Spin trapping artifacts due to the reduction of nitroso spin traps

Abstract

The use of nitroso compounds as spin traps in chemical and biological systems has become widespread, and many interesting, unstable radicals have been trapped in this manner [l-4]. However, as we will show, extreme caution must be taken when trapping radicals in a reducing medium. The technique of spin trapping involves producing the unstable free radical of interest and allowing it to react with a diamagnetic compound (the spin trap, usually a nitroso compound or a nitrone) to form a relatively stable free radical (the spin adduct) which can be observed by electron spin resonance (ESR). Observation of a stable free radical, however, is no guarantee that the radical of interest has been trapped. Spectral artifacts can arise due to nitroxide impurities or nucleophilic addition to nitroso compounds followed by oxidation to the nitroxide [5]. We report another way in which spin trapping artifacts may arise, that of direct reduction of anitroso spin trap to a nitroxide free radical. Though nitrose spin traps have been used extensively [l-4], apparently not much attention has been given to the possibility of this reduction, though reduction of the spin adduct has been proposed as a ‘decay process [3,6]. The reduction of the spin trap itself is particularly important in biological systems because of the presence of endogenous reducing agents such as ascorbate.