pH-induced coacervation in complexes of bovine serum albumin and cationic polyelectrolytes

Turbidity and light scattering measurements, along with phase contrast micr oscopy, were used to follow the processes leading to coacervation when aque ous solutions of bovine serum albumin (BSA) and poly (dimethyldiallylammoni um chloride) (PDADMAC) were brought from pH = 4 to 10. The state of macromo lecular assembly of complexes formed between BSA and PDADMAC prior to and d uring the pH-induced coacervation could be characterized by specific pH val ues at which recognizable transitions took place. In addition to the two ch aracteristic pH values (pH(crit) and pH(phi)) previously identified through turbidimetry, other transitions were explicitly established. On the basis of the pH-induced evolution of scattering intensity measurements, we conclu ded that the formation of soluble primary protein-polymer complexes is init iated at pH(crit) and proceeds until "pH'(crit)". A subsequent increase in scattering intensity at "pH(pre)" may arise from the assembly of quasi-neut ralized primary complexes as their net positive charge decreases with incre ase in pH. Subsequently, a maximum in scattering intensity at pH(phi) is ob served coincident with the appearance of turbidity and also corresponding t o the first microscopic observation of coacervate droplets. The temperature independence of pH(crit) and pH(phi) suggests that hydrophobic contributio ns are negligible for the initial BSA-PDADMAC interactions and the subseque nt coacervation process. The pH dependence of scattering intensity profiles allowed the identification of two other transitions beyond pH(phi). Spheri cal microcoacervate droplets first observed around pH(phi) subsequently dis played morphological changes at "pH(morph)", followed by the transformation to solid of flocculant substances at pH(precip).