EFFECTS OF PROTEIN CHARGE HETEROGENEITY IN PROTEIN-POLYELECTROLYTE COMPLEXATION

The interaction between globular proteins and synthetic polyelectrolytes was investigated by turbidimetry and quasielastic light scattering (QELS) techniques, in 0.10 M NaCl solutions. pH titrations were used to induce the binding of three monomeric proteins of substantially different isoelectric points (bovine serum albumin, chicken egg lysozyme, and bovine pancreas ribonuclease) to cationic and anionic polyelectrolytes with a range of charge densities. The onset of association upon addition of HCl to protein/polyanion solutions, or of NaOH to protein/polycation solutions, occurs at a well-defined pH for each polymer/protein pair. This critical pH, which could be detected by either QELS or turbidimetry, corresponds to the formation of soluble polyion-protein complexes and is followed, upon progressive pH change, by phase separation (complex coacervation). Using known protein pH titration curves, the net charge on the protein at critical conditions, Z(c), could be calculated, and it was found that Z(c) is frequently identical in sign to the charge on the polyelectrolyte. This finding is seen as evidence for the predominant role of "charge patches" on the protein in the formation of complexes with polyelectrolytes.