Cooperation and competition of basepairing and electrostatic interactions in mixtures of DNA nanostars and polylysine

TL;DR

This study investigates how electrostatic interactions and base pairing collaboratively influence phase separation in DNA nanostar and polylysine mixtures, revealing complex behaviors modulated by salt and temperature.

Contribution

It combines experimental and theoretical approaches to map the phase behavior of DNA nanostars and polylysine, highlighting the cooperative effects of electrostatics and base pairing.

Findings

Electrostatics and base pairing cooperate to stabilize coacervation at high salt and temperature.

Multiple kinetic pathways lead to nonequilibrium aggregates or droplets.

Immiscible coacervates can be created by tuning salt concentrations.

Abstract

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 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 (NS) as a function 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 timescales. 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.