Thermodynamic incompatibility of proteins at the air–water interface?

Abstract

Competitive adsorption of eight protein 1–protein 2–water ternary systems at the air–water interface has been studied. The equilibrium composition of proteins in the mixed protein films did not follow a Langmuir-type competitive adsorption model. The deviation from the Langmuir model is shown to be due to thermodynamic incompatibility of the protein species in the mixed protein film. This incompatibility changed the ratio of the binding affinity of the proteins to the air–water interface in a protein 1–protein 2–water ternary system when compared to that in a protein 1–water and protein 2–water binary systems. Two parameters—an empirical incompatibility parameter and an interaction parameter—have been determined for each of the eight ternary systems. The relevance of these two parameters in predicting the intensity of incompatibility among two proteins at the air–water interface is described. It is proposed that, because of incompatibility, proteins in a mixed protein film at interfaces may undergo two-dimensional phase separation.