Lead(II) Complex Formation with Glutathione

A structural investigation of complexes formed between the Pb²⁺ ion and glutathione (GSH, denoted AH₃ in its triprotonated form), the most abundant nonprotein thiol in biological systems, was carried out for a series of aqueous solutions at pH 8.5 and C_Pb²⁺ = 10 mM and in the solid state. The Pb L_III-edge extended X-ray absorption fine structure (EXAFS) oscillation for a solid compound with the empirical formula [Pb­(AH₂)]­ClO₄ was modeled with one Pb–S and two short Pb–O bond distances at 2.64 ± 0.04 and 2.28 ± 0.04 Å, respectively. In addition, Pb···Pb interactions at 4.15 ± 0.05 Å indicate dimeric species in a network where the thiolate group forms an asymmetrical bridge between two Pb²⁺ ions. In aqueous solution at the mole ratio GSH/Pb^II = 2.0 (C_Pb²⁺ = 10 mM, pH 8.5), lead­(II) complexes with two thiolate ligands form, characterized by a ligand-to-metal charge-transfer band (LMCT) S^– → Pb²⁺ at 317 nm in the UV–vis spectrum and mean Pb–S and Pb–(N/O) bond distances of 2.65 ± 0.04 and 2.51 ± 0.04 Å, respectively, from a Pb L_III-edge EXAFS spectrum. For solutions with higher mole ratios, GSH/Pb^II ≥ 3.0, electrospray ionization mass spectroscopy spectra identified a triglutathionyllead­(II) complex, for which Pb L_III-edge EXAFS spectroscopy shows a mean Pb–S distance of 2.65 ± 0.04 Å in PbS₃ coordination, ²⁰⁷Pb NMR spectroscopy displays a chemical shift of 2793 ppm, and in the UV–vis spectrum, an S^– → Pb²⁺ LMCT band appears at 335 nm. The complex persists at high excess of GSH and also at ∼25 K in frozen glycerol (33%)/water glasses for GSH/Pb^II mole ratios from 4.0 to 10 (C_Pb²⁺ = 10 mM) measured by Pb L_III-edge EXAFS spectroscopy.