Fast decarboxylation of aliphatic amino acids induced by 4-carboxybenzophenone triplets in aqueous solutions. A nanosecond laser flash photolysis study

Quenching of the 4-carboxybenzophenone triplet ((CB)-C-3*) 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 (CB)-C-3*, 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) x 10(8) M-1 s(-1) for N,N-dimethylglycine, (1.3 +/- 0.1) x 10(8) M-1 s(-1) Fur glycine, (1.5 +/- 0.2) x 10(8) M-1 s(-1) for alanine, (1.3 +/- 0.1) x 10(8) M-1 s(-1) for glycine ethyl ester, and (0.3 +/- 0.03) x 10(8) M-1 s(-1) For alpha-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)-C-. 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) x 10(8) M-1 s(-1) for H2N-(CMe2)-C-., (1.7 +/- 0.3) x 10(9) M-1 s(-1) for H2N-C-.(Me)H, and (1.8 +/- 0.3) x 10(9) M-1 s(-1) for H2N-(CMe2)-C-.. The transfer of protons from aminium radicals within the solvent cage gives rise to (NR)-N-.-CR2-CO2- aminyl radicals, and these are known to undergo beta-elimination of CO2.-. There was evidence for the presence of aminyl radicals in the case of alpha-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)-C-. radicals and an increase from the CO2.- species. An analysis of this effect was made assuming a diffusion-controlled rate of 1 x 10(10) 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.i-CR2-CO2- to CB.- within the solvent cage was (6.9 +/- 1.5) x 10(9) s(-1). The rate constant for the decarboxylation of the aminium species was estimated to be (8.7 +/- 0.5) x 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.