Lead(II)-Catalyzed Oxidation of Guanine in Solution Studied with Electrospray Ionization Mass Spectrometry
Bibliographic record
Abstract
The oxidation of guanine was investigated in water/methanol solution both in the absence and in the presence of Pb(II) with a variable temperature reactor coupled to a tandem mass spectrometer that allowed signature ions of solution reagents and products to be monitored by electrospray ionization (ESI). Two different oxidizing agents were employed, one strong (peroxymonosulfuric acid) and one weaker (hydrogen peroxide). Peroxymonosulfuric acid was observed to oxidize guanine rapidly at room temperature, k(app) > 10(-2) s(-1), whether in the absence or in the presence of Pb(II), to produce spiroiminohydantoin. Guanine did not show measurable oxidation by hydrogen peroxide in the absence of Pb(II) at concentrations of H(2)O(2) up to 1 M at temperatures up to 333 K (k(app) < 3 × 10(-8) s(-1) at 298 K), but in the presence of Pb(II), it was observed to produce both 5-carboxamido-5-formamido-2-iminohydantoin (2-Ih) and imidazolone (Iz) in a ratio of 2.3 ± 0.1 with a total rate enhancement of more than 4 × 10(3). The activation energy was measured to be 82 ± 11 kJ mol(-1) and is more than 120 kJ mol(-1) lower than that for the uncatalyzed oxidation with hydrogen peroxide measured to be at least 208 ± 26 kJ mol(-1). An activation energy of 113 ± 9 kJ mol(-1) has been reported by Bruskov et al. (Nucleic Acids Res.2002, 30, 1354) for the heat-induced oxidation by hydrogen peroxide of guanine embedded as guanosine in DNA which leads to the production of 8-oxo-7,8-dihydro-guanine (8-oxo-Gua). The atomic lead dication lowers the activation energy by activating the hydrogen peroxide oxidant, possibly by O-O bond activation, and by directing the oxidation, possibly through coordination to the functional groups adjacent to the carbon C5: the C6 carbonyl group and the N7 nitrogen. The coupling of tandem mass spectrometry (MS(2)) with a simple variable temperature reactor by ESI proved to be very effective for measuring reaction kinetics and activation energies in solution. Signature ions of both reagents and products, as well as the catalyst, could be identified, and the data were acquired in real time. The technique should be suitable for exploring other chemical and biochemical reactions that occur on similar time scales (minutes to hours).
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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) | 0.001 | 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".