Effect of pH on the adsorption of bovine serum albumin at the silica water interface studied by neutron reflection

In a previous study of BSA adsorption onto the hydrophilic silica/water interface using neutron reflection, we examined the concentration dependence of the surface excess of BSA at a pH close to its isoelectric point (IP). The surface excess was found to reach a plateau at a very low bulk protein concentration, suggesting a high affinity of BSA for the oxide surface. This work has now been extended to an investigation of the structure and composition of the BSA layer above and below its IF. It is found that adsorption of BSA is strongly dependent on pH, although the protein concentration has little influence on the surface excess at pH 3 and 7. Changing the pH from the IP substantially reduces the surface excess. The structure of the adsorbed layers below a bulk BSA concentration of 0.5 g dm(-3) can be fitted to a single uniform layer distribution over all pH conditions studied, which suggests that there is no significant denaturation. Denaturation generally leads to a more fragmented peptide distribution and a nonuniform density distribution normal to the surface. The thicknesses of the layers below 0.5 g dm-3 were all smaller than the dimension of the short axis of the globular solution structure for BSA, indicating that the molecules are adsorbed sideways-on with their long axes parallel to the solid surface and that adsorption onto the hydrophilic surface results in some structural deformation. The reversibility of BSA adsorption at the hydrophilic silica/water interface was also examined directly. Adsorption was found to be irreversible with respect to changes in BSA concentration but reversible with respect to solution pH at low BSA concentrations only.