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
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 E1CB 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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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.001 |
| 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 | 0.001 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.001 | 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 source (direct Gemma or distilled Codex), 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".