Absolute Rate Constants and Yields of Transients from Hydroxyl Radical and H Atom Attack on Glycine and Methyl-Substituted Glycine Anions

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The reaction of the amino acid anions, R2N−CR2−CO2- (R = H or methyl), with •OH radicals and H• atoms was quantified with respect to the site of attack, the respective absolute rate constants, and the yields of the primary transients generated in these processes. The method applied was pulse radiolysis with time-resolved optical detection. Specifically investigated amino acids were glycine, alanine, α-methylalanine and N,N-dimethylglycine. Absolute overall rate constants, as determined from the growth of UV absorptions and competition with carbonate, ranged from (1.7−3.6) × 109 M-1 s-1 for the reaction of •OH with the anions of these amino acids, and (0.1−1) × 108 M-1 s-1 for the corresponding reaction with the respective zwitterions. H• atoms react with amino acid anions containing Cα−H bonds with a rate constant of 1.4 × 108 M-1 s-1, whereas k < 107 M-1 s-1 was estimated for the reaction with α-methylalanine. The primary transient radicals from these reactions include aminyl radicals (RN•-CR2−CO2-), α-amino-α-carboxyalkyl radicals, R2N−C•R−CO2-, α-aminoalkyl radicals, R2N−C•R2, and (•CH2−) type side-chain radicals (for R = CH3 compounds). The yields of the reducing species (all but aminyl) were determined via titration with electron acceptors of different and thus distinguishing reduction potentials, namely, 4-carboxybenzophenone, methyl viologen, and hexacyanoferrate-III. On the basis of the overall rate constants and the yields of the various transients, partial rate constants were evaluated for the attack of •OH at Cα−H, at the lone electron pair at nitrogen, and at the more remote methyl groups. The results substantiate earlier conclusions that the amino nitrogen is indeed the preferred site of oxidative attack, but also that substantial amounts of R2N−C•R−CO2- type radicals are formed via direct abstraction of hydrogen from the Cα−H bond. Trends and individual data are discussed in the light of structure and substitution pattern of the amino acids investigated.