Influence of sodium polystyrene sulfonate on dynamic surface properties of bovine serum albumin solutions

Dynamic surface elasticity of solutions of bovine serum albumin (BSA)/sodium polystyrene sulfonate (PSS) complexes has been measured as depending on the age of a surface, polyelectrolyte concentration, and solution pH by the oscillating-ring method. At pH values below the isoelectric point of BSA, the rate of variations in the surface properties increases due to a decrease in the electrostatic adsorption barrier as a result of a reduction in the total charge of the protein/polyelectrolyte complex. Therewith, a local maximum arises in the kinetic dependences of the surface elasticity, this maximum indicating the onset of the breakage of the tertiary structure of the protein in the surface layer. In the pH range corresponding to like charges of the protein and polyelectrolyte, variations in the surface properties slow down. In this case, the BSA/PSS complex is also formed via the interaction of PSS with those domains of globule surface that carry a charge opposite to the total charge of a protein molecule. A higher negative charge of the complex than that of protein globules increases the electrostatic adsorption barrier and decelerates variations in the surface properties. At the same time, the dependences of the surface elasticity on the surface pressure coincide with the dependences for the protein solution. Hence, the polyelectrolyte-protein interaction affects only the adsorption kinetics, while the surface properties in the vicinity of equilibrium are governed by adsorbed protein globules.