Sequestration of Hg²⁺ by Some Biologically Important Thiols

Abstract

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.

Kinetics of Enzymatic Mercury Methylation at Nanomolar Concentrations Catalyzed by HgcAB

“…Intracellular thiolates comprise thiol functional groups on proteins (i.e., cysteine) and small molecular thiols. The concentration of thiols in cells can range from 0.1 to 10 mM (40), which is approximately 3 orders of magnitude higher than the Hg(II) concentrations used for methylation assays (0.5 nM to 60 nM). Therefore, Hg(II)-thiolate complexes are expected to be the dominant species in cells, as thiolates tend to outcompete any other ligands under these conditions.…”

“…The chemical speciation of Hg(II) is a critical factor that may impact apparent methylation rates due to the high affinity of Hg(II) for biological thiolates (SR Ϫ ) with stability constants (log ␤) of up to 45 for Hg(II)-bis-thiolate [Hg(SR) 2 ] complexes (40). Intracellular thiolates comprise thiol functional groups on proteins (i.e., cysteine) and small molecular thiols.…”

Thermodynamic stability of mercury(II) complexes formed with environmentally relevant low-molecular-mass thiols studied by competing ligand exchange and density functional theory

“…16 A similar magnitude of the stability constant for Hg II -LMM thiol complexes has been reported using a methodology in which the thermodynamics of the Hg II speciation are shifted towards a separable, identifiable, and measurable Hg II complex with the addition of a competing ligand such as iodide (I -) or bromide (Br -) ions or the lipophilic thiol dithizone. 9,[17][18][19] Even with this progress in methodological development it remains a challenge to determine the exceptionally high stability constant for Hg II -LMM thiol complexes, as exemplified by log β2 constants reported for the Hg(Cys)2 complex in recent literature, ranging between 38.2 9 and 43.5. 18 Thus, there is still a substantial uncertainty remaining before a consensus on Hg II -LMM thiols stability constants can be reached.…”

“…9,[17][18][19] Even with this progress in methodological development it remains a challenge to determine the exceptionally high stability constant for Hg II -LMM thiol complexes, as exemplified by log β2 constants reported for the Hg(Cys)2 complex in recent literature, ranging between 38.2 9 and 43.5. 18 Thus, there is still a substantial uncertainty remaining before a consensus on Hg II -LMM thiols stability constants can be reached.…”

Kinetics of enzymatic mercury methylation at nanomolar concentrations catalyzed by HgcAB

“…Hg(II) is toxic to many enzymes as a result of its high affinity for biological thiolates (SR -) with stability constants (log β) for Hg(II)-bis-thiolate [Hg(SR)2] complexes of up to 45 (40). Hg(II) added to cell lysates as HgCl2 at nanomolar concentrations is therefore expected to form highly stable complexes with intracellular thiolates.…”