Protein/polyelectrolyte coacervation: Investigating its occurrence in the lysozyme- carboxymethylcellulose system

The coacervation of carboxymethylcellulose (CMC) and hen egg white lysozyme (HEWL) was investigated. The work focused on the effects of pH, ionic strength, I, temperature, T, and mass fraction of the macromolecular components on the coacervation process by spectrophotometry, and on characteristics of the resulting coacervate phase by rheology in the linear and non-linear regimes. Coacervation was found to be highest at HEWL mass fraction ≈0.25 with very slight dependence on pH in the range from 5 to 9. The process was favored at I < 0.075 mol·L^-1 for NaCl, KCl and NaBr. For BaCl₂, the dependence was markedly distinct, reflecting differences brought about by a bivalent cation on the Debye length of the electric double layer. The coacervation process has been characterized as endothermic and entropically driven. Rheological analysis of the coacervates indicated a rather low limit of the linear viscoelastic region (0.1% strain) and the prevalence of the elastic over the viscous contribution (G´ > G´´). Despite configuring rather fragile structures, the presence of a network of strong interactions has been inferred from the observed pattern of variation of the moduli with frequency. In the non-linear regime, pseudoplasticity was observed, pointing out to the capacity of these structures in complying with the flow lines of the circumventing environment. Both apparently opposed characteristics suggest a possible evolutionary advantage of such structures in the prebiotic era.