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

Abstract

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.

Similarities between N-Acetylcysteine and Glutathione in Binding to Lead(II) Ions

“…For the Pb(II) cysteine (pH = 9.1 – 10.4) and penicillamine solutions (pH = 9.6), the ligand to metal charge transfer (LMCT) bands are somewhat blue-shifted (298 – 300 nm) relative to those of the Pb(II) N -acetylcysteine solutions (324 – 335 nm). 47, 48 Similar S − (3p) → Pb 2+ (6s) LMCT bands (320 – 335 nm) are observed for Pb(II) glutathione solutions with comparable ligand to Pb(II) mole ratios ( C Pb(II) = 10 mM, pH = 8.5). 26 …”

“…67 However, we recently obtained an isotropic 207 Pb NMR chemical shift at δ iso = 2105 ppm for solid Pb( S,N -cysteamine) 2 with the amine group as N -donor ligand. 47 This chemical shift is considerably more shielded than that of the pyridine adduct with PbS 2 N 2 coordination. 67 …”

“…52 In our earlier reports on Pb(II) complex formation with glutathione, penicillamine or cysteine, we instead used a 1.0 M Pb(NO 3 ) 2 solution in D 2 O for calibration, and incorrectly set its resonance to −2961 ppm, 26, 47, 48 a mistake made also in a previous report of the 207 Pb NMR chemical shift for a compound with PbSN 2 coordination. 68 The 207 Pb chemical shift for 1.0 M Pb(NO 3 ) 2 in D 2 O solution appears at −2990 ppm at room temperature, as reported by the Pecoraro group.…”

“…The difference is attributed to the ability of cysteine and penicillamine to form chelates as bidentate ( S,N -Cys) or tridentate ( S,N,O -Pen/Cys) ligands to the Pb(II) ion, 47, 48 in which the amine N and the carboxylate O atoms shield the 207 Pb nuclei.…”

Comparative biochemical profile of protein-rich products obtained from industrial rapeseed meal

“…The value is 3-fold higher than the limit set by EFSA (2013). Lead affinity towards protein complexation (Belatik et al, 2012;Jalilehvand et al, 2015) is the most probable explanation for the higher concentration of this element in PI compared to ASP. Cadmium, evaluated in both PI and ASP, was within (or below) the limit values set for most food products (EC No 466, 2001).…”