Sequestration of Hg2+ by Some Biologically Important Thiols

A potentiometric and 1H NMR investigation on the interactions between Hg 2+ and some biologically important sulfhydryl ligands, such as cysteine (H 2CYS), penicillamine (H 2PSH), and glutathione (H 3GSH), is reported. Equilibria were studied at T = 298.15 K, in the ionic strength range (0.1 to 1) mol·kg -1, using as ionic medium NaCl in the presence of iodide (NaI) as a competitive ligand. Results show the formation of HgL 2-z, HgLH 3-z, HgLH 2 4-z, HgL 2 2-2z, HgL 2H 3-2z, and HgL 2H 2 4-2z species (L z- = CYS 2-, PSH 2-, or GSH 3-) together with HgL 2H 3 - for GSH 3- only. Formation constant values are very high with, as an example, log β = 34.54, 34.24, and 32.05 for Hg(CYS) 0, Hg(PSH) 0, and Hg(GSH) - species, respectively (at I = 0.25 mol·kg -1, T = 298.15 K). 1H NMR measurements at I = 0.25 mol·kg -1 (NaCl) fully confirm the potentiometric findings. The speciation diagrams show that most of the metal fraction is present as complex species in a wide pH range, and the hydrolysis of the cation is completely suppressed. The sequestering ability of ligands toward Hg 2+ is very high, and it was analyzed and compared at different pH and ionic strengths.