Protein adsorption and protein-lipid interactions at the air-aqueous solution interface

Abstract

In this work we studied BSA (bovine serum albumin) adsorption and BSA-monostearin interactions at the air-aqueous solution interface. Both the surface tension-time dependence and equilibrium surface tension were determined using the Wilhelmy plate method. Temperature, protein concentration in the aqueous phase, the concentration of the lipid spread on the interface and the aqueous phase composition (ethanol and sucrose) were the variables studied. The following conclusions were drawn. (a) The rate of BSA adsorption at the interface increases with both BSA concentration in the aqueous phase and temperature. (b) With ethanol in the subphase the existence of an induction period is observed, which could reflect the existence of BSA-solute interactions in the aqueous phase and at the interface. (c) The rate of BSA adsorption increases when sucrose is present in the bulk phase. (d) The spreading of monostearin at the interface on a protein film causes a rapid reduction in surface tension which can be associated with a displacement of protein by the lipid. After the initial period, surface tension increases with time until the equilibrium surface tension — similar to that of the lipid — is reached, indicating that protein is displaced by monostearin at the interface. (e) When the amount of monostearin spread on the interface is increased, BSA-monostearin interactions both at the interface and in the aqueous bulk phase increase as well. (f) Protein-lipid interactions depend on both protein concentration and aqueous phase composition.