Stoichiometry and the Mechanism of Complex Formationin Protein-Polyelectrolyte Coacervation

The complexation and coacervation of bovine serum albumin (BSA) with poly(dimethyldiallylammonium chloride) (PDMDAAC) in pH 7.86 buffer solutions, at various ionic strengths (I), was investigated by turbidimetric titration, quasi-elastic light scattering, and electrophoretic light scattering. The results obtained support the following mechanism. Upon addition of PDMDAAC to BSA at I = 0.01 M, a stoichiometric complex is initially formed. Subsequent addition of polymer causes this complex to form a coacervate with concomitant charge neutralization. At higher ionic strengths (0.05 and 0.10 M), the initial complex is nonstoichiometric, and coacervates are formed by the aggregation of the complex. These coacervates are observed to be ∼ 700 nm by optical microscopy.