The lead(II) complexes formed with the multidentate chelator L-cysteine (H2Cys) in alkaline aqueous solution were studied using 207Pb, 13C and 1H NMR, Pb LIII-edge X-ray absorption and UV-vis. spectroscopic techniques, complemented by electro-spray ion mass spectrometry (ESI-MS). The H2Cys/Pb(II) mole ratios were varied from 2.1 to 10.0 for two sets of solutions with CPb(II) = 0.01 and 0.1 M, respectively, prepared at pH values (9.1 – 10.4) for which precipitates of Pb(II)-cysteine dissolved. At low H2Cys/Pb(II) mole ratios (2.1 – 3.0) a mixture of the dithiolate [Pb(S,N-Cys)2]2− and [Pb(S,N,O-Cys)(S-HCys)]− complexes with the average Pb-(N/O) and Pb-S distances 2.42 ± 0.04 Å and 2.64 ± 0.04 Å, respectively, was found to dominate. At high concentration of free cysteinate (> 0.7 M) a significant amount converts to the trithiolate [Pb(S,N-Cys)(S-HCys)2]2−, including a minor amount of a PbS3 coordinated [Pb(S-HCys)3]− complex. The coordination mode was evaluated by fitting linear combinations of EXAFS oscillations to the experimental spectra, and by the 207Pb NMR signals in the chemical shift range δPb = 2006 – 2507 ppm, which became increasingly deshielded with increasing free cysteinate concentration. One-pulse magic angle spinning (MAS) 207Pb NMR spectra of crystalline Pb(aet)2 (Haet = 2-aminoethanethiol or cysteamine) with PbS2N2 coordination were measured for comparison (δiso = 2105 ppm). The UV-vis. spectra displayed absorption maxima at 298 – 300 nm (S− → PbII charge transfer) for the dithiolate PbS2N(N/O) species; with increasing ligand excess a shoulder appeared at ∼ 330 nm for the trithiolate PbS3N and PbS3 (minor) complexes. The results provide spectroscopic fingerprints for structural models for Pb(II) coordination modes to proteins and enzymes.

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 CPb(II) ≈ 10 and 100 mM, varying the H2Pen/Pb(II) mole ratio (2.0, 3.0, 4.0, 10.0). 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 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(S,N,O-Pen) coordination. The Pb LIII-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 λmax ~ 298 nm for a PbIIS2NO site, are consistent with a dominating 1:2 lead(II):penicillamine complex with [Pb(S,N,O-Pen)(S-HnPen)]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.

In the title coordination polymer, [Pb(C5H9NO2S)]n {systematic name: catena-poly[(μ-2-amino-3-methyl-3-sulfido­butano­ato)lead(II)]}, the d-penicillaminate ligand coordin­ates to the metal ion in an N,S,O-tridentate mode. The S atom acts as a bridge to two neighbouring PbII ions, thereby forming a double thiol­ate chain. Moreover, the coordinating carboxyl­ate O atom forms bridges to the PbII ions in the adjacent chain. The overall coordination sphere of the PbII ion can be described as a highly distorted penta­gonal bipyramid with a void in the equatorial plane between the long Pb—S bonds probably occupied by the stereochemically active inert electron pair. The amino H atoms form N—H⋯S and N—H⋯O hydrogen bonds, resulting in a cluster of four complex units, giving rise to an R 4 4(16) ring lying in the ab plane. The crystal structure of the title compound has been reported previously [Freeman et al. (1974 ▶). Chem. Soc. Chem. Commun. pp. 366–367] but the atomic coordinates have not been deposited in the Cambridge Structural Database (refcode DPENPB). Additional details of the hydrogen bonding are presented here.