| Abstract
| - The reaction of the amino acid anions, R2N−CR2−CO2- (R = H or methyl), with •OH radicals and H• atomswas quantified with respect to the site of attack, the respective absolute rate constants, and the yields of theprimary transients generated in these processes. The method applied was pulse radiolysis with time-resolvedoptical 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 andcompetition with carbonate, ranged from (1.7−3.6) × 109 M-1 s-1 for the reaction of •OH with the anions ofthese 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 radicalsfrom 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 ofdifferent and thus distinguishing reduction potentials, namely, 4-carboxybenzophenone, methyl viologen, andhexacyanoferrate-III. On the basis of the overall rate constants and the yields of the various transients, partialrate constants were evaluated for the attack of •OH at Cα−H, at the lone electron pair at nitrogen, and at themore remote methyl groups. The results substantiate earlier conclusions that the amino nitrogen is indeed thepreferred site of oxidative attack, but also that substantial amounts of R2N−C•R−CO2- type radicals areformed via direct abstraction of hydrogen from the Cα−H bond. Trends and individual data are discussed inthe light of structure and substitution pattern of the amino acids investigated.
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