Coacervation and aggregation in lysozyme/alginate mixtures

Abstract

Upon electrostatic interaction, the mixture of oppositely charged macromolecules separates into a macromolecule-rich dense phase coexisting with a diluted phase. The associative interaction proceeds through either liquid-liquid phase separation (LLPS) forming complex coacervates or liquid-solid phase separation (LSPS) forming aggregates. We here investigate the assembly of the basic protein lysozyme (LYS) with the negatively charged polysaccharide alginate (ALG) at pH 7 in different conditions of mixing ratios, total concentration, and ionic strength using a droplet-based millifluidic device. Aggregation and coacervation are observed in this system by changing the salt concentration. A 3D phase diagram, with concentration of salt, lysozyme, and alginate as 3D coordinates, gives a thorough description of monophasic, liquid-solid, and liquid-liquid phase separation areas, and the regions where both solid and liquid phases coexist. The thermodynamic aspects behind these two kinds of complex formation are investigated using isothermal titration calorimetry (ITC). Aggregation is associated with a strong affinity between LYS and ALG, with a 100 LYS: 1 ALG stoichiometry ratio, whereas a decreased binding affinity between the two biopolymers leads to coacervation.