Oxidative Damage of Pyrimidine Nucleosides by the Environmental Free Radical Oxidant NO3• in the Absence and Presence of NO2• and Other Radical and Non-Radical Oxidants

Abstract

Analysis of the products formed in the reaction of the environmental free radical oxidant NO3• with permethylated uridine 1 and thymidine 2 in solution revealed highly complex reaction pathways following initial NO3• induced oxidative electron transfer at the pyrimidine ring. Product formation was found to depend not only on the nature of the nucleobase, but also on the presence of other free radical oxidants, namely NO2•. In the reaction of 1 with NO3•, which was generated through CAN photolysis, apart from formation of the highly oxidized nucleoside derivative 4 as the major product, cleavage of the C–N glycosidic bond did also occur, resulting in formation of ribolactone 5 and the free nucleobase 6. The suggested mechanism involves in situ generation of NO2• during the course of the reaction, which promotes conversion of the initially formed radical cation 7 to 4 in an autocatalytic fashion.When the reaction of NO2• with O3 was used to generate NO3•, the initially formed radical cation 7 in the reaction with permethylated uridine 1 is rapidly trapped by NO2• to give 5-nitrouridine 18 in a radical mediated vinylic substitution reaction. In contrast to this, under similar conditions in the reaction involving thymidine 2 the highly oxidized products 20 and 21 are obtained as major compounds, which result from addition to the C5–C6 double bond. No direct reaction between NO3• and the carbohydrate moiety in 1 and 2 was found. Also, no reaction occurred between the nucleosides and mixtures of NO2•/N2O4 and O3/O2, respectively.