Phenoxy radical detection using 31P NMR spin trapping

Recent work in our laboratory has allowed the development of quantitative P-31 NMR spin trapping techniques. These methods have been demonstrated to be effective tools for the detection and absolute quantification of many oxygen- and carbon-centered free radical species. Our methods rest on the fact that a free radical reacts with the nitroxide phosphorus compound, 5-diisopropoxy-phosphoryl-5-methyl-1-pyrroline-N-oxide (DIPPMPO), to form stable radical adducts, which are suitably detected and accurately quantified using P-31 NMR in the presence of phosphorus containing internal standards. This system was applied for the detection of phenoxy radicals, as an alternative to traditional EPR techniques. More specifically, the phenoxy radicals were produced via the oxidation of different phenols by K3Fe(CN)(6). The P-31 NMR signals for the radical adducts of phenoxy radicals (PhO center dot) were assigned and found to be located at 25.2 ppm. Subsequently, this spin trapping system was applied to the oxidation of various phenols in the presence of peroxidases and 1-hydroxybenzotriazole (HBT) as a mediator: the 2,4,6-trichlorophenol and 2,4,6-tri-tert-butylphenol were oxidized and only phenoxy radical adducts were detected, whereas during the oxidation of 2,4-dimethylphenol and isoeugenol, other adducts were detected and related to radical delocalization. These preliminary efforts demonstrate the efficacy of our methodologies, so that a variety of radical species can now be readily detected and quantified using quantitative P-31 NMR spin trapping techniques