OXIDATIVE DNA-DAMAGE BY RADICALS GENERATED IN THE THERMOLYSIS OF HYDROXYMETHYL-SUBSTITUTED 1,2-DIOXETANES THROUGH THE ALPHA-CLEAVAGE OF CHEMIEXCITED KETONES

The 3-(hydroxymethyl)-3,4,4-trimethyl-1,2-dioxetane (HTMD) highly effi ciently damages DNA compared to the merely alkyl-substituted derivativ e 3,3,4,4-tetramethyl-1,2-dioxetane (TMD). To elucidate this differenc e in oxidative reactivity, two additional hydroxymethyl-substituted 1, 2-dioxetanes, namely s-3-(hydroxymethyl)-3,4-dimethyl-4-(phenylmethyl) - (1 alpha/1 beta) and ethyl)-4,4-dimethyl-3-(phenylmethyl)-1,2-dioxet ane (2), were investigated in regard to their photochemical and photob iological properties. The high genotoxic effects of the hydroxymethyl- substituted 1,2-dioxetanes, which are reflected in the significant for mation of single-strand breaks in plasmid pBR 322 DNA and the efficien t oxidation of guanine in calf thymus DNA and the nucleoside 2'-deoxyg uanosine (dGuo), are for the first time understood in terms of radical chemistry. The reactivity order of the dioxetanes 1 alpha/1 beta > HT MD > 2 >> TMD to damage DNA parallels the propensity of these dioxetan es to generate radicals. These reactive species are formed in the ther molysis of the dioxetanes through alpha cleavage of the intermediary t riplet-excited alpha-hydroxy- and alpha-phenylsubstituted carbonyl pro ducts. The presence of radicals was confirmed by spin-trapping experim ents with 5,5-dimethyl-1-pyrroline N-oxide and by laser-flash photolys is. These carbon-centered radicals are efficiently scavenged by molecu lar oxygen to produce peroxyl radicals, which are proposed as the acti ve DNA-damaging species in the thermal decomposition of the hydroxymet hyl-substituted 1,2-dioxetanes HTMD, 1 alpha/1 beta, and 2.