CADMIUM BINDING BY BIOLOGICAL LIGANDS .5. SOLUTION STUDIES OF CADMIUM AND ZINC-BINDING BY SULFHYDRYL LIGANDS N,N'-DIMETHYL-N,N'-BIS(2-MERCAPTOETHYL)ETHYLENEDIAMINE AND (2-MERCAPTOETHYL)AMINE

Extensive anaerobic alkalimetric titrations of cadmium and zinc with the tetradentate N2S2-donor disulfhydryl ligand NN'-dimethyl-NN'-bis(2-mercaptoethyl)ethylenediamine (dsdm) and of zinc with the monosulfhydryl NS-donor (2-mercaptoethyl)amine in aqueous solutions have revealed polynuclear complexes with cadmium but not with zinc. Nearly 1600 pH measurements (glass electrode, 25-degrees-C, 0.2 M (KNO3) ionic strength), in 39 separate titrations, were collected with a computer-controlled titrator. The solutions consisted of 1-8 mM cadmium or zinc nitrate and a 1-8-fold excess in ligand. The functional form of the data was in agreement with the least-squares refined equilibrium models consisting of the following principal species: Zn(dsdm)2H42+ (log-beta = 53.9), Zn(dsdm) (log-beta = 19.3); Cd4(dsdm)4H44+ (log-beta = 109.7), Cd3(dsdm)3H22+ (log-beta 77.8), Cd2(dsdm)2H+ (log-beta 49.1), Cd(dsdm) (log-beta 20.5); Zn(mea)2H22+ (log-beta 30.6), Zn(mea)2H+ (log-beta 24.9), Zn(mea)3H2+ (log-beta 39.4), Zn(mea)2 (log-beta 17.7). Cd-dsdm polynuclear species form between pH 3 and 6 but break down to the mononuclear 1:1 complex above pH 7. At pH 7, the dsdm ligand forms a more stable cadmium complex than does edta. Simulation calculations suggest that dsdm can effectively compete for cadmium binding with the sulfhydryl-rich protein metallothionein.