Lead(II) Complex Formation with Glutathione

2016-02-20T20:28:29Z (GMT) by Vicky Mah Farideh Jalilehvand
A structural investigation of complexes formed between the Pb<sup>2+</sup> ion and glutathione (GSH, denoted AH<sub>3</sub> 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 <i>C</i><sub>Pb<sup>2+</sup></sub> = 10 mM and in the solid state. The Pb L<sub>III</sub>-edge extended X-ray absorption fine structure (EXAFS) oscillation for a solid compound with the empirical formula [Pb­(AH<sub>2</sub>)]­ClO<sub>4</sub> 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 Pb<sup>2+</sup> ions. In aqueous solution at the mole ratio GSH/Pb<sup>II</sup> = 2.0 (<i>C</i><sub>Pb<sup>2+</sup></sub> = 10 mM, pH 8.5), lead­(II) complexes with two thiolate ligands form, characterized by a ligand-to-metal charge-transfer band (LMCT) S<sup>–</sup> → Pb<sup>2+</sup> 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<sub>III</sub>-edge EXAFS spectrum. For solutions with higher mole ratios, GSH/Pb<sup>II</sup> ≥ 3.0, electrospray ionization mass spectroscopy spectra identified a triglutathionyllead­(II) complex, for which Pb L<sub>III</sub>-edge EXAFS spectroscopy shows a mean Pb–S distance of 2.65 ± 0.04 Å in PbS<sub>3</sub> coordination, <sup>207</sup>Pb NMR spectroscopy displays a chemical shift of 2793 ppm, and in the UV–vis spectrum, an S<sup>–</sup> → Pb<sup>2+</sup> 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<sup>II</sup> mole ratios from 4.0 to 10 (<i>C</i><sub>Pb<sup>2+</sup></sub> = 10 mM) measured by Pb L<sub>III</sub>-edge EXAFS spectroscopy.