A Picosecond Flash-Photolysis Study of the Biphotonic Ionization of Psoralen Derivatives and Ethidium Bromide

Singlet molecular oxygen (1O2) was generated in aqueous solution (H2O or D2O) at 37 degrees C by the thermal dissociation of the endoperoxide of 3,3'-(1,4-naphthylidene) dipropionate (NDPO2). Guanosine and deoxyguanosine quench 1O2 with overall quenching rate constants of 6.2 X 10(6) M-1 s-1 and 5.2 X 10(6) M-1 s-1, respectively. Reaction with 1O2 results in the formation of 8-hydroxyguanosine (8-OH-Guo) and 8-hydroxydeoxyguanosine (8-OH-dGuo), respectively, with a yield of 1.5% at 1 mM substrate with an NDPO2 concentration of 40 mM; a corresponding 8-hydroxy derivative is not formed from deoxyadenosine. In D2O the yield of 8-OH-Guo is 1.5-fold that in H2O. Sodium azide suppresses 8-OH-Guo and 8-OH-dGuo production. In contrast, the hydroxyl radical scavengers, tert-butanol, 2-propanol, or sodium formate, do not decrease the production of the 8-OH derivatives. The formation of 8-OH derivatives is significantly increased (2-5-fold) by thiols such as dithiothreitol, glutathione, cysteine, and cysteamine. With use of a plasmid containing a fragment of the mouse metallothionein I promoter (pMTP3') and a novel end-labeling technique, the position of 1O2-induced single-strand breaks in DNA was examined. Strand breaks occur selectively at dGuo; no major differences (hot spots) were observed between individual guanines.