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 α/ 1 β) 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 α/ 1 β > 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 α cleavage of the intermediary triplet-excited α-hydroxy- and α-phenyl-substituted 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 α/ 1 β, and 2 .