Absolute Rate Constants and Yields of Transients from Hydroxyl Radical and H Atom Attack on Glycine and Methyl Substituted Glycine Anions (CROSBI ID 92955)
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Štefanić, Igor ; Bonifačić, Marija ; Asmus, Klaus-Dieter ; Armstrong, David A.
engleski
Absolute Rate Constants and Yields of Transients from Hydroxyl Radical and H Atom Attack on Glycine and Methyl Substituted Glycine Anions
The reaction of the amino acid anions, R2N-CR2-CO2 (R = H or alkyl) 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, 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 M1 s1 for the reaction of OH with the anions of these amino acids, and (0.1 -1) ´ 108 M1 s1 for the corresponding reaction with the respective zwitterions. H-Atoms react with amino acid anions containing CaH bonds with a rate constant of 1.4 ´ 108 M1 s1 whereas k < 107 M1 s1 was estimated for the reaction with a-methylalanine. The primary transient radicals from these reactions include aminyl radicals (RN-CR2-CO2), a-amino-a-carboxyalkyl radicals, R2N-CR-CO2, a-aminoalkyl radicals, R2N-CR2, 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. Based on the overall rate constants and the yields of the various transients, partial rate constants were evaluated for the attack of OH at CaH, 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-CR-CO2 type radicals are formed via direct abstraction of hydrogen from the CaH bond. Trends and individual data are discussed in the light of structure and substitution pattern of the amino acids investigated.
amino acid anions; pulse radiolysis; radicals; rate constants
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