engleski

Mechanism of Free Radical Induced Decarboxylation of Aliphatic Amino Acids

Radiation chemical methods were used to investigate the reactions of aliphatic amino acids (a.a.) with hydroxyl, methyl, and i-propyl radicals. A major and most significant product from all of these processes is carbon dioxide. Pulse-radiolysis revealed that the initial step in the OH-induced radical mechanism is oxidation of the amino group, producing aminium radical and aminyl radical with yields of 63% and 37% respectively. The amino radical cation, suffers fast (< 100 ns) fragmentation into carbon dioxide and amino methyl radical. The other primary radical, the aminyl radical, can also be converted into the decarboxylating aminium radical by reaction with proton donors such as phosphate or the a.a. zwitterion, but only on much longer (tipically ƒÝs to ms) time scale. Competitively, the aminyl radical transforms into a carbon-centered radical either by an intramolecular 1,2-H-atom shift or by bimolecular reaction with a.a. anion. Although the a.a. C-centered radical is not generated in a direct H-atom abstraction by hydroxyl radical, this radical appears to be the exclusive product in the reaction of a.a. anion with hydroxyl, methyl, and i-propyl radicals. A most significant findings is that C-centered radical can be converted into the decarboxylating N-centered aminium radical by reaction reaction with proton donors such as a.a. zwitterion or phosphate and thus also becomes a source of carbon dioxide. The amino methyl radical-induced route establishes, in fact, a chain mechanism which could be proven through dose rate effect experiments and suppresion of the chain upon addition of hexacyanoferrate(III) ion or methyl viologen as a scavenger for the reducing precursor radicals. The possible initiation of a.a. decarboxylation by C-centered radicals and the assistance of proton donors at various stages within the overall mechanism are considered to be of general significance and interest in chemical and biological systems.

amino acids; decarboxylation; pulse radiolysis; radicals

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