A comparative study of meso- and rac-2,3-dimercaptosuccinic acids and their zinc complexes in aqueous solution.

The conformations of meso-2,3-dimercaptosuccinic acid (meso-DMSA) in aqueous solution have been postulated from proton NMR titrations. The complexes formed with zinc(II) and the ligands rac-DMSA and meso-DMSA have been postulated from potentiometric titrations of solutions containing varying ratios of zinc:ligand. The complex formation behavior of rac-DMSA with zinc(II) is dramatically different from that of meso-DMSA. These differences are reflected in the complex formation constants of the zinc(II) species and their distributions in solution as a function of zinc:ligand ratios and as a function of pH. On the basis of these results we have predicted that rac-DMSA is more effective than meso-DMSA in mobilizing lead in vivo and that the 1:2 zinc complex of rac-DMSA can be used effectively for the treatment of heavy metal poisoning because endogenous zinc will not be depleted by this chelation treatment.