Phenoxy radical detection using 31P NMR spin trapping

摘要

Recent work In our laboratory has allowed the development of quantitative 31P NMR spin trapping techniques.These methods have been demonstrated to be effective tools for the detection and absolute quantification of manyoxygen- and carbon-centered free radical species. Our methods rest on the fact that a free radical reacts with thenitroxlde phosphorus compound, 5-diisopropoxy-phosphory1-5-methyl-1-pyrroline-N-oxide (DIPPMPO), to formstable radical adducts, which are suitably detected and accurately quantified using 31P NMR in the presence ofphosphorus containing internal standards. This system was applied for the detection of phenoxy radicals, as analternative to traditional EPR techniques. More specifically, the phenoxy radicals were produced via the oxidation ofdifferent phenols by K3Fe(CN)6. The 31P NMR signals for the radical adducts of phenoxy radicals (Ph0.) were assignedand found to be located at 25.2 ppm. Subsequently, this spin trapping system was applied to the oxidation of variousphenols in the presence of peroxidases and 1-hydroxybenzotriazole (HBT) as a mediator: the 2,4,6-trichlorophenoland 2,4,6-tri-tert-butylphenol were oxidized and only phenoxy radical adducts were detected, whereas during theoxidation 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 nowbe readily detected and quantified using quantitative 31P NMR spin trapping techniques.