Nature of the antigen-antibody interaction. Primary and secondary bonds: optimal conditions for association and dissociation.

All antigen-antibody (AG-AB) bonds are weak physical bonds; covalent bonds are not encountered. The main bonds involved are: (I) Coulombic bonds; (II) Ca2+-bridges; (III) hydrogen bonds; (IV) Lifshitz--van der Waals bonds. Combinations of III and IV occur as the "bonds" usually alluded to as hydrophobic (H phi) interactions. In primary bonds, mainly types I and IV occur; types II and III are quite rare. Secondary bonds, which evolve after a certain time-lapse (varying from minutes to days), mainly involve type IV bonds and H phi interactions, while hydrogen bonds sensu stricto have been known to play a role in rare instances. In affinity chromatography involving AG-AB interactions, complete elution with mild eluents usually is desirable. It would thus appear essential to let little time elapse between AG-AB complex formation and the elution step, to minimize strengthening of the AG-AB interaction by secondary bond formation. For expeditious elution, it also is important to avoid using dehydrating agents (which tend to decrease the bond distance between AG and AB and thus could strengthen the bond in a number of ways). The degree of involvement of types I and IV bonds as well as of H phi interactions varies considerably among different AG-AB systems. These components thus have to be measured separately in order to determine the optimal conditions for elution. The parameters of the liquid medium that may be modulated to influence dissociation (or association) are, for example, surface tension, pH, ionic strength, dielectric constant, temperature, admixture of dehydrating agents or of chaotropic salts. The influence of variations in each of these parameters on types I, III and IV bonds (and thus also on H phi interactions) will be delineated. Because of the misleading implications of the term "hydrophobic interactions", it seems preferable henceforth to allude to them as "interfacial forces".