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

Abstract

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 solution), which immediately decarboxylate and leave strongly reducing α-aminoalkyl radicals, •CR2NH2. 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 105−108 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 Cα-bound H atom (applicable for glycine and alanine), thereby generating C-centered α-amino-α-carboxyl radicals. A rate constant of 1.7 × 105 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 β-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 × 107 M-1 s-1). Rate constants for the β-fragmentation itself have been determined to be 2.3 × 104 s-1 for HN•−CH(CH3)−CO2 -, and 7.4 × 104 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.

Hydrogen Atom Transfer in Model Reactions

“…Aminyl radicals have also been detected indirectly during the reaction of hydroxyl radicals (HO ) or their conjugated base ( O À ) with the free amino group of amino acids (Reactions (3.9) and (3.10)) [40][41][42][43], and identified by time-resolved EPR experiments [44]. Similar reactions may be expected for peptides.…”

“…In their studies on the radical-induced decarboxylation of Gly anion, Bonifačić et al observed a protoncatalyzed decarboxylation of H 2 NaC HaCO 2 À , which may proceed via Reactions (3.16)-(3.18)[40,41]. In their studies on the radical-induced decarboxylation of Gly anion, Bonifačić et al observed a protoncatalyzed decarboxylation of H 2 NaC HaCO 2 À , which may proceed via Reactions (3.16)-(3.18)[40,41].…”

The NOx System in Homogeneous and Heterogeneous Nuclear Waste

“…Our studies on radiolytic and photolytic oxidation reactions of aminocarboxylate ions have provided a significant advance in understanding the mechanism of OH attack. [5][6][7][8] Complementary electronic structure calculations have yielded insight into the mechanism of the radiolytic oxidation in the model complexants glycine and its methylated derivatives. 5,6 Attack by the hydroxyl radical in basic solution had previously been shown to produce a carbon-centered radical, presumably via hydrogen abstraction.…”

“…[5][6][7][8] Complementary electronic structure calculations have yielded insight into the mechanism of the radiolytic oxidation in the model complexants glycine and its methylated derivatives. 5,6 Attack by the hydroxyl radical in basic solution had previously been shown to produce a carbon-centered radical, presumably via hydrogen abstraction. 9 Theoretical predictions of prompt decarboxylation following electron transfer to give the aminomethyl radical, 6 and suggestions that competitive H-abstraction would produce the aminyl radicals previously inferred from pulse radiolysis studies, 5 have been verified by observations of both radical products by time-resolved EPR spectroscopy (TRESR) and spin trapping experiments.…”

“…5,6 Attack by the hydroxyl radical in basic solution had previously been shown to produce a carbon-centered radical, presumably via hydrogen abstraction. 9 Theoretical predictions of prompt decarboxylation following electron transfer to give the aminomethyl radical, 6 and suggestions that competitive H-abstraction would produce the aminyl radicals previously inferred from pulse radiolysis studies, 5 have been verified by observations of both radical products by time-resolved EPR spectroscopy (TRESR) and spin trapping experiments. 7,8 β-Scission of the aminyl radical has been theoretically described, 6 and the mechanism of formation of the previouslydetected carboxyl radicals 10 has thus been discovered.…”