Fast Decarboxylation of Aliphatic Amino Acids Induced by 4-Carboxybenzophenone Triplets in Aqueous Solutions. A Nanosecond Laser Flash Photolysis Study

Abstract

Quenching of the 4-carboxybenzophenone triplet (3CB*) by H2N−CH2−CO2−Et and by amino acid anions of the general formula NR2−CR2−CO2-, 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 3CB*, producing the CB•- radical anion and the R2N•+−CR2−CO2- 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) × 108 M-1 s-1 for N,N-dimethylglycine, (1.3 ± 0.1) × 108 M-1 s-1 for glycine, (1.5 ± 0.2) × 108 M-1 s-1 for alanine, (1.3 ± 0.1) × 108 M-1 s-1 for glycine ethyl ester, and (0.3 ± 0.03) × 108 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 R2N−•CR2 species produced from the decarboxylation of the R2N•+−CR2−CO2- aminium species. The second-order rate constants for CB reduction by the aminoalkyl radicals are (3.2 ± 0.4) × 108 M-1 s-1 for H2N−•CH2, (1.7 ± 0.3) × 109 M-1 s-1 for H2N−•C(Me)H, and (1.8 ± 0.3) × 109 M-1 s-1 for H2N−•CMe2. The transfer of protons from aminium radicals within the solvent cage gives rise to •NR−CR2−CO2- aminyl radicals, and these are known to undergo β-elimination of CO2•-. 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 CO2•-, was observed. The magnitude of this growth matched the yield of (CBH)• from the spectral analysis of the primary products. In further experiments, the R2N•+−CR2−CO2- 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 R2N−•CR2 radicals and an increase from the CO2•- species. An analysis of this effect was made assuming a diffusion-controlled rate of 1 × 1010 M-1 s-1 for the transfer of the proton from R2N•+−CR2−CO2- to OH-. It indicated that the rate constant for transfer of the proton from R2N•+−CR2−CO2- to CB•- within the solvent cage was (6.9 ± 1.5) × 109 s-1. The rate constant for the decarboxylation of the aminium species was estimated to be (8.7 ± 0.5) × 1010 s-1. The latter rate is at least 1 order of magnitude above those observed for decarboxylations of aliphatic acyloxyl radicals in aqueous media.