ABSORPTION-SPECTROSCOPY OF THE COMPLEXATION BETWEEN SUPERPOTENT GUANIDINIUM SWEETENERS AND SPECIFIC MONOCLONAL-ANTIBODIES

Spectroscopic studies of antibody-antigen interactions can provide useful information about the interactive motifs and energetics involved in the intermolecular association process. In this study we used absorption spectroscopy to examine the interactions between five different monoclonal antibodies (mAb) and four superpotent ligand sweeteners. Quantitative changes in the absorption spectra in the wavelength range of 230-800 mn were utilized for the determination of intrinsic association constants and thermodynamic parameters of the mAb-ligand complexes. The intrinsic association constants for the mAb-ligand complexes were found to be in the range of 10(7)-10(5)lM(-1) and were in agreement with previous radioimmunoassay determinations. For two mAb, qualitative changes in the spectra in the 340 nm range could be identified and were related to the presence of charge-transfer interaction between the guanidinium ligand and aromatic residues in the binding site of the mAb. A charge transfer spectra was observed in mAb NC10.8 with two different sweetener ligands. The thermodynamic parameters of the ligand-mAb interactions were analyzed by van't Hoff plots and in almost all cases the reactions were found to be enthalpically driven. The determinations of intrinsic affinity and thermodynamic parameters may be useful in computer-aided molecular modelling studies of the antibody binding pocket and predicted ligand docking orientations. Antibody NC6.8 was found to react with this set of sweetener ligands in a rank order that is related to their sweetness potencies and the spectroscopic findings for NC6.8 are in agreement with the X-ray diffraction data of the Fab-ligand crystal structures. These studies may shed light on the molecular nature of these superpotent sweetener ligands and their interactions with putative taste receptors.