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 positivel y charged Al2O3 particles was studied as a function of surface coverage (th eta). The adsorption isotherm of BSA exhibited saturation (theta = 1) and t he existence of an inflection point at theta of 0.82. The relative numbers of ionic groups on a BSA molecule interacting with the Al2O3 surface at var ious theta were monitored by measuring the relative adsorption density of H + and OH-, ([Gamma(H+) - Gamma(OH-)]), for BSA-adsorbed Al2O3 using potenti ometric titration. The [Gamma(H+) -Gamma(OH-)] curves for Al2O3, BSA, and B SA-adsorbed Al2O3 at various KNO3 concentrations showed a common intersecti on point (cip) which was the pH giving the acid-base equivalence point, res pectively. Compared with the cip's of Al2O3 (5.6) and BSA (5.2), the cip's of BSA-adsorbed Al2O3 were situated at points corresponding to more alkalin e pH values over the theta range of 0.13 to 1.0. These results suggested th at negatively charged groups, mainly carboxyl groups, on the BSA molecule e lectrostatically interacted with the Al2O3 surface. The degree of shift in the cip increased gradually with increasing theta from 0.13 to 0.70, while it decreased markedly over the theta range of 0.82 to 1.0. The variation in the cip reflected the change in the total number of ion pairs formed betwe en BSA molecules and Al2O3 The initial rates of BSA desorption during alkal i cleaning were low and almost constant over the theta range of 0.13 to 0.7 0, but increased markedly at theta higher than 0.82. It is suggested that t he conformational changes of BSA adsorbed on Al2O3, involving changes in th e relative magnitude of electrostatic interaction forces, occur discretely at theta of approximately 0.8.