In Situ Radiolysis Steady-State ESR Study of Carboxyalkyl Radical Trapping by 5,5-Dimethyl-1-pyrroline-N-oxide: Spin Adduct Structure and Stability

Abstract

Short-lived carboxyalkyl radicals formed in the reaction of three mono- and two dicarboxylic acids with radiolytically produced hydroxyl radicals or hydrated electrons were trapped successfully with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) in dilute aqueous solution. The in situ radiolysis steady-state ESR spectra of the spin adducts were analyzed to determine accurate ESR parameters for these spin adducts in a uniform environment. DMPO spin adducts of the five carboxyalkyl radicals were identified for the first time. Parent radicals include carboxymethyl, dicarboxymethyl, 1,2-dicarboxyethyl, 1-amino-1-carboxymethyl (glycine radical), and 2-carboxyethyl radicals. ESR parameters for DMPO spin adducts of carboxyalkyl radicals are rather similar with nitrogen hyperfine coupling constants of approximately 16 G and with g values around 2.0054 except for glycine radical. Proton hyperfine coupling constants due to C2−H vary between 21 and 25 G. Nitrogen and C2-proton hyperfine coupling constants are smaller than those of alkyl radical adducts with larger g values. These changes are ascribed to the spread of the π-conjugation system to include the carboxyl groups. Moreover, the ESR spectrum of both the carboxymethyl and the 1,2-dicarboxyethyl radical is a superposition of spectra from adduct radicals with protonated and deprotonated carboxyl groups in the spin addend. The protonated radical adducts have small additional proton hyperfine couplings due to these carboxyl protons. These carboxyalkyl radicals exhibit intramolecular hydrogen bonding between the hydroxyl function and the aminoxyl oxygen in aqueous solution, which has a profound effect upon the stability of their DMPO spin adducts against spin adduct rearrangement and fragmentation. From carboxymethyl and dicarboxymethyl−DMPO adducts, (CH3)2C(CH2CHCH2)N(O·)CH2CO2H and (CH3)2C(CH2CHCH2)N(O·)CH(CO2-)CO2H are proposed to form following the opening of the DMPO ring. A similar fragmentation reaction is proposed to occur from DMPO−dicarboxyhydroxymethyl adduct formed in γ-irradiated tartronic acid (hydroxymalonic acid) solution. DMPO spin adducts of glycine and 2-carboxyethyl radicals also show the small proton hyperfine splitting, revealing the intramolecular hydrogen bonding.