Scavenging of Alkylperoxyl Radicals by Addition to Ascorbate: An Alternative Mechanism to Electron Transfer

Vitamin C (ascorbate; Asc) is a biologically important antioxidant that scavenges reactive oxygen species such as deleterious alkylperoxyl radicals (ROO•), which are generated by radical-mediated oxidation of biomolecules in the presence of oxygen. The radical trapping proprieties of Asc are conventionally attributed to its ability to undergo single-electron transfers with reactive species. According to this mechanism, the reaction between Asc and ROO• results in the formation of dehydroascorbate (DHA) and the corresponding hydroperoxides (ROOH). When studying the reactivity of DNA 5-(2′-deoxyuridinyl)methylperoxyl radicals, we discovered a novel pathway of ROO• scavenging by Asc. The purpose of this study is to elucidate the underlying mechanism of this reaction with emphasis on the characterization of intermediate and final decomposition products. We show that the trapping of ROO• by Asc leads to the formation of an alcohol (ROH) together with an unstable cyclic oxalyl-l-threonate intermediate (cOxa-Thr), which readily undergoes hydrolysis into a series of open-chain oxalyl-l-threonic acid regioisomers. The structure of products was determined by detailed MS and NMR analyses. The above transformation can be explained by initial peroxyl radical addition (PRA) onto the C2=C3 enediol portion of Asc. Following oxidation of the resulting adduct radical, the product subsequently undergoes Baeyer-Villiger rearrangement, which releases ROH and generates the ring expansion product cOxa-Thr. The present investigation provides robust clarifications of the peroxide-mediated oxidation chemistry of Asc and DHA that has largely been obscured in the past by interference with autooxidation reactions and difficulties in analyzing and characterizing oxidation products. Scavenging of ROO• by PRA onto Asc may have beneficial consequences since it directly converts ROO• into ROH, which prevents the formation of potentially deleterious ROOH, although it induces the breakdown of Asc into fragments of oxalyl-l-threonic acid.