Phase separation via simultaneous interactions: the cooperation of electrostatics and base pairing

We investigate the interplay of two specific interactions, base pairing and electrostatics, in driving biomolecular phase separation. Particularly, we study condensation of a mixture containing poly-L-Lysine (PLL) and branched, self-hybridizing DNA particles. Electrostatic-driven phase separation arises from the attraction between PLL and DNA. Simultaneously, the DNA forms branched structures, nanostars (NSs), which are capable of base-pairing via self-complementary, single-stranded sequences at the end of each arm. We observe four distinct phases using fluorescent imaging of material structures at varying salt concentrations, charge ratios, and temperatures. This includes a complex coacervate-like transition from gel-like aggregates to liquid droplets with increasing salt; a high salt limit where the NSs form liquid drops that exclude PLL; and a regime where NS-only droplets coexist with NS/PLL coacervates. Overall, this work finds an extraordinary diversity of phase behaviors over a relatively narrow range of conditions, apparently due to a subtle balance between the electrostatic and base pairing interactions.