β-Fragmentation and Other Reactions Involving Aminyl Radicals from Amino Acids

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Aminyl radicals, HN•−CR2−CO2- (R = H, CH3) have been identified as significant transients in the hydroxyl radical-induced oxidation of the anions of glycine (Gly-), alanine (Ala-), and α-methylalanine (MeAla-) at relative yields of 37%, 43%, and 56%, respectively. Quantification has been achieved by two independent pulse radiolysis methods. Direct titration of the aminyl radicals made use of their capability to oxidize hydroquinone to the easily detectable semiquinone radical. The rate constant for this reaction involving the aminyl radical from alanine is 1.1 × 108 M-1 s-1 (at pH 11.0). An alternative method relied on the titration with methyl viologen and 4-carboxybenzophenone of the reducing radicals, which are formed as the result of secondary reactions of the aminyl radicals. In fact, several processes may occur and compete with each other in this case: (i) In the presence of proton donors the aminyl radicals can effectively be converted into H2N•+−CR2−CO2- radical zwitterions (even in basic solutio...