A potentiometric and 1H NMR investigation on the interactions between Hg2+ and some biologically important sulfhydryl ligands, such as cysteine (H2CYS), penicillamine (H2PSH), and glutathione (H3GSH), 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 HgL2–z , HgLH3–z , HgLH2 4–z , HgL2 2–2z , HgL2H3–2z , and HgL2H2 4–2z species (L z– = CYS2–, PSH2–, or GSH3–) together with HgL2H3 – for GSH3– 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 Hg2+ is very high, and it was analyzed and compared at different pH and ionic strengths.

A thermodynamic picture is reported for cys–Pb2+, psh–Pb2+, gsh–Pb2+, and gsh–Zn2+systems, and equilibrium constants can be predicted for any experimental condition.

A study on the interactions between CH(3)Hg(+) and some S, N and O donor ligands (2-mercaptopropanoic acid (thiolactic acid (H(2)TLA)), 3-mercaptopropanoic acid (H(2)MPA), 2-mercaptosuccinic acid (thiomalic acid (H(3)TMA)), D,L-penicillamine (H(2)PSH), L-cysteine (H(2)CYS), glutathione (H(3)GSH), N,N'-bis(3-aminopropyl)-1-4-diaminobutane (spermine (SPER)), 1,2,3,4,5,6-benzenehexacarboxylic acid (mellitic acid (H(6)MLT)) and ethylenediaminetetraacetic acid (H(4)EDTA)) is reported. The speciation models in aqueous solution and the possible structures of the complexes formed are discussed on the basis of potentiometric, calorimetric, UV spectrophotometric and electrospray mass spectrometric results. For the CH(3)Hg(+)-S donor ligand systems, the formation of ML(1-z) and MLH(2-z) complex species is observed, together with a diprotonated MLH(2)(3-z) species for CYS(2-), PSH(2-) and GSH(3-) and the mixed hydrolytic one ML(OH)(-z) for TLA(2-) and MPA(2-). The dependence of the stability on ionic strength and on temperature is also analysed. In the other CH(3)Hg(+)-L systems (L = MLT(6-), SPER and EDTA(4-)), ML(1-z), MLH(2-z) and MLH(2)(3-z) complex species are formed, together with the MLH(3)(4-z) species for SPER, the mixed hydrolytic ML(OH)(-z) one for SPER and EDTA, and the M(2)L(2-z) for EDTA only. On the basis of the speciation models proposed, the sequestering ability of the ligands towards methylmercury(II) cation is evaluated. All S donor ligands show a good sequestering power (at 10(-11) mol L(-1) level, in the pH range 4 to 8) following the trend MPA(2-) < PSH(2-) < GSH(3-) < TLA(2-) < CYS(2-) < TMA(3-), while significantly lower is the sequestering ability of MLT, SPER and EDTA (at 10(-3)-10(-5) mol L(-1) level, in the pH range 4 to 8).

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