Studies on the nature of intermolecular bonding in antigen-antibody complexes

The formation and dissociation of BSA:anti-BSA complexes was studied under two dissociating conditions: (i) low pH, which disrupts Coulombic bonds, and (ii) low surface tension, which disrupts van der Waals bonds. Soluble complexes formed in marked (thirty-two times) antigen excess were almost totally dissociated at low pH. By contrast, complexes formed near equivalence or in the zone of antibody excess, while becoming smaller, were not dissociated at low pH. Lowering surface tension at neutral pH likewise resulted in smaller complexes but not in dissociation of antigen from antibody. Dissociation of complexes formed near equivalence or in antibody excess was achieved by simultaneously lowering pH and surface tension. These data suggest that complexes formed in marked antigen excess involve predominantly Coulombic bonding while complexes formed at other antigen:antibody ratios involve both Coulombic and van der Waals bonding.