ADSORPTION OF MONOMERIC BOVINE SERUM-ALBUMIN ON SULFONATED POLYSTYRENE MODEL COLLOIDS .1. ADSORPTION-ISOTHERMS AND EFFECT OF THE SURFACE-CHARGE DENSITY

The adsorption of monomeric bovine serum albumin (m-BSA) onto sulfonated polystyrene model colloids was studied by determining the adsorption isotherms under various pH and ionic strength conditions. The amount of m-BSA adsorbed was higher for the latex with a higher surface charge density for all the pH values (pH 3-8) and ionic strengths (2 and 50 mM) studied. The maximum amount adsorbed was obtained at pH 5 (near the isoelectric point (IEP) of the protein), when the ionic strength was 2 mM, and shifted to pH 4 when the ionic strength was 50 mM. This amount (3.4 mg m-2) was higher compared with that obtained with a conventional sulfate latex, but lower in comparison with that obtained with oligomeric BSA (o-BSA) (5.5 mg m-2). No significant desorption of m-BSA and o-BSA was detected after the redispersion of the latex-protein complexes under several, including extreme, experimental conditions. These results together with those obtained from the adsorption isotherms (with only one plateau) seem to indicate no multilayer formation or aggregation of BSA molecules. The flexibility of the BSA molecules and the asymmetry of the charge distribution seem to be the reasons for the high amount of protein adsorbed on sulfonated latexes.