Lead(II) Complex Formation with <scp>l</scp>-Cysteine in Aqueous Solution
Bibliographic record
Abstract
The lead(II) complexes formed with the multidentate chelator L-cysteine (H2Cys) in an alkaline aqueous solution were studied using (207)Pb, (13)C, and (1)H NMR, Pb LIII-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 CPb(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(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 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 examining the (207)Pb 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) (207)Pb 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(-) → 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.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".