Protein-protein interaction at the air-water interface

A method of measuring the interaction between a protein monofilm and a soluble protein in the subsolution is described. This is measured in terms of monofilm expansion at constant pressure. Soluble proteins will collect at an interface with the formation of a monolayer. At certain higher concentrations, these soluble proteins will invade and expand their own monofilm. This spontaneous protein film expansion occurring in a monofilm on a subsolution of the same protein has been compared with the film expansion observed between dissimilar proteins in the monolayer and in the subsolution. We believe that this comparison of monofilm expansion rates demonstrates the existence of a specific protein-protein interaction and is related to differences in molecular structure. Protein interaction was shown to occur between pork insulin monofilm and alpha globulin reactant at compression pressures between 4.5 dyne-cm. to 16 dyne-cm. (collapse pressure of insulin). The specific expansion observed followed a first-order law. Further, the specific interaction between insulin monofilm and porcine albumin occurred at pH 7.4 and pH 5.1, but not at pH 2.2. Confirmation for this pH dependence was obtained from the pressure-area isotherm studies of equiweight binary mixtures of albumin and insulin.