  In many biopolymer solutions, attractive interactions that stabilize
finite-sized clusters at low concentrations also promote phase separation at
high concentrations. Here we study a model biopolymer system that exhibits the
opposite behavior: Self-assembly of DNA oligonucleotides into finite-sized,
stoichiometric clusters, known as "DNA nanostars", tends to inhibit phase
separation of the oligonucleotides at high temperatures. We use
microfluidics-based experiments to map the phase behavior of DNA nanostars at
high concentrations of divalent cations, revealing a novel phase transition in
which the oligonucleotides condense upon increasing temperature. We then show
that a theoretical model of competition between self-assembly and phase
separation quantitatively predicts changes in experimental phase diagrams
arising from DNA sequence perturbations. Our results point to a general
mechanism by which self-assembly shapes phase boundaries in complex biopolymer
solutions.
