Micro- and macro-phase behavior in protein-polyelectrolyte complexes

Negatively charged polyelectrolytes such as carboxymethylcellulose, pectin, and alginate are commonly present in food products. These polyelectrolytes serve a variety of functions such as controlling viscosity and stabilizing emulsions. Proteins are also present in many food formulations. Because of their high charge density, polyelectrolytes can be expected to interact wi th these proteins. Hence, an understanding of the parameters controlling pr otein-poryelectrolyte interactions is useful. The formation of protein-polyelectrolyte complexes (PCs) is driven by elect rostatic interactions, and electrostatic parameters such as protein surface charge density, polymer linear charge density, and ionic strength can infl uence both the formation and the stability of the PPC. However, the electro static attractive forces in PPCs are countered by a loss in polyelectrolyte conformational freedom. Hence, polymer chain parameters, e.g. the inherent chain stiffness (bare persistence length), are also important. For some pr otein-polyelectrolyte systems, the loss of polymer conformational freedom c an become so large as to overcome electrostatic effects and prohibit PPC fo rmation. In addition to electrostatic effects, the structure of PPCs is also control led by stoichiometry, i.e. the average number of bound proteins per polymer chain (n) increases with total protein concentration. At the same time, ch anges in the pH and ionic strength will be reflected in the mass action equ ilibrium constant, with an increase in the electrostatic interaction energy shifting the equilibrium (PrUnbound --> PrBound) toward the bound state. Macroscopic phase separation can arise when PPCs are intrinsically insolubl e due to local charge neutralization. It can also be a consequence of the a ggregation of electroneutral soluble PPCs. Hence, an understanding of the i nteractions and composition of the soluble complexes will enhance our abili ty to predict and control phase behavior in protein-polyelectrolyte systems .