THE EFFECT OF OXYGEN AT PHYSIOLOGICAL LEVELS ON THE DETECTION OF FREE-RADICAL INTERMEDIATES BY ELECTRON-PARAMAGNETIC-RESONANCE

It is well known that oxygen enhances the relaxation of free radical E PR probes through spin lattice and Heisenberg spin-spin interactions w ith consequent effect on the line height and width. The two relaxation processes have opposing effects on the signal heights and depend on t he concentration of oxygen, the incident microwave power, and the pres ence of other paramagnetic species. During EPR studies of chemical, bi ochemical, and cellular processes involving free radicals, molecular o xygen has significant magnetic influence on the EPR signal intensity o f the free radical species under investigation in addition to affectin g the rates of production of the primary species and the stability of the spin adduct nitroxides. These effects are often overlooked and can cause artifacts and lead to erroneous interpretation. In the present study, the effects of oxygen and ferricyanide on the EPR signal height of stable and persistent spin adduct nitroxides at commonly employed microwave powers were examined. The results show that under commonly a dopted EPR spectrometer instrumental conditions, artifactual changes i n the EPR signal of spin adducts occur and the best way to avoid them is by keeping the oxygen level constant using a gas-permeable cell.