Copper-Mediated Peptide Radical Ions in the Gas Phase
Bibliographic record
Abstract
Molecular radical cations, M •+, of amino acids and oligopeptides are produced by collision-induced dissociation of mixed complex ions, [Cu II (dien)M] •2+, that contain Cu II, an amine, typically diethylenetriamine (dien), and the oligopeptide, M. With dien as the amine ligand, abundant M •+ formation is observed only for the amino acids tryptophan and tyrosine, and oligopeptides that contain either the tryptophanyl or tyrosyl residue. Dissociation of the M •+ ion is rich and differs considerably from that of protonated amino acids and peptides. Facile fragmentation occurs around the α-carbon of the tryptophanyl residue. Cleavage of the N−C α bond and proton transfer from the exocyclic methylene group in the side chain to the N-terminal residue results in formation of the [z n − H] •+ ion and elimination of the N-terminal fragment as ammonia or an amide, depending on the position of the tryptophanyl residue. Cleavage of the C α −C bond of an oligopeptide containing a C-terminal tryptophan residue and proton transfer from the carboxylic group to the N-terminal fragment (a carbonyl oxygen atom) results in formation of the [a n + H] •+ ion and elimination of carbon dioxide. Both types of fragmentation have no analogous reactions in protonated peptides. For the M •+ of tryptophanylglycylglycine, WGG, elimination of the tryptophanyl side chain results in GGG •+ . This radical cation fragments by eliminating its C-terminal glycine to give the [b 2 − H] •+ ion, which is an oxazolone and shares much of the structure and reactivity of the b 2 + ion from protonated triglycine. Density functional theory shows the mechanism of forming the [b 2 − H] •+ ion is similar to that of the b 2 + ion, although the free-energy barrier at 29.4 kcal/mol is lower. The [b 2 − H] •+ ion eliminates CO readily to give the [a 2 − H] •+ ion, which is an iminium radical ion.
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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 | 0.000 |
| 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 | 0.001 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.003 | 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".