CADMIUM BINDING BY BIOLOGICAL LIGANDS .4. POLYNUCLEAR COMPLEXES OF CADMIUM WITH 2,3-DIMERCAPTOPROPANE-1-SULFONIC ACID

Extensive alkalimetric anaerobic titrations of cadmium and 2,3-dimercaptopropane-1-sulphonic acid (H3dmps) in aqueous solutions have revealed a family of polynuclear complexes. Over 750 pH measurements [glass electrode, 25-degrees-C, I = 0.2 mol dm-3 (KNO3)], in 17 separate titrations, were collected with a computer-controlled titrator. The solutions consisted of 0.6-4.0 mmol dm-3 cadmium nitrate and at least a 2.5-fold excess of H3dmps. The functional form of the data was consistent with the equilibrium model [Cd3(dmps)3]3- (log beta = 59.9), [Cd(dmps)2]4- (log beta = 28.27), [Cd3(dmps)4]6- (log beta = 71.9), [Cd5(dmps)6]8- (log beta = 114.3) and [Cd7(dmps)8]10- (log beta = 156.7). Polynuclear species form below pH 2, but break down to the mononuclear bis complex above pH 7. Simulations suggest that H3dmps can effectively compete for cadmium binding with the sulphydryl-rich protein metallothionein.