Nature of specific ligand-receptor bonds, in particular the antigen-antibody bond.

Abstract

The binding forces involved in the specific interactions between antigens (Ag) and antibodies (Ab), lectins and carbohydrates, ligands and their receptors and, in most cases, enzymes and their substrates, are of a noncovalent, purely physicochemical nature. The same (attractive) physicochemical forces that govern their binding also constitute the (repulsive) forces which prevent i t [e.g., freely suspended peripheral blood cells to approach each other more closely than a given minimum distance (van Oss, 1989, 1990a)l and which allow proteins and other biopolymers to remain in aqueous solution (van Oss et al., 1986b). The three primary physical forces that play a role in Ag-Ab bonds are Lifshitz-van der Waals (LW), electrostatic (EL), and Lewis acid-base (AB), or electron-acceptor-electron-donor interactions. While it is usually possible to distinguish between LW, EL, and AB contributions to Ag-Ab bonding, for example, by observing the effect of changes in ionic strength (which influence EL, but not AB or LW forces), or of the admixture of polar organic solvents (which mainly influence AB forces), it remains difficult to discriminate between general AB interactions (which, in water, usually are mainly due to hydrogen-bonding), and direct hydrogen bond formation. It is important to be able to distinguish between the three primary forces. The difference in the rate of decay as a function of distance of the three primary physical forces involved in cell interactions as well as in Ag-Ab bonds, is a major reason for treating the three forces individually. The rate of decay with distance of the three forces involved in specific bonds plays an important role in the study of these interactions, because Ags and Abs and other ligands and receptors must be able to attract each other from a distance, which can be of the order of 3-8 nm. It should also be realized that various physicochemical changes one can make in the conditions under which Ags and Abs (and other receptors and ligands) interact (such as ionic strength, pH, temperature, content of organic solvent or other solute), can have a different influence on LW, or EL, or AB interaction forces.