Semi-quantitative detection of free radicals using spin trapping and phosphorus-31 NMR spectroscopy

In this study the effectiveness of quantitative 31P nuclear magnetic resonance (NMR) spectroscopy in conjunction with spin trapping to detect and quantify various free radical species was investigated. Initially the research was focussed on assigning the 31P NMR signals for hydroxyl and superoxide radicals and on identifying the quantitative reliability of the technique. The radical adduct reaction products (diamagnetic species), of hydroxyl (HO·) and superoxide (O2 -·) radicals with phosphorus containing nitroxides, were determined to be relatively stable and as a result the 31P NMR chemical shifts for these species were determined. Their structures were verified and quantified using 31P NMR in the presence of a phosphorus containing internal standard. The 31P NMR chemical shifts for 5-diisopropylphosphoryl-5-methyl-1-pyrroline-N-oxide (DIPPMPO) and 5-diethoxyphosphoryl-5-methyl-1-pyrroline-N-oxide (DEPMPO) spin traps with a variety of carbon centered radicals were also determined.A novel equilibrium reaction involving molecular oxygen and alkaline hydrogen peroxide was investigated and it was found that the concentration of superoxide radicals present in a system of alkaline hydrogen peroxide is affected by the nature of gas with which it is in equilibrium. The concentration of superoxide radicals was determined to be significantly larger than that of hydroxyl radicals at elevated pH values.Experiments have shown that NMR in conjunction with spin trapping can be a valuable tool for the detection and quantification of radical species