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

The lead(II) complexes formed with the multidentate chelator l-cysteine (H(2)Cys) in an alkaline aqueous solution were studied using Pb-207, C-13, and H-1 NMR, Pb L-III-edge X-ray absorption, and UVvis spectroscopic techniques, complemented by electrospray ion mass spectrometry (ESI-MS). The H(2)Cys/Pb-II mole ratios were varied from 2.1 to 10.0 for two sets of solutions with C-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 H(2)Cys/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 average Pb-(N/O) and Pb-S distances of 2.42 +/- 0.04 and 2.64 +/- 0.04 angstrom, 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 examining the Pb-207 NMR signals in the chemical shift range delta(Pb) = 2006-2507 ppm, which became increasingly deshielded with increasing free cysteinate concentration. One-pulse magic-angle-spinning (MAS) Pb-207 NMR spectra of crystalline Pb(aet)(2) (Haet = 2-aminoethanethiol or cysteamine) with PbS2N2 coordination were measured for comparison (delta(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 similar to 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.