Adsorption of monomeric bovine serum albumin on sulfonated polystyrene model colloids 3. Colloidal stability of latex—protein complexes

Abstract

The present work deals with the study of the colloidal stability of sulfonated polystyrene latex particles covered by a model protein such as bovine serum albumin (BSA). Two BSA forms, monomeric (m-BSA) and oligomeric (o-BSA), have been previously adsorbed onto two sulfonated latexes with the same particle size but having different surface charge densities. The latex particles, fully or partially covered by the protein (termed latex—protein complexes), were resuspended under several sets of conditions (different pH values and ionic strengths) and their colloidal stability against the addition of electrolyte was studied by turbidity measurements. The latex—protein complexes (with m-BSA and o-BSA) were stable at pH 7, when the protein is negatively charged, owing to the electrostatic repulsion between the particles. The estimation of the zeta potential of the complexes versus the degree of coverage by the proteins permits the confirmation of the electrostatic contribution to the colloidal stability. However, at pH 5, when the protein is uncharged, the complexes with m-BSA on their surface were unstable, while those with o-BSA were stable even at high ionic strength. The stability of the o-BSA—sulfonated latex complexes at pH 5 was confirmed by measuring the particle size of the complexes with a light scattering technique: the size of the complexes was similar to that of the bare particles. Again by zeta potential estimation it is possible to detect that there is no electrostatic stabilization for the m-BSA— or o-BSA—sulfonated polystyrene latex complexes at pH 5, and therefore there would be an additional steric stabilization due to the size of the oligomeric molecules (probably tetrameric) which are adsorbed on the particle surface.