Pyrroles As Antioxidants: Solvent Effects and the Nature of the Attacking Radical on Antioxidant Activities and Mechanisms of Pyrroles, Dipyrrinones, and Bile Pigments
Bibliographic record
Abstract
The absolute rate constants, k(inh), and stoichiometric factors, n, of pyrroles, 2-methyl-3-ethylcarboxy-4,5-di-p-methoxyphenylpyrrole, 6, 2,3,4,5-tetraphenylpyrrole, 7, and 2,3,4,5-tetra-p-methoxyphenylpyrrole, 8, compared to the phenolic antioxidant, di-tert-butylhydroxyanisole, DBHA, during inhibited oxidation of cumene initiated by AIBN at 30 degrees C gave the relative antioxidant activities (k(inh)) DBHA > 8 > 7 > 6 and n = 2, whereas in styrene, 8 > DBHA. These results are explained by hydrogen atom transfer, HAT, from the N-H of pyrroles to ROO(*) radicals. The k(inh) values in styrene of dimethyl esters of the bile pigments of bilirubin ester (BRDE), of biliverdin ester (BVDE), and of a model compound (dipyrrinone, 1) gave k(inh) in the order pentamethylhydroxychroman (PMHC) >> BRDE > 1 > BVDE. These antioxidant activities for BVDE and the model compound, 1, and PMHC dropped dramatically in the presence of methanol due to hydrogen bonding at the pyrrolic N-H group. In contrast the k(inh) of BRDE increased in methanol. We now show that pyrrolic compounds may react by HAT, proton-coupled electron transfer, PCET, or single electron transfer, SET, depending on their structure, the nature of the solvent, and the attacking radical. Compounds BVDE and 1 react by the HAT or PCET pathway (HAT/PCET) in styrene/chlorobenzene with ROO(*) and with the DPPH(*) radical in chlorobenzene according to N-H/N-D kH/kD of 1.6, whereas the DKIE with BRDE was only 1.2 with ROO(*). The antioxidant properties of polypyrroles of the BVDE class and model compounds (e.g., 1) are controlled by intramolecular H bonding which stabilizes an intermediate pyrrolic radical in HAT/PCET. According to kinetic polar solvent effects on the monopyrrole, 8, and BRDE, which gave increased rates in methanol, some pyrrolic structures are also susceptible to SET reactions. This conclusion is supported by some calculated ionization potentials. The antioxidant mechanism for BRDE with peroxyl radicals is described by the PCET reaction. Experiments using the 2,6-di-tert-butyl-4-(4'-methoxyphenyl)phenoxyl radical (DBMP(*)) showed this to be a better radical to monitor HAT activities in stopped-flow kinetics compared to the use of the more popular DPPH(*) radical.
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How this classification was reachedexpand
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 | 0.001 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| 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 itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, 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".