Fast Decarboxylation of Aliphatic Amino Acids Induced by 4-Carboxybenzophenone Triplets in Aqueous Solutions. A Nanosecond Laser Flash Photolysis Study
Bibliographic record
Abstract
Quenching of the 4-carboxybenzophenone triplet ( 3 CB*) by H 2 N−CH 2 −CO 2 −Et and by amino acid anions of the general formula NR 2 −CR 2 −CO 2 -, where R is H or Me, has been investigated in basic aqueous solution. Spectral analysis of the primary products on the microsecond time scale showed that the major quenching process was electron transfer to 3 CB*, producing the CB •- radical anion and the R 2 N •+ −CR 2 −CO 2 - zwitterion aminium radical. However, on a nanosecond time scale, a small amount of (CBH) • was also formed, and this was attributed to a rapid proton transfer from about 10% of the aminium radicals to the CB •- anion radicals within the primary solvent cage. The values of the overall primary quenching rate constants were (8.5 ± 0.9) × 10 8 M -1 s -1 for N, N -dimethylglycine, (1.3 ± 0.1) × 10 8 M -1 s -1 for glycine, (1.5 ± 0.2) × 10 8 M -1 s -1 for alanine, (1.3 ± 0.1) × 10 8 M -1 s -1 for glycine ethyl ester, and (0.3 ± 0.03) × 10 8 M -1 s -1 for α-methylalanine. The introduction of methyl groups into the glycine structure resulted in a pattern of reactivity similar to that observed for amines. Except in the case of glycine ethyl ester, there were strong secondary growths of CB •- . This was attributed to the reduction of CB by the R 2 N− • CR 2 species produced from the decarboxylation of the R 2 N •+ −CR 2 −CO 2 - aminium species. The second-order rate constants for CB reduction by the aminoalkyl radicals are (3.2 ± 0.4) × 10 8 M -1 s -1 for H 2 N− • CH 2, (1.7 ± 0.3) × 10 9 M -1 s -1 for H 2 N− • C(Me)H, and (1.8 ± 0.3) × 10 9 M -1 s -1 for H 2 N− • CMe 2 . The transfer of protons from aminium radicals within the solvent cage gives rise to • NR−CR 2 −CO 2 - aminyl radicals, and these are known to undergo β-elimination of CO 2 •- . There was evidence for the presence of aminyl radicals in the case of α-methylalanine, where a small tertiary growth of CB •-, due to the reduction of CB by CO 2 •-, was observed. The magnitude of this growth matched the yield of (CBH) • from the spectral analysis of the primary products. In further experiments, the R 2 N •+ −CR 2 −CO 2 - zwitterion aminium radicals were deprotonated by bulk OH - when NaOH was added at concentrations in the range of 1−4 M. As expected, this produced a lowering of the CB •- yield from the R 2 N− • CR 2 radicals and an increase from the CO 2 •- species. An analysis of this effect was made assuming a diffusion-controlled rate of 1 × 10 10 M -1 s -1 for the transfer of the proton from R 2 N •+ −CR 2 −CO 2 - to OH - . It indicated that the rate constant for transfer of the proton from R 2 N •+ −CR 2 −CO 2 - to CB •- within the solvent cage was (6.9 ± 1.5) × 10 9 s -1 . The rate constant for the decarboxylation of the aminium species was estimated to be (8.7 ± 0.5) × 10 10 s -1 . The latter rate is at least 1 order of magnitude above those observed for decarboxylations of aliphatic acyloxyl radicals in aqueous media.
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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 | 0.001 |
| 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".