Furocoumarin dioxetanes and hydroperoxides as novel photobiological DNA damaging agents

Abstract

Photoexcitation of psoralen, an excellent DNA intercalator, was achieved in the dark by thermal decomposition of psoralen-functionalized dioxetane 2. The 3,4 monoadduct of psoralen was formed when dioxetane 2 was treated with calf thymus DNA in H₂0 at 50°C. Photobinding of psoralen to DNA was also observed when alkyl-substituted dioxetane 1 was thermally decomposed in the presence of psoralen and DNA. The dioxetanes of trimethyl-substituted furocoumarins 4 showed strong mutafenic activity in the Salmonella typhimurium strain T AI00 (sensitive to alkylating agents and UV²⁶⁰ radiation). The mutagenicity of furocoumarin dioxetanes 4 is comparable to that of benzofuran dioxetanes 5, the latter constitute potent mutagens. Presumably DNA adducts of the intermediary furocoumarin epoxides, generated in situ by deoxygenation of the dioxetanes, are responsible for the mutagenicity. The furocoumarin hydroperoxides 8,9, which were synthesized by photooxygenation of alloimperatorin (6) and imperatorin (7), are efficient photoactive DNA-damaging agents. They induced single strand breaks and endonuclease-sensitive modifications in supercoiled bacteriophage PM2 DNA upon irradiation at 360 nm. The DNA damage profiles exhibited by furocoumarin hydroperoxides 8,9 are similar to those of hydroxyl radicals, generated either by 'Y radiation or by xanthine and xanthine oxidase in the presence of Fe(III)-EDT A. tert-Butanol, an efficient hydroxyl radical scavenger, reduced the formation of DNA modifications significantly. Moreover, in the absence of near UV radiation, the furocoumarin hydroperoxides did not cause any DNA damage. These results indicate that the hydroxyl radicals produced in the photolysis of the hydroperoxides 8,9 are the ultimate DNA-damaging agents.