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

[Image: see text] A spectroscopic investigation of the complexes formed between the Pb(II) ion and d-penicillamine (H(2)Pen), a chelating agent used in the treatment of lead poisoning, was carried out on two sets of alkaline aqueous solutions with C(Pb(II)) ≈ 10 and 100 mM, varying the H(2)Pen/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 (13)C NMR chemical shift for the penicillamine COO(–) group confirmed Pb(II) coordination. The (207)Pb 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 (207)Pb NMR isotropic chemical shift of 909 ppm obtained for crystalline penicillaminatolead(II) with Pb(S,N,O-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((207)Pb) ≈ 1870 ppm and λ(max) ≈ 298 nm for a Pb(II)S(2)NO site, are consistent with a dominating 1:2 lead(II):penicillamine complex with [Pb(S,N,O-Pen)(S-H(n)Pen)](2–n) (n = 0–1) coordination in alkaline solutions, and provide useful structural information on how penicillamine can function as an antidote against lead toxicity in vivo.