Dynamic and static properties of proteins adsorbed at the air/water interface

Abstract

The adsorption of β- and κ-casein at the air/water interface from solutions containing concentrations ranging from 10^–5 to 10^–1 weight % has been monitored using both radiotracer and ellipsometric techniques. The surface pressure and dilatational modulus have also been measured both during the adsorption process and at steady state conditions. Differences between the surface properties of β- and κ-casein are related to the structures of the molecules. The adsorption data cannot be described by Gibbs' adsorption equation. The adsorption isotherms and film thicknesses are discussed in terms of the distribution of amino-acid residues between trains and loops. At low surface coverage where loop formation is not pronounced, theories based on polymer statistics describe the relationships between film pressure, dilatational modulus and surface coverage satisfactorily. At low surface concentrations of protein the dilatational modulus is purely elastic whereas at higher surface concentrations it has both elastic and viscous components.