Comparison between antigen-antibody binding energies and interfacial free energies

Antigen-antibody binding energies derived from equilibrium data are compared with the binding energies resulting from the interfacial free energies obtained from contact angle measurements of antigens and antibodies. From these interfacial free energies two sorts of theoretical antigen-antibody binding energies can be derived, as well as the Hamaker constants for most antigen-antibody systems. For interaction in vacuo the Hamaker constants obtained are between 4 and 6 X 10(-13) ergs, while these constants for hydrated antigen antibody interactions are less than 10(-14) ergs. For interactions in vacuo, interfacial free energies yield binding energies (delta Fa) that lie between -120 and -140 ergs/cm2. For interactions in the aqueous phase (with interstitial water still present), much lower binding energies (delta Fb) are derived, of the order of -.01 and -1 ergs/cm2. In comparison, dextran-anti-dextran interactions show a binding energy derived from equilibrium data (delta Feq) of the order of -10 ergs/cm2. In general the equilibrium binding energies delta Feq of most antigen-antibody systems would vary between -1 and -20 ergs/cm2. The implications of this comparison are discussed in the light of the influence of residual water between antigenic determinant and antibody-active site, as well as in the light of the degree of perfection of fit between these sites.