Mechanisms for Low-Energy (0.5−30 eV) Electron-Induced Pyrimidine Ring Fragmentation within Thymine- and Halogen-Substituted Single Strands of DNA
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Bibliographic record
Abstract
Low-energy (0.5−30 eV) electron irradiation of the thymine- and halogen-substituted nonamers Au/Cy* 6 T 3 and Au/Cy* 6 (FU,BrU,IU) 3, chemisorbed onto a gold surface, leads to the desorption of neutral CN and OCN from fragmentation of the DNA base ring. No neutral halogen-containing species are detected. The cross sections at 15 eV for CN and OCN desorbing from the substituted oligomers bromouracil, fluorouracil, iodoracil, and thymine are measured to be 7.5-, 4.5-, (2.5−3.0)-, and (3.0−3.5) × 10 -17 cm 2, respectively; the difference between T and BrU is about 2- to 3-fold over the investigated energy range. The decrease of neutral species produced, in the sequence BrU > FU > IU ≈ T, is observed over the entire 0.5−30 eV range. Unimolecular dissociation of resonant and nonresonant electron-induced uracil-like radicals is likely to be responsible for the formation/desorption of these neutral species. Above 20 eV incident electron energy, the initial process essentially involves nonresonant interactions, whereas within the 5−20 eV energy range, core-excited resonances are implicated. At lower energy, the neutral fragments, which are observed only for bromouracil-substituted oligonucleotides, are induced via a shape resonance. The behavior of the magnitude of the CN and OCN yields with incident electron energy, along with empirical threshold calculations for neutral fragment formation and considerations of transient anion dissociation dynamics, led us to propose possible mechanisms for pyrimidine ring fragmentation by slow electron impact.
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Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| 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) | 0.000 | 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 it