Fast Decarboxylation of Aliphatic Amino Acids Induced by 4-Carboxybenzophenone Triplets in Aqueous Solutions. A Nanosecond Laser Flash Photolysis Study
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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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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) | 0.001 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 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 it