Salt-dependent properties of a coacervate-like, self-assembled DNA liquid

TLDR

Coacervation, the liquid-liquid phase separation of polymer-rich phases, has been observed across diverse biomolecular systems, yet its properties remain poorly understood due to component complexity and interaction intricacies. This study investigates a single-component DNA nanostar system that condenses into a liquid via basepairing interactions and compares its behavior to other coacervates. The authors measured density, viscosity, particle self-diffusion, and surface tension of the nanostar liquid droplets. The DNA nanostar liquid’s density, viscosity, diffusion, and surface tension are governed mainly by sequence- and salt-dependent basepairing thermodynamics, with transport limited by single-bond breaking, few surface bonds, electrostatic screening compaction, a breakdown of the Stokes–Einstein relation, and a low surface tension that together imply a heterogeneous, clustered structure enabled by particle flexibility.

Abstract

Liquid-liquid phase separation of a polymer-rich phase from a polymer-dilute solution, known generally as coacervation, has been observed in a variety of biomolecular systems. Understanding of this process, and the properties of the resulting liquid, has been hampered in typical systems by the complexity of the components and of the intermolecular interactions. Here, we examine a single-component system comprised entirely of DNA, in which tetravalent DNA nanostar particles condense into liquids through attractive bonds formed from basepairing interactions. We measure the density, viscosity, particle self-diffusion, and surface tension of NS-liquid droplets. The sequence- and salt-dependent thermodynamics of basepairing accounts for most properties, particularly indicating that particle transport is an activated process whose barrier is the breaking of a single bond, and that very few bonds are broken at the surface. However, more complex effects are also seen. The relation of density to salt shows that electrostatic screening compacts the NS particles. Further, the interrelation of the transport properties indicates a breakdown of the Stokes-Einstein relation. This observation, in concert with the low surface tension and single-bond transport barrier, suggests this DNA liquid has a heterogeneous, clustered structure that is likely enabled by internal NS particle flexibility. We discuss these results in comparison to other coacervate systems.