Conformation of adsorbed bovine serum albumin governing its desorption behavior at alumina-water interfaces

The mode of initial adsorption of bovine serum albumin (BSA) onto positively charged Al 2O 3 particles was studied as a function of surface coverage (θ). The adsorption isotherm of BSA exhibited saturation ( θ= 1) and the existence of an inflection point at θ of 0.82. The relative numbers of ionic groups on a BSA molecule interacting with the Al 2O 3 surface at various θ were monitored by measuring the relative adsorption density of H + and OH −, ([Γ H + − Γ OH − ]), for BSA-adsorbed Al 2O 3 using potentiometric titration. The [Γ H + − Γ OH − ] curves for Al 2O 3, BSA, and BSA-adsorbed Al 2O 3 at various KNO 3 concentrations showed a common intersection point (cip) which was the pH giving the acid-base equivalence point, respectively. Compared with the cip's of Al 2O 3 (5.6) and BSA (5.2), the cip's of BSA-adsorbed Al 2O 3 were situated at points corresponding to more alkaline pH values over the θ range of 0.13 to 1.0. These results suggested that negatively charged groups, mainly carboxyl groups, on the BSA molecule electrostatically interacted with the Al 2O 3 surface. The degree of shift in the cip increased gradually with increasing θ from 0.13 to 0.70, while it decreased markedly over the θ range of 0.82 to 1.0. The variation in the cip reflected the change in the total number of ion pairs formed between BSA molecules and Al 2O 3. The initial rates of BSA desorption during alkali cleaning were low and almost constant over the θ range of 0.13 to 0.70, but increased markedly at θ higher than 0.82. It is suggested that the conformational changes of BSA adsorbed on Al 2O 3, involving changes in the relative magnitude of electrostatic interaction forces, occur discretely at θ of approximately 0.8.