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 us ed to identify free radicals that are generated photochemically using dyes as photosensitizers. When oxygen is present in such systems, sing let oxygen (O-1(2)) may be produced and can react with DMPO. We have s tudied the reaction of DMPO with O-1(2) in aqueous solutions over a wi de range of pH, using micellar Rose Bengal (pH 2-13) and anthrapyrazol e (pH < 2) as photosensitizers. We found that DMPO quenches O-1(2) pho sphorescence (k(q) = 1.2 x 10(6) M(-1) s(-1)), thereby initiating oxyg en consumption that is slow at pH 10 but increases about 10-fold at pH < 6. This oxygen consumption is a composite process that includes eff icient oxidation of both DMPO and its degradation products. The oxidat ion products include both products in which the DMPO pyrroline ring re mains intact (DMPO/(OH)-O-. and 5,5-dimethyl-2-oxo-pyrroline-1-oxyl (D MPOX) radicals) and those in which it becomes opened (nitro and nitros o products). The nitroso product itself strongly quenched O-1(2) phosp horescence, while (photo)decomposition of the nitroso group, presumabl y to nitric oxide (NO.), produced nitrite as a minor product. We propo se that O-1(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) I t may be protonated and rearrange to a strongly oxidizing nitronium-li ke moiety, which could be reduced to the DMPO hydroperoxide radical DM PO/(O2H)-O-. while oxidizing another DMPO moiety to ultimately form DM POX. The DMPO/ (O2H)-O-. could undergo further redox decomposition, e. g. via the known Fenton-like reaction, to produce both free (OH)-O-. r adical and the DMPO/(OH)-O-. radical. (ii) The biradical >C(OO.)-N-.(O ) may cyclize to a 1,2,3-trioxide (ozonide), which could open the pyrr oline ring to form 4-methyl-4-nitropentan-1-al and 4-methyl-4-nitrosop entanoic acid. Because the oxidation of DMPO by O-1(2) leads to both r apid O-2 depletion and the formation of transients and products that m ight interfere with trapping and identification of free radicals, DMPO should be used with caution in systems where O-1(2) is produced.