TRANSITION-STATE THEORY AND SECONDARY FORCES IN ANTIGEN-ANTIBODY COMPLEXES

Secondary forces, defined as those interactions between the antigen (e pitope including the surrounding environment) and areas immediately ad jacent to the antibody active site, were investigated using monofluore scein-derivatized synthetic peptides of varying electrostatic properti es. Secondary forces were quantitated by measuring the unimolecular ra te constants at two different temperatures using the high-affinity ant i-fluorescein monoclonal antibody 4-4-20 complexed with fluorescein-de rivatized synthetic Unimolecular rate constants were correlated with t ransition-state theory to explain secondary An acidic peptide produced a large temperature-dependent effect upon binding including a signifi cant enthalpic factor (+33.28 kcal/mol) relative to the binding of flu orescein ligand (+23.96 kcal/mol). Binding of a basic peptide produced both a relatively smaller temperature effect and enthalpy factor than fluorescein ligand. The antibody-ligand binding results were interpre ted invoking the concepts of thermally averaged metatypic (liganded) s tates of the antibody as well as potential biochemical interactions be tween the antigen and accessible surface regions of the antibody's com plementarity determining regions.