Lead(II) Complex Formation with l‑Cysteine in Aqueous Solution

Abstract

The lead­(II) complexes formed with the multidentate chelator l-cysteine (H₂Cys) in an alkaline aqueous solution were studied using ²⁰⁷Pb, ¹³C, and ¹H NMR, Pb L_III-edge X-ray absorption, and UV–vis spectroscopic techniques, complemented by electrospray 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 <i>C</i>_Pb^II = 0.01 and 0.1 M, respectively, prepared at pH values (9.1–10.4) for which precipitates of lead­(II) cysteine dissolved. At low H₂Cys/Pb^II mole ratios (2.1–3.0), a mixture of the dithiolate [Pb­(<i>S</i>,<i>N</i>-Cys)₂]^2– and [Pb­(<i>S</i>,<i>N</i>,<i>O</i>-Cys)­(<i>S</i>-HCys)]⁻ complexes with average Pb–(N/O) and Pb–S distances of 2.42 ± 0.04 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(<i>S</i>,<i>N</i>-Cys)­(<i>S</i>-HCys)₂]^2–, including a minor amount of a PbS₃-coordinated [Pb­(<i>S</i>-HCys)₃]⁻ complex. The coordination mode was evaluated by fitting linear combinations of EXAFS oscillations to the experimental spectra and by examining the ²⁰⁷Pb NMR signals in the chemical shift range δ_Pb = 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 (δ_iso = 2105 ppm). The UV–vis spectra displayed absorption maxima at 298–300 nm (S^– → Pb^II 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 lead­(II) coordination modes to proteins and enzymes