Lead(II) Binding to the Chelating Agent d‑Penicillamine in Aqueous Solution

Abstract

A spectroscopic investigation of the complexes formed between the Pb­(II) ion and d-penicillamine (H₂Pen), a chelating agent used in the treatment of lead poisoning, was carried out on two sets of alkaline aqueous solutions with <i>C</i>_Pb(II) ≈ 10 and 100 mM, varying the H₂Pen/Pb­(II) molar ratio (2.0, 3.0, 4.0, 10.0). Ultraviolet–visible (UV-vis) spectra of the 10 mM Pb­(II) solutions consistently showed an absorption peak at 298 nm for S^– → Pb­(II) ligand-to-metal charge-transfer. The downfield ¹³C NMR chemical shift for the penicillamine COO^– group confirmed Pb­(II) coordination. The ²⁰⁷Pb NMR chemical shifts were confined to a narrow range between 1806 ppm and 1873 ppm for all Pb­(II)-penicillamine solutions, indicating only small variations in the speciation, even in large penicillamine excess. Those chemical shifts are considerably deshielded, relative to the solid-state ²⁰⁷Pb NMR isotropic chemical shift of 909 ppm obtained for crystalline penicillaminatolead­(II) with Pb­(<i><i>S,N,O</i></i>-Pen) coordination. The Pb L_III-edge extended X-ray absorption fine structure (EXAFS) spectra obtained for these solutions were well-modeled with two Pb–S and two Pb-(N/O) bonds with mean distances 2.64 ± 0.04 Å and 2.45 ± 0.04 Å, respectively. The combined spectroscopic results, reporting δ­(²⁰⁷Pb) ≈ 1870 ppm and λ_max ≈ 298 nm for a Pb^IIS₂NO site, are consistent with a dominating 1:2 lead­(II):penicillamine complex with [Pb­(<i><i>S,N,O</i></i>-Pen)­(<i>S</i>-H_<i>n</i>Pen)]^2–<i>n</i> (<i>n</i> = 0–1) coordination in alkaline solutions, and provide useful structural information on how penicillamine can function as an antidote against lead toxicity <i>in vivo</i>