Adsorption of bovine serum albumin onto TiO2 particles

The adsorption of bovine serum albumin (BSA) at the TiO2-aqueous NaCl interface was studied as a function of pH and electrolyte concentration using kinetic and steady-state measurements. The electrokinetic properties of BSA-covered TiO2 particles and the acid-base character of the protein were monitored by electrophoretic mobilities and acid-base potentiometric titration, respectively. Some experiments were also outlined to analyze the different affinities of BSA monomers, dimers, and polymers for the surface. The adsorption process is fast; less than 10 min is enough to achieve steady-state conditions. All measured isotherms showed high initial slopes even under adverse electrostatic conditions, resulting in high-affinity isotherms. Adsorption plateau values (Gamma(max)) had a great pH dependence; Gamma(max)-pH curves reached a maximum at around the isoelectric point of BSA and showed a drastic decrease on the acid side. Both structural and electrostatic effects must be invoked to explain the diminution of adsorbed BSA on either side of the isoelectric point. Structural effects were related to the different conformational states that BSA molecules adopt with pH changes, whereas electrostatic effects were analyzed assuming that BSA molecules behave as soft particles. This reasoning also allows us to explain the independence of the Gamma(max)-pH curves from electrolyte concentration. (C) 1997 Academic Press.