Dynamic equilibrium of β-conglycinin/lysozyme heteroprotein complex coacervates

Complex coacervate is a classic charged polymer material in food science, personal care, and cellular biology. Nevertheless, time is often an important factor that is overlooked in the study of complex coacervates. This study analyzed the dynamic equilibrium/relaxation behavior of β-conglycinin/lysozyme (β-CG/LYS) heteroprotein coacervates after mixing, especially the effect of NaCl concentration, total protein concentration, and pH on this equilibrium behavior, only using two simple but effective methods (turbidity kinetics and bright-field microscopy). Although the turbidity kinetics of the β-CG/LYS coacervates experienced rapid initial growth like soluble complexes and amorphous precipitates, it subsequently showed a unique decline behavior. Through bright-field microscopy, we found that the drop in turbidity was closely related to the dynamic equilibrium of β-CG/LYS coacervates, including the shape-transformation from non-spherical clusters to spherical coacervates and the gradual merging of small coacervates. Adding NaCl (0, 20, and 40 mM), increasing the total protein concentration (1 and 5 mg/mL), and lowering pH (6 and 8) all accelerated the dynamic equilibrium of the β-CG/LYS coacervate and reduced the relaxation time. Turbidity kinetics can be used as a screening method to evaluate the effect of various factors on the dynamic equilibrium/relaxation behavior of coacervates.