Light scattering study of complex formation between protein and polyelectrolyte at various ionic strengths

Formation of protein-polyelectrolyte complexes (PPCs) between bovine serum albumin (BSA) and potassium poly (vinyl alcohol) sulfate (KPVS) was studied at pH 3 as a function of ionic strength. Turbidimetric titration was employed by a combination of dynamic light scattering (DLS) and electrophoretic light scattering (ELS). The formal charge (ZPPC) of the resulting PPCs at different ionic strengths were estimated from ELS data by assuming the free draining and the non-free draining model. The radius of a BSA molecule in the complex was used in the former model for calculation of ZPPC with the Henryʹs equation, while in the latter case the hydrodynamic radius of a PPC particle determined from DLS was employed. The results obtained were compared with the ZPPC values calculated using a relation of Z PPC = n b Z BSA + α Z KPVS , where ZBSA (≥0) and ZKPVS (≤0) denote the formal charge of BSA and KPVS, respectively. Moreover, nb is the number of bound proteins per complex composed of α polymer chains. It was suggested that the PPC between BSA and KPVS behaves as a free draining molecule during the electrophoresis, at least at a high ionic strength. Also suggested is that the PPC formation at low ionic strength follows a 1:1 stoichiometry in the charge neutralization.