Lead chelates of meso- and racemic-dimercaptosuccinic acid

Lead chelates of racemic- and meso-dimercaptosuccinic acid (DMSA) were synthesized and isolated from aqueous solutions and characterized by potentiometric measurements and infrared spectroscopy. Two types of lead chelates of racemic DMSA were isolated: one in which racemic-DMSA is coordinated to Pb+2 via one oxygen and one sulfur atom and the other in which the Pb2+ is coordinated via two sulfur atoms. The latter form of the chelate is converted into the former upon dissolution in dimethylsulfoxide. Only one type of Pb2+ chelate of the meso form of the ligand was formed. In this case, meso-DMSA is coordinated to Pb+2 via one oxygen and one sulfur atom. Meso- and racemic-DMSA have very different solubilities in aqueous solutions. Meso-DMSA is slightly soluble in water, whereas racemic-DMSA is very soluble in water even in the presence of strong acids. The solubilities of the chelates were found to be pH dependent. When the uncoordinated sulfhydryl and carboxylic acid groups dissociate, the chelates dissolve and remain in aqueous solution. The infrared spectra of meso- and racemic-DMSA show distinct features that can be used to detect the presence of either diastereoisomer.