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(2 H )-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, 1 H and 13 C 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 efficiently photooxidize 8-oxodGuo 1 via a type II mechanism ( 1 O 2 ), whereas riboflavin and benzophenone, which act mostly by a type I mechanism, were far less efficient.