Glycine Decarboxylation:  The Free Radical Mechanism

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 HPO42-/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)63- 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)63-. 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)63- 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.