Proteins at liquid interfaces. V. Shear properties

Abstract

The shear rheological properties of films of β-casein, bovine serum albumin (BSA), and lysozyme adsorbed at air-water and oil-water interfaces have been measured with a surface viscoelastometer. The globular proteins BSA and lysozyme form viscoelastic films when adsorbed to both air-water and oil-water interfaces whereas the disordered β-casein molecule exhibits viscoelastic properties which are so low that they are only just detectable. The viscoelastic properties are related to mechanical models comprising elastic springs and viscous dashpots. The viscoelastic properties exhibited by BSA and lysozyme are dependent upon the surface concentration and reach maximum values at about saturation monolayer coverage. Lysozyme films exhibit greater resistance to shear than BSA films at similar surface concentrations; the maximum surface viscosity coefficient, η 0, for lysozyme at the air-water interface is greater than 10 4 mN s m −1 while the equivalent figure for BSA is 4 × 10 2 mN s m −1. This difference results from the greater cohesion and degree of structure of lysozyme films. The viscoelastic properties of BSA films are maximum at the isoelectric point of the proteins. The rheological properties of the films at air-water and oil-water interfaces are compared; the unit area of flow and activation energy of flow are larger at the latter interface. The adsorbed, surface denatured protein film can be considered as a thin (∼100 Å) protein gel layer.