engleski

Oxidation Mechanism of Aliphatic alfa-Amino Acids in Aqueous Solutions ; Electron Transfer from Amino vs. Carboxylate Functional Group

Time-resolved Fourier transform electron paramagnetic resonance, FT EPR, and optical spectroscopy were used to study oxidation mechanism of simple aliphatic  -amino acids by the triplet state of anthraquinone-2, 6-disulfonate in aqueous solution. Reductive electron transfer was always the major primary quenching reaction irrespective of pH  3. The deprotonated amino group is the preferential site of oxidative attack leading to the formation of aminium radicals as short living precursors. Electron transfer from the carboxylic functional group and formation of acyloxyl radical intermediates occurs at least ten times slower and is therefore operative only for amino acid zwitterions. Both primary transients, aminium and acyloxyl radicals, undergo fast one step fragmentations into the same products, CO2 and  -aminoalkyl radicals. However, they do not constitute resonance mesomeric forms of one and the same species and the decarboxylation of aminium radicals is not preceded by the intramolecular carboxylate to amino group electron transfer. This is concluded from the polarization pattern analysis of the successor  -aminoalkyl radicals, the first FT EPR detectable products on 100 ns time scale. Whereas aminium precursor leads to the triplet and radical pair polarized  -aminoalkyl radicals, only radical pair polarization could be detected for the same radicals produced over the acyloxyl route. This strongly indicates that the acyloxyl radical, with the unpaired electron located on oxygen, is considerably longer living species than the aminium radical, loosing its triplet polarization by spin lattice relaxation before undergoing decarboxylation.

amino acids; FT EPR; electron transfer; radicals; mechanism

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