Fragmentation Chemistry of [Met-Gly]<sup>•+</sup>, [Gly-Met]<sup>•+</sup>, and [Met-Met]<sup>•+</sup> Radical Cations
Bibliographic record
Abstract
Radical cations [Met-Gly](•+), [Gly-Met](•+), and [Met-Met](•+) have been generated through collision-induced dissociation (CID) of [Cu(II)(CH3CN)2(peptide)](•2+) complexes. Their fragmentation patterns and dissociation mechanisms have been studied both experimentally and theoretically using density functional theory at the UB3LYP/6-311++G(d,p) level. The captodative structure, in which the radical is located at the α-carbon of the N-terminal residue and the proton is on the amide oxygen, is the lowest energy structure on each potential energy surface. The canonical structure, with the charge and spin both located on the sulfur, and the distonic ion with the proton on the terminal amino group, and the radical on the α-carbon of the C-terminal residue have similar energies. Interconversion between the canonical structures and the captodative isomers is facile and occurs prior to fragmentation. However, isomerization to produce the distonic structure is energetically less favorable and cannot compete with dissociation except in the case of [Gly-Met](•+). Charge-driven dissociations result in formation of [b(n) - H](•+) and a(1) ions. Radical-driven dissociation leads to the loss of the side chain of methionine as CH3-S-CH=CH2 producing α-glycyl radicals from both [Gly-Met](•+) and [Met-Met](•+). For [Met-Met](•+), loss of the side chain occurs at the C-terminal as shown by both labeling experiments and computations. The product, the distonic ion of [Met-Gly](•+), NH3 (+)CH(CH2CH2SCH3)CONHCH(•)COOH dissociates by loss of CH3S(•). The isomeric distonic ion NH3 (+)CH2CONHC(•)(CH2CH2SCH3)COOH is accessible directly from the canonical [Gly-Met](•+) ion. A fragmentation pathway that characterizes this ion (and the distonic ion of [Met-Met](•+)) is homolytic fission of the Cβ-Cγ bond to lose CH3SCH2(•).
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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) | 0.000 | 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 | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| 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".