated the effect of added multivalent salts on the interaction and stability of biomimetic inorganic-organic polymeric coacervates. We have previously demonstrated the coacervate formation between inorganic polytungstate nanoclusters bearing 8 negative charges, {W₁₂} and neutral polyethylene oxide (PEO) in LiCl aqueous solution. By adding divalent cations, such as Ca²⁺ and Sr²⁺ above a critical concentration (>100 mM), we have observed that PEO-{W₁₂} coacervate droplets can be stabilized in salted water over a long time period (>overnight) without ripening. Moreover, it is interesting to observe by real-time confocal microscopy that the coacervate droplets can be attracted and move directionally to each other into intimate contact yet without coalescence. Interfacial molecular exchange upon droplet contacts and necking is examined by dual-color fluorescence microscopy. We speculate that such multivalent counterion mediated electrostatic interaction and stabilization of coacervates could give insight to the interfacial composition and structural organization of LLPS-based protocells.