β-fragmentation and other reactions involving aminyl radicals from amino acids

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 alpha-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 x 10(8) 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 solution), which immediately decarboxylate and leave strongly reducing alpha-aminoalkyl radicals, (CR2NH2)-C-.. The rate-determining step in this reaction sequence has been shown to be the protonation of the aminyl function by, e.g., the respective zwitterions of the amino acids, or hydrogen phosphate. Absolute rate constants for these proton-transfer processes cover a 10(5)-10(8) M-1 s(-1) range (including previously published values for the glycine-derived aminyl radical). (ii) The aminyl radicals are further capable of abstracting a C-alpha-bound H atom (applicable for glycine and alanine), thereby generating C-centered alpha-amino-alpha-carboxyl radicals. A rate constant of 1.7 x 10(5) M-1 s(-1) has been obtained for this process in the alanine system. (ii) A third competing process of significance has been identified to be beta-fragmentation of the aminyl radicals into the respective imines and CO2.-. The latter radical anion was identified through its electron transfer to 4-carboxybenzophenone (k = 3.3 x 10(7) M-1 s(-1)). Rate constants for the beta-fragmentation itself have been determined to be 2.3 x 10(4) s(-1) for HN.-CH(CH3)-CO2-, and 7.4 x 10(4) s(-1) for HN.-C(CH3)(2)-CO2- They follow the trend predicted by density functional theory (DFT) calculations on the free energies of reaction and activation.