Flash photolysis of misonidazole and metronidazole

Laser flash photolysis at 355 nm of misonidazole or metronidazole in aqueous solutions produced the relatively long-lived nitro radical anion as the only observable transient species. When 266 nm excitation was used, a small yield of solvated electron was observed. It is suggested that the nitroimidazole first undergoes photoionization and the photoelectrons are scavenged by ground state nitroimidazole molecules to produce the nitro radical anion. Alternatively, added EDTA or carbonate ion acted as an electron donor to the excited state nitroimidazole molecule, thereby increasing the yield of nitro radical anion. The transient yield from metronidazole was about half that from misonidazole, while the phosphorescence intensity of metronidazole in an ethanol glass was about 20 times that of misonidazole. The misonidazole n, pi* triplet state is more easily reduced than that of metronidazole and, in the presence of an electron donor, the radical anion is postulated to result from electron transfer to the triplet state of the nitroimidazole.