Polarimetric and nuclear magnetic resonance studies of the complexation of mercury by thiols

The complexation of Hg(II) by glutathione has been studied by polarimetry under conditions of excess ligand with the objective of characterizing formation of the 3:1 complex, Hg(glutathione)3. The optical rotatory power of solutions containing glutathione only and of solutions containing glutathione and Hg(II) at ratios of 2:1, 2.5:1, 3:1, and 4:1 was measured as a function of pH. Acid dissociation constants for the ammonium and thiol groups of glutathione and for the two ammonium groups of Hg(glutathione)2 and the formation constant of the 3:1 complex (Hg(glutathione)2 + glutathione [Formula: see text] Hg(glutathione)3) were determined from the pH dependence of the optical rotatory power. The value obtained for the formation constant, Kf = 1.5 × 103, indicates that binding of the third ligand to form Hg(glutathione)3 is much weaker than binding of the first two glutathione ligands. However, calculations indicate that binding is sufficiently strong that a significant fraction of Hg(II) is present as Hg(glutathione)3 under physiological conditions. Equilibrium constants were also determined by polarimetry and by 13C nuclear magnetic resonance for the displacement of one thiolate ligand by another (RSHgSR + R′SH [Formula: see text] RSHgSR′ + RSH; RSHgSR′ + R′SH [Formula: see text] R′SHgSR′ + RSH). The results indicate that, at pH 5.5 and at physiological pH, the relative stability increases in the order Hg(glutathione)2 < Hg(penicillamine)2 < Hg(mercaptoethylamine) 2. However, when competitive protonation of free ligand is accounted for, it is shown that the intrinsic stability of the complexes increases in the order Hg(penicillamine)2 < Hg(mercaptoethylamine)2 < Hg(glutathione)2, which parallels the order of the Brønsted basicity of the thiolate ligands.