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

The reaction of singlet molecular oxygen with 2'-deoxyguanosine and DN A was studied. Emphasis was placed on the identification and character ization of the main methylene blue mediated type II (singlet oxygen) o xidation products of 2'-deoxyguanosine and its corresponding 3',5'-di- O-acetylated derivative. Two major oxidation products of 2'-deoxyguano sine were isolated and characterized by mass spectrometry analysis and extensive H-1 and C-13 NMR measurements as the two 4R and 4S* diaste reomers 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 correspond ing 3',5'-di-O-acetylated derivative. Methylene blue mediated photosen sitization of 2'-deoxyguanosine led also to the production of 7,8-dihy dro-8-oxo-2'-deoxyguanosine, but in a relatively lower yield with resp ect to the two above diastereomers. The participation of singlet oxyge n in the mechanism of formation of these oxidation products was confir med. A reasonable mechanism involving the transient formation of an un stable endoperoxide produced through a Diels-Alder 1,4-cycloaddition o f singlet oxygen to the purine ring is suggested. Quantitative analysi s allowed us to demonstrate that the two diastereomers of 4,8-dihydro- 4-hydroxy-8-oxo-2'-deoxyguanosine are the main singlet oxygen oxidatio n products of the guanine moiety within nucleosides, whereas 7,8-dihyd ro-8-oxoguanine was found to be the major O-1(2) oxidation product of guanine in double-stranded DNA.