Evidence for β scission in the oxidation of amino acids

Time-resolved ESR and steady-state spin trapping were used to identify the radicals and their kinetics following the radiolytic oxidation by the (OH)-O-circle radical of the anion of a-methylalanine, H2N-C(CH3)(2)-CO2-. Spectra of two new radicals were identified in aqueous solution, the aminyl radical, HNcircle-C(CH3)(2)-CO2-, and the decarboxylated radical, H2N-circleC(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)-O-circle radical on o.-methylaianine. Measurements of the radical yields indicated that the aminyl radical was the major radical product from the initial (OH)-O-circle attack. The observations (a) that the aminyl radical decays via a first-order process (lifetime about 14 mus) and (b) that there is a large presence Of (CO2-)-C-circle spin adducts with CH2=NO2- are consistent with beta scission being a prominent reaction of the aminyl radical from alpha-methylalanine. Aspects of the radiolytic oxidations by the (OH)-O-circle of alpha-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 alpha-methylaianine anion and preferential formation of the N-centered radical via H abstraction.