Oxidative DNA damage by radicals generated in the thermolysis of hydroxymethyl-substituted 1,2-dioxetanes through the α cleavage of chemiexcited ketones

The 3-(hydroxymethyl)-3,4,4-trimethyl-1,2-dioxetane (HTMD) highly efficiently damages DNA compared to the merely alkyl-substituted derivative 3,3,4,4-tetramethyl-1,2-dioxetane (TMD). To elucidate this difference in oxidative reactivity, two additional hydroxymethyl-substituted 1,2-dioxetanes, namely cis/trans-3-(hydroxymethyl)-3,4-dimethyl-4-(phenylmethyl)- (1 alpha/1 beta) and 3-(hydroxymethyl)-4,4-dimethyl-3-(phenylmethyl)-1,2-dioxetane (2), were investigated in regard to their photochemical and photobiological properties. The high genotoxic effects of the hydroxymethyl-substituted 1,2-dioxetanes, which are reflected in the significant formation of single-strand breaks in plasmid pBR 322 DNA and the efficient oxidation of guanine in calf thymus DNA and the nucleoside 2'-deoxyguanosine (dGuo), are for the first time understood in terms of radical chemistry. The reactivity order of the dioxetanes 1 alpha/1 beta > HTMD > 2 >> TMD to damage DNA parallels the propensity of these dioxetanes to generate radicals. These reactive species are formed in the thermolysis of the dioxetanes through alpha cleavage of the intermediary triplet-excited alpha-hydroxy- and alpha-phenylsubstituted carbonyl products. The presence of radicals was confirmed by spin-trapping experiments with 5,5-dimethyl-1-pyrroline N-oxide and by laser-flash photolysis. These carbon-centered radicals are efficiently scavenged by molecular oxygen to produce peroxyl radicals, which are proposed as the active DNA-damaging species in the thermal decomposition of the hydroxymethyl-substituted 1,2-dioxetanes HTMD, 1 alpha/1 beta, and 2.