Photosensitized Reaction of 8-Oxo-7,8-dihydro-2‘-deoxyguanosine: Identification of 1-(2-Deoxy-β-d-erythro-pentofuranosyl)cyanuric Acid as the Major Singlet Oxygen Oxidation Product

The main photosensitized oxidation product in aqueous solution of 8-oxo-7,8-dihydro-2‘-deoxyguanosine (1) has been isolated by HPLC and characterized as 1-(2-deoxy-β-d-erythro-pentofuranosyl)-cyanuric acid (7) and its precursor, 1,3,5-triazine-1(2H)-carboximidamide, 3-(2-deoxy-β-d-erythro-pentofuranosyl)tetrahydro-2,4,6-trioxo- (6). This was achieved by carrying out extensive spectroscopic measurements including FAB mass spectrometry, 1H and 13C NMR analyses. The formation of the photooxidized nucleoside 7 is accounted for by a type II mechanism through initial [2+2] cycloaddition across the C-4−C-5 ethylenic bond of 1. The 1,2 bond cleavage of the transient dioxetanes gives rise to the modified nucleoside 6 through a nine-membered ring oxidized intermediate nucleoside. The derivative 6 further decomposes into the stable cyanuric acid nucleoside 7 by the hydrolysis of the guanidino residue with the concomitant release of urea 8. Photosensitizers such as methylene blue and rose Bengal were found to efficie...