Cadmium binding by biological ligands. Part 4. Polynuclear complexes of cadmium with 2,3-dimercaptopropane-1-sulphonic acid

Abstract

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°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 β= 59.9), [Cd(dmps)₂]^4–(log β= 28.27), [Cd₃(dmps)₄]^6–(log β= 71.9), [Cd₅(dmps)₆]^8–(log β= 114.3) and [Cd₇(dmps)₈]^10–(log β= 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.