An effective strategy for structural elucidation of oligosaccharides through NMR spectroscopy combined with peracetylation using doubly <sup>13</sup>C-labeled acetyl groups
Bibliographic record
Abstract
The use of NMR spectroscopy for the elucidation of larger carbohydrate structures isolated from natural sources is principally limited by severe overlap of 1H signals, poor sensitivity when experiments involve 13C nuclei, and difficulties in conclusively establishing linkage positions. Peracetylation of oligosaccharides with doubly 13C-labeled acetyl groups provides several major advantages for their structural elucidation when combined with specifically tailored NMR pulse sequences. The 2.54.7 Hz J-coupling constants between acetyl carbonyl-13C nuclei and protons of the sugar ring at the sites of acetylation enables these sites to be readily assigned. By inference, glycosidic linkage positions on monosaccharides can be unambiguously determined. This can be used in lieu of permethylation analysis, yet does not require degradation of oligosaccharides. Spectral dispersion in the directly detected (1H) dimension is increased ~2.62.7-fold due to the downfield shifting of sugar-ring protons at the positions of acetylation. Peracetylation also introduces three new frequency dimensions for NMR studies, namely the 13CO, 13CMe, and 1HMe frequencies of the acetyl groups. These frequencies can be correlated to sugar protons, either independently or in combination, in alternative 2-, 3-, or 4-D experiments. The use of HartmannHahn coherence transfer combined with zero-quantum dephasing periods permits purely absorptive in-phase multiplets to be extracted and enables accurate scalar couplings between ring protons to be measured, even in multidimensional experiments. Results are illustrated on a nonasaccharide-alditol derived from N-linked glycoproteins and on some smaller structures containing sialic acids and N-acetylhexosamines. Methods for small-scale sample acetylation using the superacylation catalyst, 4-dimethylamino pyridine, are described. A brief historical perspective pertinent to the fundamental contributions of Dr. R.U. Lemieux to the field of carbohydrate NMR is also presented.Key words: NMR, oligosaccharides, peracetylation, doubly 13C-labeled acetyl groups, tailored pulse sequences, heteronuclear HartmannHahn.
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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 | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.001 | 0.001 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.001 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.000 | 0.001 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 0.002 |
| Insufficient payload (model declined to judge) | 0.002 | 0.001 |
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".