Proteins at liquid interfaces: Role of the molten globule state

Abstract

Recent advances in understanding the structure and dynamics of proteins at liquid interfaces are reviewed with particular reference to adsorbed layers at the oil—water interface in protein-stabilized emulsions. We discuss the importance of molecular flexibility in determining the properties of adsorbed layers and the ease of exchange of protein molecules between bulk and surface in mixed systems. While a statistical model of a nearly random copolymer can be used to describe the adsorption of a disordered protein such as β-casein, such a representation is unrealistic for an adsorbed globular protein, which is typically a compact deformable macromolecular particle having a structure lying somewhere between the native state and the completely unfolded form. It is proposed that such a state of an adsorbed globular protein at a liquid interface is close to what is now called the “molten globule” state. This is the partially denatured state of a globular protein which retains the ordered secondary structure but not the tertiary structure of the native protein. We describe the various ways of producing the molten globule state, and we review the experimental evidence for the molten globule state of α-lactalbumin in some detail. In the final part of the paper, we discuss some new results on the surface activity of α-lactalbumin and the competitive adsorption of α-lactalbumin and β-lactoglobulin in emulsions at acidic pH or in the presence of ethylenediaminetetraacetic acid. This discussion shows how the concept of the molten globule state provides new insight into the relationship between protein structure and the properties of adsorbed layers at liquid interfaces.