engleski

One Electron Oxidation of Amino Acid Anions. Primary Sites, Transients, Yields, and Rate Constants

The one electron oxidation of anions of simple amino acids (glycine, alanine, a-methyl alanine, N,N-dimethylglycine) was re-examined by time-resolved radiation and photochemical techniques. These studies have provided new, unambiguous and quantitative information on the nature and reactivity of the various transients generated in this process. The most important findings in anaerobic aqueous solutions, as exemplified for glycine, are as follows: Hydroxyl radical attack leads, with about equal probability, to H2N-ˇCH-CO2- radical and an oxidation of the deprotonated NH2 group, which results in a caged [HO- / +ˇNH2-CH2-CO2-] ion pair. The oxidation by 4-carboxybenzophenone excited triplets, on the other hand, yielded [CBˇ- / +ˇNH2-CH2-CO2-] exclusively. The fate of the N-centered aminium radical species in the solvent cage appears to be controlled by a competition between decarboxylation, with formation of ˇCH2NH2, and proton transfer to the nearby OH- or CBˇ-, which produces the ˇNH-CH2-CO2- aminyl radical. The effects of specific methyl substitution at the N and a-C sites have been examined with the amino acids other than glycine. New results are also presented on a number of secondary processes, such as the enhancement of decarboxylation by proton donors and the rates of b-scission of the aminyl radicals into imines and the reducing ˇCO2- radical anion.

amino acids; electron oxidation; mechanisms; rate constants

nije evidentirano