Fast fragmentation and slow protonation: a buffer‐dependent isotope effect in reactions of <i>N</i>‐methyl hydroxy(benzylthiamine) analyzed by the Keeffe–Jencks equations
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Bibliographic record
Abstract
Abstract The fragmentation of 2‐(1‐hydroxybenzyl)thiamine in neutral solution (to cleave the pyrimidine and thiazolium) has been shown to compete very effectively with elimination of benzaldehyde to produce thiamine in neutral solution. The fragmentation is believed to involve protonation competing with C—N bond cleavage in the C2α conjugate base as a rate‐determining step. We report that proton removal from C2α of N1 ′‐methyl‐2‐(1‐hydroxybenzyl)thiamine (MHBnT) is rate‐limiting in low concentrations of pH 6 phosphate buffer: reprotonation competes with the subsequent fragmentation step (cleaving the pyrimidine–thiazolium bridge derived from thiamine) at higher buffer concentrations. Comparison of the observed rates of reaction of protio and C2α‐deutero MHBnT reveals a non‐linear variation of the kinetic isotope effect that fits precisely to a ratio derived from the Keeffe–Jencks rate law formulation for E 1 CB reactions. The fragmentation step is clearly distinct from the proton removal step and the isotope sensitivity is limited to the initial step. The variation of the isotope effect is a result of changes due to differing contributions from the hydroxide and buffer‐catalyzed reaction mechanisms. Copyright © 2004 John Wiley & Sons, Ltd.
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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 |
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