REACTION OF SINGLET OXYGEN WITH 2'-DEOXYGUANOSINE AND DNA - ISOLATION AND CHARACTERIZATION OF THE MAIN OXIDATION-PRODUCTS

The reaction of singlet molecular oxygen with 2'-deoxyguanosine and DNA was studied. Emphasis was placed on the identification and characterization of the main methylene blue mediated type II (singlet oxygen) oxidation products of 2'-deoxyguanosine and its corresponding 3',5'-di-O-acetylated derivative. Two major oxidation products of 2'-deoxyguanosine were isolated and characterized by mass spectrometry analysis and extensive H-1 and C-13 NMR measurements as the two 4R* and 4S* diastereomers of 4,8-dihydro-4-hydroxy-8-oxo-2'deoxyguanosine. The addition of O-1(2) was also found to occur to the base moiety of the corresponding 3',5'-di-O-acetylated derivative. Methylene blue mediated photosensitization of 2'-deoxyguanosine led also to the production of 7,8-dihydro-8-oxo-2'-deoxyguanosine, but in a relatively lower yield with respect to the two above diastereomers. The participation of singlet oxygen in the mechanism of formation of these oxidation products was confirmed. A reasonable mechanism involving the transient formation of an unstable endoperoxide produced through a Diels-Alder 1,4-cycloaddition of singlet oxygen to the purine ring is suggested. Quantitative analysis allowed us to demonstrate that the two diastereomers of 4,8-dihydro-4-hydroxy-8-oxo-2'-deoxyguanosine are the main singlet oxygen oxidation products of the guanine moiety within nucleosides, whereas 7,8-dihydro-8-oxoguanine was found to be the major O-1(2) oxidation product of guanine in double-stranded DNA.