This review discusses recent aspects of oxidation reactions of DNA and model compounds involving mostly OH radicals, one-electron transfer process and singlet oxygen (1O2). Emphasis is placed on the formation of double DNA lesions involving a purine base on one hand and either a pyrimidine base or a 2-deoxyribose moiety on the other hand. Structural and mechanistic information is also provided on secondary oxidation reactions of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodGuo), a major DNA marker of oxidative stress. Another major topic which is addressed here deals with recent developments in the measurement of oxidative base damage to cellular DNA. This has been mostly achieved using the accurate and highly specific HPLC method coupled with the tandem mass spectrometry detection technique. Interestingly, optimized conditions of DNA extraction and subsequent work-up allow the accurate measurement of 11 modified nucleosides and bases within cellular DNA upon exposure to oxidizing agents, including UVA and ionizing radiations. In addition, the modified comet assay, which involves the use of bacterial DNA N-glycosylases to reveal two main classes of oxidative base damage, is applicable to isolated cells and is particularly suitable when only small amounts of biological material are available. Finally, recently available data on the substrate specificity of DNA repair enzymes belonging to the base excision pathways are briefly reviewed.

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The hydroxyl radical-mediated formation of two tandem base lesions within DNA including N-(2deoxy-β-D-erythro-pentofuranosyl)formylamine-8-oxo-7,8-dihydro-2′-deoxyguanosine (dβF-8-oxodGuo) and 8-oxodGuo-dβF is reported in this study. A specific enzymatic processing was developed to quantitatively release the tandem lesions as dinucleoside monophosphates from DNA. Then, the resulting hydrolyzed DNA samples were analyzed using liquid chromatography coupled to electrospray ionization tandem mass spectrometry. The simultaneous measurement of the two vicinal lesions was performed in the negative mode using the accurate and sensitive multiple reaction monitoring technique. For this purpose, two characteristic ions arising from the fragmentation of the pseudo-molecular ion [M -H]were monitored. Both dβF-8-oxodGuo and 8-oxodGuo-dβF damage were found to be generated in γ-irradiated DNA as a significant fraction of • OH radical-induced base damage. Interestingly, 8-oxodGuo-dβF was produced in a much higher yield than the reversed sequence lesion. Indirect evidence is provided for the formation of other tandem lesions involving 8-oxodGuo, but that still remain to be fully identified. Insights into the mechanism of formation of the DNA damage were gained from several experiments including DNA photosensitization, γ-irradiation in the presence of iron, and exposure to Fenton reagents. This allowed refinement of the proposed pathways for the formation of dβF-8-oxodGuo and 8-oxodGuo-dβF tandem base damage.

On the basis of molecular modeling studies, the 7-nitroindole nucleoside 1 was selected as a suitable photochemical precursor for photochemical generation of the C1' deoxyribosyl radical under irradiation, which led to 2'-deoxyribonolactone. The nitro-indole nucleoside derivatives 1a and 1b were prepared and their conformation was determined by X-ray crystallography and NMR spectroscopy. The photoreaction of these nucleosides gave the corresponding deoxyribonolactone derivatives efficiently, with release of 7-nitrosoindole. This reaction was successfully applied to synthesis of oligonucleotides containing the deoxyribonolactone lesion.