ADSORPTION OF MONOMERIC BOVINE SERUM-ALBUMIN ON SULFONATED POLYSTYRENE MODEL COLLOIDS .3. COLLOIDAL STABILITY OF LATEX-PROTEIN COMPLEXES

The present work deals with the study of the colloidal stability of su lfonated polystyrene latex particles covered by a model protein such a s bovine serum albumin (BSA). Two BSA forms, monomeric (m-BSA) and oli gomeric (o-BSA), have been previously adsorbed onto two sulfonated lat exes with the same particle size but having different surface charge d ensities. The latex particles, fully or partially covered by the prote in (termed latex-protein complexes), were resuspended under several se ts of conditions (different pH values and ionic strengths) and their c olloidal 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, owin g 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 contribu tion to the colloidal stability. However, at pH 5, when the protein is uncharged, the complexes with m-BSA on their surface were unstable, w hile those with o-BSA were stable even at high ionic strength. The sta bility of the o-BSA-sulfonated latex complexes at pH 5 was confirmed b y 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 wo uld be an additional steric stabilization due to the size of the oligo meric molecules (probably tetrameric) which are adsorbed on the partic le surface.