A Pulse Radiolysis Study of Some Dicarboxylic Acids of the Citric Acid Cycle. The Kinetics and Spectral Properties of the Free Radicals Formed by Reaction with the OH Radical

The free radicals formed by H-atom abstraction from many hydroxy-, monoand polycarboxylic acids have been characterized by a variety of fastkinetic techniques [1—4]. These studies led to some general conclusions, formulated by Hayon et al. [2], concerning the spectra and pK’s of the radicals. i) The absorption spectra of a-carboxyalkyl radicals have maxima in the spectral range of 320 to 350 nm with molar absorbances less than 1000 M -1cm_1. The absorption spectra of a-carboxy-a-hydroxyalkyl radicals have maxima near 250 nm with molar absorbances ranging from 5000 to 9000 M _1cm_1. The absorption spectra of polycarboxylic acid radi­ cals are in general combinations of the characteristics of the corresponding monocarboxylic acid radicals. ii) The dissociation constants for the carboxylic groups of polycarboxylic radicals are generally too close to be resolved but are usually higher than those of the parent compounds. iii) The pK of the hydroxyl group on the same carbon as the radical site is generally 3—4 pH units lower than that of the parent compound while that on a carbon adjacent to the radical site is relatively unchanged. The dicarboxylic acids studied here are fumaric and maleic acids (H 0 2CCH=CH C02H, the trans and cis isomers, respectively), malic acid (H 0 2CCH(0H)CH2C 0 2H) and succinic acid (H 0 2CCH2CH2C 0 2H). We undertook to determine the rates of reaction of these acids with the OH radi-