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Oxidation of the Spin Trap 5,5-Dimethyl-1-pyrroline N -Oxide by Singlet Oxygen in Aqueous Solution

1996; American Chemical Society; Volume: 118; Issue: 6 Linguagem: Inglês

10.1021/ja952140s

1943-2984

Piotr Bilski, K. Reszka, Monika Bilska‐Markowska, Colin F. Chignell,

Analytical Chemistry and Sensors

The spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) is frequently used to identify free radicals that are generated photochemically using dyes as photosensitizers. When oxygen is present in such systems, singlet oxygen (1O2) may be produced and can react with DMPO. We have studied the reaction of DMPO with 1O2 in aqueous solutions over a wide range of pH, using micellar Rose Bengal (pH 2−13) and anthrapyrazole (pH < 2) as photosensitizers. We found that DMPO quenches 1O2 phosphorescence (kq = 1.2 × 106 M-1 s-1), thereby initiating oxygen consumption that is slow at pH 10 but increases about 10-fold at pH < 6. This oxygen consumption is a composite process that includes efficient oxidation of both DMPO and its degradation products. The oxidation products include both products in which the DMPO pyrroline ring remains intact (DMPO/•OH and 5,5-dimethyl-2-oxo-pyrroline-1-oxyl (DMPOX) radicals) and those in which it becomes opened (nitro and nitroso products). The nitroso product itself strongly quenched 1O2 phosphorescence, while (photo)decomposition of the nitroso group, presumably to nitric oxide (NO•), produced nitrite as a minor product. We propose that 1O2 adds to the >CN(O) bond in DMPO, producing a biradical, >C(OO•)N•(O). This biradical may follow one of two pathways: (i) It may be protonated and rearrange to a strongly oxidizing nitronium-like moiety, which could be reduced to the DMPO hydroperoxide radical DMPO/•O2H while oxidizing another DMPO moiety to ultimately form DMPOX. The DMPO/•O2H could undergo further redox decomposition, e.g. via the known Fenton-like reaction, to produce both free •OH radical and the DMPO/•OH radical. (ii) The biradical >C(OO•)N•(O) may cyclize to a 1,2,3-trioxide (ozonide), which could open the pyrroline ring to form 4-methyl-4-nitropentan-1-al and 4-methyl-4-nitrosopentanoic acid. Because the oxidation of DMPO by 1O2 leads to both rapid O2 depletion and the formation of transients and products that might interfere with trapping and identification of free radicals, DMPO should be used with caution in systems where 1O2 is produced.