Proteins at liquid interfaces. IV. Dilatational properties

Abstract

The changes in the surface pressure of films of the three proteins β-casein, bovine serum albumin (BSA), and lysozyme adsorbed at the air-water interface when subjected to sinusoidal changes in area have been monitored by the Wilhelmy plate technique. The dilatational moduli were determined at different substrate concentrations of proteins, at different distances from the moving barrier, and at different compression-expansion frequencies. The dilatational modulus of a β-casein film at the air-water interface lies in the range 5–30 mN m −1 and is maximal when the film comprises either close-packed amino acid trains or close-packed loops and tails. The dependence of the modulus on the frequency of compression-expansion is due to molecular relaxations within the film; this is due to the reversible movement of segments of the flexible β-casein molecule from trains to loops on compression with a relaxation time of about 10 −8 sec. Because β-casein films at the oil-water interface exist primarily in a loop configuration, the dependence of dilatational modulus on substrate protein concentration is different from the air-water system. In contrast to β-casein, the films of the globular proteins BSA and lysozyme at the air-water interface are essentially rigid and have moduli which are independent of frequency; the dilatational moduli for positions close to the moving barrier lie in the ranges 60–400 and 200–600 mN m −1 for BSA and lysozyme, respectively.