OH Radical Induced Decarboxylation of Amino Acids. Decarboxylation vs Bond Formation in Radical Intermediates

The *OH radical reaction with exo-2-amino-endo-6-(methylthio)bicyclo[2.2.1]heptane-endo-2-carboxylic acid primarily affords oxidation of the sulfur center in the molecule. The subsequent pathway strongly depends on pH. A transient radical with interaction between the sulfur and the carboxylate moieties is stabilized particularly in acid solutions with maximum yield at pH 3. It is characterized by a sulfur-carboxyl bond, which exhibits typical features of 2σ/1σ* three-electron bonds. It exhibits an optical absorption (λ_max 340 nm) and decays with t_1/2 ≈ 26 µs via deprotonation to an α-thioalkyl carbon-centered radical. This transient bond formation between the carboxyl group and the oxidized sulfur at low pH successfully prevents a competing process, namely, decarboxylation, which takes over at pH > 4. The underlying mechanism is considered to be a concerted action involving an electron transfer from the anionic carboxylate to the oxidized sulfur atom, homolytic carbon-carboxyl bond breakage, and deprotonation of the amino group. Related studies indicate that this kind of radical-induced decarboxylation can be generalized and receives its driving force to a significant extent from the resonance stabilization of the α-amino radical remaining after CO₂ cleavage.