Abstract
Liquid-liquid phase separation has emerged as one of the important paradigms in the chemical physics as well as biophysics of charged macromolecular systems. We elucidate an equilibrium phase separation mechanism based on charge regulation, i.e., protonation-deprotonation equilibria controlled by pH, in an idealized macroion system which can serve as a proxy for simple coacervation. First, a low-density density-functional calculation reveals the dominance of two-particle configurations coupled by ion adsorption on neighboring macroions. Then a binary cell model, solved on the Debye-H\"uckel as well as the full nonlinear Poisson-Boltzmann level, unveils the charge-symmetry breaking as inducing the phase separation between low- and high-density phases as a function of pH. These results can be identified as a charge symmetry broken complex coacervation between chemically identical macroions.
Highlights
The importance of complex coacervation in polymers, colloids, and proteins that exhibit an associative liquid-liquid phase separation (LLPS), driven by electrostatic interactions between oppositely charged macroions, has been recognized for about a century [1,2], though its fundamental role in compartmentalization and intracellular phase transitions in biological systems has been identified only recently [3]
We elucidate an equilibrium phase separation mechanism based on charge regulation, i.e., protonation-deprotonation equilibria controlled by pH, in an idealized macroion system which can serve as a proxy for simple coacervation
A binary cell model, solved on the Debye-Hückel as well as the full nonlinear Poisson-Boltzmann level, unveils the charge symmetry breaking as inducing the phase separation between low- and high-density phases as a function of pH
Summary
The importance of complex coacervation in polymers, colloids, and proteins that exhibit an associative liquid-liquid phase separation (LLPS), driven by electrostatic interactions between oppositely charged macroions, has been recognized for about a century [1,2], though its fundamental role in compartmentalization and intracellular phase transitions in biological systems has been identified only recently [3]. The dependence of the associative LLPS on the bathing environment conditions, such as the solution pH [14], has lacked a comprehensive theoretical elucidation based on relevant microscopic models That these effects are important in protein solutions [12] is clear from the fact that the protein charge is not fixed, but is a result of the proton-mediated dissociation of amino-acid (AA) groups at the solvent accessible surface [15], whose chemical equilibrium depends on the bathing environment parameters such as the solution pH [16]. We show that the LLPS is based on a symmetry breaking transition of the macroion charge distribution, characterized by a spatially alternating sign of the macroion charge In that respect this CR system driving a complex coacervation behaves not unlike the alternating multilayer structure of the electrical double layer in ionic liquids [26], except that here the charge alternation is driven by CR and not by the presence of different ion species. We identify this spatial charge layering, stemming from a symmetry broken charge distribution and leading to phase behavior that exhibits features of complex coacervation phenomenology, as charge symmetry broken complex coacervation between chemically identical macroions
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