A binding model for the precipitation of proteins by carboxymethyl cellulose

Abstract

The precipitation of ovalbumin andlysozyme by carboxymethyl cellulose (CMC) is modeled as a multiple equilibrium binding phenomenon. The binding is shown to be negatively cooperative, with the cooperativity resulting from electrostatic interactions between the oppositely charged protein and polymer. The binding constant is the product of a charge-independent quantity, the intrinsic binding constant, and a cooperativity function. The ionic strength and pH dependence of the binding constant is modeled using the Debye-Huckel theory to account for the electrostatic free energy of the interacting species. The intrinsic association constant varies linearly with ionic strength; the slopes of these plots are the sum of hydrophobic and electrostatic (ion-dipole) contributions. For proteins with a high surface charge density, the ion-dipole interaction dominates, but for proteins with lower charge density, hydrophobic considerations become more important. The resolubilization of protein-polyelectrolyte precipitates at high polymer dosage levels is seen to occur at a high complex charge density. This residual charge hinders aggregation of the complexes to form insoluble primary particles, resulting in lower protein recoveries.