Evidence for β Scission in the Oxidation of Amino Acids

Abstract

Time-resolved ESR and steady-state spin trapping were used to identify the radicals and their kinetics following the radiolytic oxidation by the •OH radical of the anion of α-methylalanine, H2N−C(CH3)2−CO2-. Spectra of two new radicals were identified in aqueous solution, the aminyl radical, HN•−C(CH3)2−CO2-, and the decarboxylated radical, H2N−•C(CH3)2. Time-resolved ESR spectra of the two new radicals were obtained, and the hyperfine couplings and g factors were determined. The radical formation times were consistent with both of the radicals being formed in the initial attack of the •OH radical on α-methylalanine. Measurements of the radical yields indicated that the aminyl radical was the major radical product from the initial •OH attack. The observations (a) that the aminyl radical decays via a first-order process (lifetime about 14 μs) and (b) that there is a large presence of •CO2- spin adducts with CH2NO2- are consistent with β scission being a prominent reaction of the aminyl radical from α-methylalanine. Aspects of the radiolytic oxidations by the •OH of α-methylalanine and glycine are contrasted with respect to initial radical formation and the subsequent reactions of the resulting radicals. These experimental findings corroborate predictions from density functional theory calculations of prompt decarboxylation following electron loss from the α-methylalanine anion and preferential formation of the N-centered radical via H abstraction.