pH-Induced Coacervation in Complexes of Bovine Serum Albumin and Cationic Polyelectrolytes

Turbidity and light scattering measurements, along with phase contrast microscopy, were used to follow the processes leading to coacervation when aqueous solutions of bovine serum albumin (BSA) and poly(dimethyldiallylammonium chloride) (PDADMAC) were brought from pH = 4 to 10. The state of macromolecular assembly of complexes formed between BSA and PDADMAC prior to and during the pH-induced coacervation could be characterized by specific pH values at which recognizable transitions took place. In addition to the two characteristic pH values (pHcrit and pHφ) previously identified through turbidimetry, other transitions were explicitly established. On the basis of the pH-induced evolution of scattering intensity measurements, we concluded that the formation of soluble primary protein−polymer complexes is initiated at pHcrit and proceeds until “pH‘crit”. A subsequent increase in scattering intensity at “pHpre” may arise from the assembly of quasi-neutralized primary complexes as their net positive charge decreases with increase in pH. Subsequently, a maximum in scattering intensity at pHφ is observed coincident with the appearance of turbidity and also corresponding to the first microscopic observation of coacervate droplets. The temperature independence of pHcrit and pHφ suggests that hydrophobic contributions are negligible for the initial BSA−PDADMAC interactions and the subsequent coacervation process. The pH dependence of scattering intensity profiles allowed the identification of two other transitions beyond pHφ. Spherical microcoacervate droplets first observed around pHφ subsequently displayed morphological changes at “pHmorph”, followed by the transformation to solid or flocculant substances at pHprecip.