Cooperation and Competition of Base Pairing and Electrostatic Interactions in Mixtures of DNA Nanostars and Polylysine.

Phase separation in biomolecular mixtures can result from multiple physical interactions, which may act either complementarily or antagonistically. In the case of protein-nucleic acid mixtures, charge plays a key role but can have contrasting effects on phase behavior. Attractive electrostatic interactions between oppositely charged macromolecules are screened by added salt, reducing the driving force for coacervation. By contrast, base pairing interactions between nucleic acids are diminished by charge repulsion and are thus enhanced by added salt, promoting associative phase separation. To explore this interplay, we combine experiment and theory to map the complex phase behavior of a model solution of poly-l-lysine (PLL) and self-complementary DNA nanostars (NSs) as functions of temperature, ionic strength, and macromolecular composition. Despite having opposite salt dependences, we find that electrostatics and base pairing cooperate to stabilize NS-PLL coacervation at high ionic strengths and temperatures, leading to two- or three-phase coexistence under various conditions. We further observe a variety of kinetic pathways to phase separation at different salt concentrations, resulting in the formation of nonequilibrium aggregates or droplets whose compositions evolve on long time scales. Finally, we show that the cooperativity between electrostatics and base pairing can be used to create immiscible coacervates that partition various NS species at intermediate salt concentrations. Our results illustrate how the interplay between distinct interaction modes can greatly increase the complexity of the phase behavior relative to systems with a single type of interaction.