Radiation chemical methods were used to investigate the reactions of glycine anions, H2NCH2CO2 - (Gly-), with •OH, (CH3)2C•OH, and •CH3 radicals. A major and most significant product from all of these processes is CO2. Pulse-radiolysis revealed that the initial step in the •OH-induced mechanism is oxidation of the amino group, producing +H2N•-CH2-CO2 - and HN•-CH2-CO2 - with yields of 63% and 37%, respectively. The amino radical cation, +H2N•-CH2-CO2 -, suffers fast (≤100 ns) fragmentation into CO2 + •CH2NH2. The other primary radical, HN•-CH2-CO2 -, can also be converted into the decarboxylating +H2N•-CH2-CO2 - by reaction with proton donors such as phosphate (H2PO4 -/k = 7.4 × 107 M-1 s-1, and HPO4 2-/k = 2.5 × 105 M-1 s-1) or the glycine zwitterion, Gly± (k = 3.9 × 105 M-1 s-1), but only on a much longer (typically μs to ms) time scale (k ≈ 4 × 105 M-1 s-1). Competitively, the HN•-CH2-CO2 - transforms into a carbon-centered radical H2N-C•H-CO2 - either by an intramolecular 1,2-H-atom shift (k = (1.2 ± 1.0) × 103 s-1) or by bimolecular reaction with Gly- (k = (3.0 ± 0.2) × 104 M-1 s-1). Both C-centered radicals, H2N-C•H-CO2 - and •CH2NH2, are reductants as verified through their reactions with Fe(CN)6 3- and methyl viologen (MV2+) in pulse-radiolysis experiments (k ≈ 4 × 109 M-1 s-1). The eventual complete transformation of all primary radicals into H2N-C•H-CO2 - and •CH2NH2 was further substantiated by γ-radiolytic reduction of Fe(CN)6 3-. In the presence of suitable electron donors, the HN•-CH2-CO2 - radical acts as an oxidant. This was demonstrated through its reaction with hydroquinone (k = (7.4 ± 0.5) × 107 M-1 s-1). Although the C-centered H2N-C•H-CO2 - radical is not generated in a direct H-atom abstraction by •OH, this radical appears to be the exclusive product in the reaction of Gly- with (CH3)2C•OH, •CH2NH2, and •CH3 (k ≈ 102 M-1 s-1). A most significant finding is that H2N-C•H-CO2 - can be converted into the decarboxylating N-centered radical cation +H2N•-CH2-CO2 - by reaction with proton donors such as Gly± (k ≈ 3 × 103 M-1 s-1) or phosphate and thus also becomes a source of CO2. The •CH2NH2-induced route establishes, in fact, a chain mechanism which could be proven through dose rate effect experiments and suppression of the chain upon addition of Fe(CN)6 3- or MV2+ as a scavenger for the reducing precursor radicals. The possible initiation of amino acid decarboxylation by C-centered radicals and the assistance of proton donors at various stages within the overall mechanism are considered to be of general significance and interest in chemical and biological systems.

The reaction of the amino acid anions, R2N−CR2−CO2 - (R = H or methyl), with •OH radicals and H• atoms was quantified with respect to the site of attack, the respective absolute rate constants, and the yields of the primary transients generated in these processes. The method applied was pulse radiolysis with time-resolved optical detection. Specifically investigated amino acids were glycine, alanine, α-methylalanine and N,N-dimethylglycine. Absolute overall rate constants, as determined from the growth of UV absorptions and competition with carbonate, ranged from (1.7−3.6) × 109 M-1 s-1 for the reaction of •OH with the anions of these amino acids, and (0.1−1) × 108 M-1 s-1 for the corresponding reaction with the respective zwitterions. H• atoms react with amino acid anions containing Cα−H bonds with a rate constant of 1.4 × 108 M-1 s-1, whereas k < 107 M-1 s-1 was estimated for the reaction with α-methylalanine. The primary transient radicals from these reactions include aminyl radicals (RN•-CR2−CO2 -), α-amino-α-carboxyalkyl radicals, R2N−C•R−CO2 -, α-aminoalkyl radicals, R2N−C•R2, and (•CH2−) type side-chain radicals (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-carboxybenzophenone, methyl viologen, and hexacyanoferrate-III. On the basis of the overall rate constants and the yields of the various transients, partial rate constants were evaluated for the attack of •OH at Cα−H, at the lone electron pair at nitrogen, and at the more remote methyl groups. The results substantiate earlier conclusions that the amino nitrogen is indeed the preferred site of oxidative attack, but also that substantial amounts of R2N−C•R−CO2 - type radicals are formed via direct abstraction of hydrogen from the Cα−H bond. Trends and individual data are discussed in the light of structure and substitution pattern of the amino acids investigated.