THE ROLE OF CHEMICAL-POTENTIAL IN THE ADSORPTION OF LYSOZYME AT THE AIR-WATER-INTERFACE

The adsorption of C-14-labeled lysozyme at the air-water interface has been studied at various temperatures using the radiotracer method. It was observed that the surface concentration of lysozyme decreased with time during the first 60-100 min of adsorption, followed by a rapid increase at later stages. Concomitant with the decrease in surface concentration, the surface tension of the protein solution increased during the initial period, followed by only a gradual increase at later stages. The results indicated that at the moment of creation of the interface, the undenatured lysozyme tends to desorb from the interface owing to its high electrochemical potential at the interface. At the subsurface, lysozyme undergoes partial unfolding, which subsequently facilitates positive adsorption at later stages. The effect of temperature on the kinetics of adsorption indicated that the activation energy barrier for adsorption of lysozyme at the air-water interface was about 12 kcal/mol. On the basis of the data presented, a general mechanism for protein adsorption is proposed, which invokes that it is the chemical potential gradient rather than concentration gradient that acts as the driving force for adsorption of proteins at interfaces.