Photochemical and Photobiotogical Studies with Acridine and Phenanthridine Hydroperoxides in Cell‐free DNA

Abstract— The acridine and phenanthridine hydroperoxides 3 and 7 were synthesized as photochemical hydroxyl radical sources for oxidative DNA damage studies. The generation of hydroxyl radicals upon UVA irradiation (Λ. = 350 nm) was verified by trapping experiments with 5,5‐di‐methyl‐1‐pyrroline N‐oxide and benzene. The enzymatic assays of the damage in cell‐free DNA from bacteriophage PM2 caused by the acridine and phenanthridine hydroperoxides 3 and 7 under near‐UVA irradiation revealed a wide range of DNA modifications. Particularly, extensive single‐strand break formation and DNA base modifications sensitive to formamidopyrimidine DNA glycosylase (Fpg protein) were observed. In the photooxida‐tion of calf thymus DNA, up to 0.69±0.03% 8‐oxo‐7,8‐dihydroguanine was formed by the hydroperoxides 3 and 7 on irradiation, whose yield was reduced up to 40% in the presence of the hydroxyl radical scavengers mannitol and fert‐butanol. The acridine and phenanthridine hydroperoxides 3 and 7 also induce DNA damage through the type I photooxidation process, for which photoinduced electron transfer from 2′‐deoxyguanosine to the singlet states of 3 and 7 was estimated by the Rehm‐Weller equation to possess a negative Gibb's free energy of cα ‐5 kcal/ mol. Control experiments with the sensitizers acridine 1 and the acridine alcohol 4 in calf thymus and PM2 DNA confirmed the photosensitizing propensity of the UVA‐ab‐sorbing chromophores. The present study emphasizes that for the development of selective and efficient photochemical hydroxyl radical sources, chromophores with low photosensitizing ability must be chosen to avoid type I and type II photooxidation processes.