Absolute rate constants and yields of transients from hydroxyl radical andH atom attack on glycine and methyl-substituted glycine anions

The reaction of the amino acid anions, R2N-CR2-CO2- (R = H or methyl), with (OH)-O-. radicals and H* atoms was quantified with respect to the site of attack, the respective absolute rate constants, and the yields of the prima ry transients generated in these processes. The method applied was pulse ra diolysis with time-resolved optical detection. Specifically investigated am ino acids were glycine, alanine, alpha -methylalanine and N,N-dimethylglyci ne. Absolute overall rate constants, as determined from the growth of UV ab sorptions and competition with carbonate, ranged from (1.7-3.6) x 10(9) M-1 s(-1) for the reaction of (OH)-O-. with the anions of these amino acids, a nd (0.1 - 1) x 10(8) M-1 s(-1) for the corresponding reaction with the resp ective zwitterions. H* atoms react with amino acid anions containing C-alph a-H bonds with a rate constant of 1.4 x 10(8) M-1 s(-1), whereas k < 10(7) M-1 s(-1) was estimated for the reaction with alpha -methylalanine. The pri mary transient radicals from these reactions include aminyl radicals (RN.-C R2-CO2-), alpha -amino-alpha -carboxyalkyl radicals, R2N-(CR)-R-.-CO2-, alp ha -aminoalkyl radicals, R2N-(CR2)-R-., and ((CH2-)-C-.) type side-chain ra dicals (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-carboxybenzophenon e, methyl viologen, and hexacyanoferrate-III. On the basis of the overall r ate constants and the yields of the various transients, partial rate consta nts were evaluated for the attack of (OH)-O-. at C-alpha-H, at the lone ele ctron pair at nitrogen, and at the more remote methyl groups. The results s ubstantiate earlier conclusions that the amino nitrogen is indeed the prefe rred site of oxidative attack, but also that substantial amounts of R2N-(CR )-R-.-CO2- type radicals are formed via direct abstraction of hydrogen from the C,,,-H bond. Trends and individual data are discussed in the light of structure and substitution pattern of the amino acids investigated.