Influence of anionic compounds on adsorption behavior of bovine serum albumin at oxide-water interfaces

The influence of anionic compounds on the adsorption behavior of bovine serum albumin (BSA) at oxide-water interfaces was studied in buffer solutions composed of carboxylic acids, phosphate, or KNO3 at the isoelectric point (IEP) of BSA, using silicon dioxide (SiO2), titanium dioxide (TiO2), and aluminum oxide (Al2O3) particles. The amount of adsorbed BSA (Gamma) decreased in the presence of buffer salt ions, depending on the kind of buffer anion. The degree of decrease in Gamma was more pronounced on Al2O3 Which has a higher positive surface charge density, and less on negatively charged SiO2. The intrinsic viscosity of BSA increased with increasing ionic strength in all the buffer solutions. The densities of the positive surface charges of the oxides decreased markedly, accompanied by shifts in their IEP's to acidic values, as a result of preincubation in carboxylic acid- and phosphate-buffered solutions, but not in KNO3 solution. The charge profiles of the BSA-covered oxides obtained in the citrate-buffered solution were more acidic in character, giving more acidic IEP's, compared with the profiles obtained in KNO3 solution. It was also found that partial desorption of BSA preadsorbed on Al2O3 took place during incubation in carboxylic acid- and phosphate-buffered solutions but not in KNO3 solution. From these results, the repression of BSA adsorption onto oxides was ascribed to the masking of adsorbing sites on the oxide surfaces and partial desorption of adsorbed BSA induced by adsorption of anions of carboxylic acids and phosphate, as well as to the expansion of the BSA molecule due to the charge-shielding action of buffer salt ions.