Fragmentation Mechanisms of Product Ions from Protonated Tripeptides
Bibliographic record
Abstract
Dissociation chemistries of the primary fragment ions from the tripeptides, GAG and AGG, were examined both experimentally and theoretically, and compared with those from GGG [El Aribi, H.; Rodriquez, C. F.; Almeida, D. R. P.; Ling, Y.; Mak, W. W.-N.; Hopkinson, A. C.; Siu, K. W. M. J. Am. Chem. Soc. 2003, 125, 9229−9236]. Findings in this study with GAG and AGG confirm and extend those in the earlier study on GGG. Fragmentation of the b 2 to a 2 ion from GAG and AGG is qualitatively similar to that from GGG; stabilization by the methyl group, however, results in generally lower energies for GAG. Fragmentation of the a 2 ion from GAG produces both the a 1 (protonated methanimine) and the internal iminium ion (protonated ethanimine). By contrast, fragmentation of the a 2 ion from AGG produces only the a 1 ion (protonated ethanimine). In GAG, formation of the internal iminium ion requires an intramolecular proton transfer in the proton-bridged complex after cleavage of CO, which is absent in AGG. The a 1 ion from GAG is postulated to form via cleavage of the vibrationally excited proton-bridged complex prior to proton transfer, favored under higher collision-energy conditions. The critical transition state in the fragmentation of the b 2 to a 1 ion is best described as a complex between the a 1 ion and a carbene. Although details differ, in both GAG and AGG, there is a proton transfer from the terminal amino group to the carbene carbon and a second proton transfer from the ring nitrogen back to the terminal amino group. Separation of the components then yields the a 1 ion and a neutral oxazolone.
Fetched live from OpenAlex and de-inverted. Abstracts are not stored in this database: the inverted indexes are 8.6 GB of the frame’s 9.3 GB of text, and the host has 13 GB free.
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 | 0.001 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.001 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.002 | 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".