TYPE-I AND TYPE-II PHOTOSENSITIZED OXIDATIVE MODIFICATION OF 2'-DEOXYGUANOSINE (DGUO) BY TRIPLET-EXCITED KETONES GENERATED THERMALLY FROM THE 1,2-DIOXETANE HTMD

The nucleoside 2'-deoxyguanosine (dGuo) was treated with 3-(hydroxymet hyl)-3,4,4-trimethyl-1,2-dioxetane (HTMD), the latter generates effici ently triplet-excited carbonyl products on thermal decomposition in th e dark. The type I photooxidation products, 2, D-erythro-pentofuranosy l)-4-amino]-5(2H)-oxazolone (oxazolone) and the cyclic nucleoside -bet a-D-erythro-pentofuranosyl)-5-guanidinylidene- 2-hydroxy-4-oxoimidazol idine (oxoimidazolidine), as well as the type II photooxidation produc ts 4-(R)- and (S)*-4-hydroxy-8-oxo-4,8-dihydro-2'-deoxyguanosine (4-H O-8-oxodGuo) and 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodGuo), were quantitatively determined by appropriate selective and sensitive HPLC assays. The concentration and time profiles revealed that about 40% o f the triplet ketones derived from the thermal decomposition of HTMD l ed to photooxidation of dGuo. Essentially equal amounts of type I and type II photooxidation products were found, as could be established by comparison with predominant type I (benzophenone, riboflavin) and typ e II (Rose Bengal, methylene blue) photosensitizers. The participation of singlet oxygen (type II activity) was confirmed by the substantial D2O effect in the formation of 8-oxodGuo. The results demonstrate tha t dioxetanes, particularly HTMD, are efficient photooxidants of dGuo o n thermal activation in the dark and constitute excellent chemical too ls to study photobiological processes without the use of light, in the present case, photogenotoxicity.