ESR spectral change of radicals produced in L-alanine-3,3,3-d3 and L-alanine-d4. -A new pathway to produce the de-hydrogen radical and the hydrogen exchange reactions of the de-amino radical-

Abstract

Radicals produced in L-alanine-3,3,3-d3 and L-alanine-d4 by X-irradiation and their spectral change were investigated using X-band ESR. Apparently, two major radicals appeared. One is the radical formed by deamination from a protonated anion and the other is produced by the loss of an alpha hydrogen. Before the appearance of the de-hydrogen radical, the precursor radical with a singlet feature was observed at 148–158 K. The signal of the precursor decreased and that of the de-hydrogen radical increased with temperature. Because the feature of the precursor radical was a singlet, it will be produced by deprotonation of the alpha hydrogen from the deprotonated cation radical observed at 77–80 K. Protonation to the amine moiety of the deprotonated radical will be a new pathway to produce the de-hydrogen radical.In L-alanine-3,3,3-d3, the hydrogen exchange reaction between the alpha hydrogen in the de-amino radical and the beta deuterium in a neighboring molecule occurred at the first-order rate of 7 × 10−4 s−1 at 294 K. The activation energy was 25 kcal/mol. By annealing at 393 K, the spectral features have obviously changed by the hydrogen exchange reactions in the de-amino radicals. Finally, all the de-amino radicals changed to •CHCH3COOH(D) and the methyl deuterium in the de-hydrogen radicals changed to hydrogen by a hydrogen exchange reaction between the C-D deuteriums of the radical and the N–H hydrogens of a crystal molecule after 100 min of annealing at 393 K. Many of the N–H hydrogens in the crystalline molecules changed to deuterons leading to the deuteration of the amino hydrogens in the de-hydrogen radicals.