Multiligand interactions at the combining site of anti-fluorescyl antibodies. Molecular recognition and connectivity

Article abstract of DOI:10.1021/ja00198a058

Partitioning of binding energy among the constituent subsites in a ligand-antibody complex can be useful in assessment of bimolecular processes in the combining site. Binding affinities of five anti-fluorescyl monoclonal antibodies for fluorescein (1) and two compounds representing its component parts, xanthenone (2) and benzoate, were studied by the fluorescence quenching technique. Dissociation constants were found to be in the range 10^-12- 10^-10 M for 1, 10^-8- 10^-7 M for 2, and 10^-3- 10^-1 M for benzoate. Binding at both subsites was found to depend strongly on the recognition of polar or charged groups. The carboxylate accounts for ca. 4 kcal/mol for binding at the benzoate site whereas two hydroxyl groups contribute at least 5.5 kcal/mol toward binding of xanthene derivatives. The thermodynamic changes associated with ligand binding were described in terms of the intrinsic (ΔG(i)) and the 'connection' (ΔG(S)) free energies. The connection free energies, estimated at 1.0-2.5 kcal/mol, represent a lower limit for reduction of the entropic energy barrier to bimolecular reactions that would ensue from binding of two reactants prior to chemical transformation. Study of this model system provides a thermodynamic rationale for the utility of monoclonal antibodies as catalysts for bimolecular processes.