Radical cations of amino acids and peptides: Structures and stabilities
Bibliographic record
Abstract
Amino acid and peptide radical cations, M*+, are formed by oxidative dissociations of [Cu(auxiliary ligand)(M)]*2+ and [Metal(III)(salen)(M)]+ complexes. The most easily formed radicals contain either an aromatic or basic amino acid residue. Aromatic amino acids have low ionization energies, are easily oxidized and delocalize the charge and spin over the ring systems; basic amino acids facilitate formation of alpha-radicals that have captodative structures in which the charge and spin are formally separated, although feeding back some of the charge onto the amide or carboxyl group adjacent to the radical center through hydrogen bonding enriches the electron-withdrawing properties and is highly stabilizing. DFT calculations located five isomers of His*+ with an alpha-radical with a captodative structure at the global minimum in a deep potential well. An IRMPD spectrum confirmed that this isomer is the experimentally observed "long-lived" isomer. When both charge and spin are on the peptide backbone, as in [GGG]*+, captodative structures have the lowest energies; the barriers to interconversion between the three isomeric alpha-radicals of [GGG]*+ are high as the charge impedes migration of a hydrogen atom. Dissociation of [GGG]*+ is charge-driven. In peptide radical cations containing a basic amino acid residue the charge is sequestered on the side chain and the radical center, either on the backbone or on another side chain, initiates the fragmentation.
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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 | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.003 | 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".