Photochemistry of aqueous solutions of benzazolo[3,2-a]quinolinium salts. A spin-trapping study using 17O-enriched water and oxygen

Abstract The photolysis of buffered aqueous solutions containing the quinolinium salts, 3-nitro-7-ethyl-benzimidazolo[3,2-1]quinolinium perchlorate (NEBQClO 4 ) and 3-nitrobenzothiazolo[3,2- a ]quinolinium chloride (NBQCl), at 344 and 365 nm, respectively, was studied in the presence of the spin trap 5,5-dimethyl-1-pyrroline-1-oxide (DMPO). Only a water-derived DMPO-OH · spin-adduct was obtained for both of these salts, at a DMPO concentration of 14 mM, as confirmed by H 2 17 O-enriched water experiments. A photosolvated intermediate is postulated as the OH donating species. Lowe steady-state concentrations of the spin adduct were obtained in argon-saturated solutions, implying that oxygen gas is at least partially necessary in the water DMPO-OH formation. Evidence for superoxide ion formation was obtained by the DMPO- 17 OH spin-adduct formation during the photolysis of NBQCl in an 17 O-enriched oxygen atmosphere in the presence of 150 mM DMPO. An increase in the DMPO-OH steady-state concentration was observed if the photolysis of NBQCl was performed in the presence of superoxide dismutase (SOD). Our results suggest that this effect is due to the SOD inhibition of the destruction of DMPO-OH · by superoxide ion.