Oxidative Damage of Thymidines by the Atmospheric Free-Radical Oxidant NO3•

Abstract

Analysis of the products formed in the reaction of nitrate radicals, NO3 •, with the N- and O-methylated and acetylated thymidines 1a and 1b revealed, for the first time, insight regarding how this important atmospheric free-radical oxidant can cause irreversible damage to DNA building blocks. Mechanistic studies indicated that the initial reaction step likely proceeds via NO3 • induced electron transfer at the pyrimidine ring, followed by deprotonation of the methyl group at C5. The oxidation ultimately leads to formation of nitrates 2, aldehydes 4 and, in the case of high [NO3 •], also to carboxylic acids 5. In addition to this, through a very minor pathway, loss of the methyl group at C5 also occurred to give the respective 2′-deoxyuridines 6. The nitrates 2 are highly labile compounds that undergo rapid hydrolysis during work-up and purification of the reaction mixtures, which could lead to serious misinterpretation of the experimental findings and reaction mechanism. Products resulting from NO3 • addition to the C5=C6 double bond in the pyrimidine ring were not observed. Also, no reaction of NO3 • with the 2′-deoxyribose moiety was detected.