engleski

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

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 CBˇ- radical anion and the +ˇNR2-CR2-CO2- zwitterion aminium radical. However, on a nanosecond time scale, a small amount of (CBH)ˇ was also formed, and this was attributed to a rapid proton transfer from about 10% of the aminium radicals to the CBˇ- anion radicals within the primary solvent cage. The values of the overall primary quenching rate constants were (8.5 ą 0.9) x 108 M-1 s-1 for N, N-dimethylglycine, (1.3 ą 0.1) x 108 M-1 s-1 for glycine, (1.5 ą 0.2) x 108 M-1 s-1 for alanine, (1.3 ą 0.1) x 108 M-1 s-1 for glycine ethyl ester, and (0.3 ą 0.03) x 108 M-1 s-1 for a-methylalanine. The introduction of methyl grous into the glycine structure resulted in the pattern of reactivity similar to that observed for amines. Except in the case of glycine ethyl ester, there were strong secondary growths of CBˇ-. This was attributed to the reduction of CB by R2N-ˇCR2 species produced from the decarboxylation of the +ˇNR2-CR2-CO2- aminium species. The second-order rate constants for CB reduction by the aminoalkyl radicals are (3.2 ą 0.4) x 108 M-1 s-1 for H2N-ˇCH2, (1.7 ą 0.3) x 109 M-1 s-1 for H2N-ˇC(Me)H, and (1.8 ą 0.3) x 109 M-1 s-1 for H2N-ˇCMe2. The transfer of protons from aminium radicals within the solvent cage gives rise to ˇNR-CR2-CO2- aminyl radicals, and these are known to undergo b-elimination of ˇCO2-. There was evidence for the presence of aminyl radicals in the case of a-methylalanine, where a small tertiary growth of CBˇ-, due to the reduction of CB by ˇCO2-, was observed. The magnitude of this growth matched the yield of (CBH)ˇ from the spectral analysis of the primary products. In further experiments, the +ˇNR2-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 CBˇ- yield from the R2N-ˇCR2 radicals and an increase from ˇCO2- species. An analysis of this effect was made assuming a diffusion-controlled rate of 1 x 1010 M-1 s-1 for the transfer of the proton from +ˇNR2-CR2-CO2- to OH-. It indicated that the rate constant for transfer of the proton from +ˇNR2-CR2-CO2- to CBˇ- within the solvent cage was (6.9 ą 1.5) x 109 s-1. The rate constant for the decarboxylation of the aminium species was estimated to be (8.7 ą 0.5) x 1010 s-1. The latter rate is at least one order of magnitude above those observed for decarboxylations of aliphatic acyloxyl radicals in aqueous media.

aliphatic amino acids; radicals; reaction mechanism

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