INFLUENCE OF ELECTROSTATIC FORCES ON THE ADSORPTION OF SUCCINYLATED BETA-LACTOGLOBULIN AT THE AIR-WATER-INTERFACE

beta-Lactoglobulin was succinylated to various extents by reacting with succinic anhydride. The net charge, hydrodynamic radii, and conformational characteristics of succinylated beta-lactoglobulins were analyzed. Adsorption studies at the air-water interface showed that the rate of change of surface pressure as well as steady-state surface pressure decreased progressively with increase of the extent of succinylation. This was related to the electrostatic potential energy barrier to adsorption at the interface. Studies with C-14-labeled native and 69% succinylated beta-lactoglobulin revealed that the adsorption of these proteins followed square-root-of-time kinetics. However, the apparent diffusion coefficient calculated from the adsorption data was an order of magnitude greater than the conventional diffusion coefficient. The experimental surface equation of state for the native and 69% succinylated beta-lactoglobulin was compared with the Davies' equation of state for the charged monolayers. For both the native and succinylated proteins, the Davies' equation overestimated the electrostatic contribution to the surface pressure. This suggested that not all the charged residues were located at the surface even at low surface coverage; a significant portion of the charged residues were suspended into the aqueous phase in the form of loops. The data also suggested that while the surface pressure of the native beta-lactoglobulin film arose from the electrostatic, kinetic, and cohesive forces, the surface pressure of the succinylated beta-lactoglobulin arose mainly from the electrostatic and kinetic forces.