Stability Constants for Dimercaptosuccinic Acid with Bismuth(III), Zinc(II), and Lead(II)

Abstract

Stability constants for the complexation of zinc(II), lead(II), and bismuth(III) by the vicinal dithiolate chelating agent meso-dimercaptosuccinic acid (DMSA) have been determined by a combination of potentiometric titration and spectrophotometric competition at 25°C and 0.1 M ionic strength. The spectrophotometric studies use the shifts in the ultraviolet bands of the thiol groups to quantitate metal binding to DMSA in the presence of competitive aminocarboxylic acids. Bismuth(III) forms a bis(DMSA) chelate with an exceptionally high stability constant of 10^43,87. This complex undergoes a series of protonations over the pH range 10 to 2, but there appears to be no measurable dissociation of ligand over this pH range. The zinc-DMSA system is dominated by a Zn₂(DMSA)₂ dimer, which has a protonation constant of 10⁶ and dissociates completely at lower pH. No more than 20% of total zinc exists as a monomeric complex at any pH. Lead forms a 1:1 complex with a stability constant of 10^17,4. Insoluble protonated lead complexes precipitate at pH < 6.5. Speciation calculations have been used to evaluate the potential competition from serum zinc to the binding of Pb²⁺ and Bi³⁺ by DMSA. The results indicate that DMSA should be relatively effective for the in vivo chelation of both Bi³⁺ and Pb^{2 +}.