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

Aminyl radicals, HN.-CR2-CO2- (R = H, CH3) have been identified as signific ant transients in the hydroxyl radical-induced oxidation of the anions of g lycine (Gly(-)), alanine (Ala(-)), and alpha-methylalanine (MeAla(-)) at re lative yields of 37%, 43%, and 56%, respectively. Quantification has been a chieved by two independent pulse radiolysis methods. Direct titration of th e aminyl radicals made use of their capability to oxidize hydroquinone to t he easily detectable semiquinone radical. The rate constant for this reacti on involving the aminyl radical from alanine is 1.1 x 10(8) M-1 s(-1) (at p H 11.0). An alternative method relied on the titration with methyl viologen and 4-carboxybenzophenone of the reducing radicals, which are formed as th e result of secondary reactions of the aminyl radicals. In fact, several pr ocesses may occur and compete with each other in this case: (i) In the pres ence of proton donors the aminyl radicals can effectively be converted into H2N.+-CR2-CO2- radical zwitterions (even in basic solution), which immedia tely decarboxylate and leave strongly reducing alpha-aminoalkyl radicals, ( CR2NH2)-C-.. The rate-determining step in this reaction sequence has been s hown to be the protonation of the aminyl function by, e.g., the respective zwitterions of the amino acids, or hydrogen phosphate. Absolute rate consta nts 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 radi cal). (ii) The aminyl radicals are further capable of abstracting a C-alpha -bound H atom (applicable for glycine and alanine), thereby generating C-ce ntered 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-frag mentation of the aminyl radicals into the respective imines and CO2.-. The latter radical anion was identified through its electron transfer to 4-carb oxybenzophenone (k = 3.3 x 10(7) M-1 s(-1)). Rate constants for the beta-fr agmentation 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 tre nd predicted by density functional theory (DFT) calculations on the free en ergies of reaction and activation.