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

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 ( 1 O 2 ) may be produced and can react with DMPO. We have studied the reaction of DMPO with 1 O 2 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 1 O 2 phosphorescence ( k q = 1.2 × 10 6 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 1 O 2 phosphorescence, while (photo)decomposition of the nitroso group, presumably to nitric oxide (NO • ), produced nitrite as a minor product. We propose that 1 O 2 adds to the >C N(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/ • O 2 H while oxidizing another DMPO moiety to ultimately form DMPOX. The DMPO/ • O 2 H 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 1 O 2 leads to both rapid O 2 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 1 O 2 is produced.