6-Nitroquinolones in dimethylsulfoxide: Spectroscopic characterization and photoactivation of molecular oxygen

Abstract

Abstract The synthesized 6-nitroquinolone derivatives were characterized by UV/vis, NMR spectroscopy and the EPR spin trapping technique was applied to monitor the photoinduced generation of non-persistent radical species. The NMR spectra of ethyl 1,4-dihydro-6-nitro-4-oxoquinoline-3-carboxylate ( 1 ) and 1,4-dihydro-6-nitro-4-oxoquinoline-3-carboxylic acid ( 3 ) confirmed the dominance of the oxo-tautomer in dimethylsulfoxide (DMSO) solutions. The electronic absorption spectra of 6-nitroquinolones 1 and 3 with the amino hydrogen at nitrogen of the enaminone system (N–C=C–C=O) revealed the interaction of >NH with the aprotic DMSO solvent, which leads to the formation of the corresponding N -deprotonated species. The photoinduced reactions of 6-nitroquinolones in the aerated DMSO solutions were investigated by monitoring the electronic spectra upon a steady-state irradiation. While the ethyl 1,4-dihydro-1-ethyl-6-nitro-4-oxoquinoline-3-carboxylate ( 2 ) possessing an ethyl group at the nitrogen of the 4-pyridone moiety exhibits a rather low stability upon UVA photoexcitation, the changes in the electronic spectra observed for the derivatives 1 and 3 were attributed to the alterations in the concentration of their N − /NH forms caused by the water absorption in the hygroscopic DMSO, with no evidence of photodecomposition. The photoexcitation (365 nm or 400 nm monochromatic radiation) of 6-nitroquinolones in aerated DMSO solutions containing spin trapping agents revealed the generation of superoxide radical anion spin-adducts, along with further spin-adducts of carbon- and oxygen-centered radicals produced by the interaction of photoexcited nitroquinolone molecules with the solvent. In the scope of various biological activities of quinolone-like compounds, a cytotoxic effect of ethyl 1,4-dihydro-1-ethyl-6-nitro-4-oxoquinoline-3-carboxylate ( 2 ) on cancer human (HeLa) and murine (B16) and non-cancer (NIH-3T3) cell lines was evaluated, demonstrating a rather low cytotoxicity towards studied cell lines.