[23] Spin trapping of superoxide and hydroxyl radicals

Publisher Summary Many free radicals of biological interest are highly reactive and never reach a concentration high enough to be detected by electron paramagnetic resonance (EPR). An example of this is the hydroxyl radical, which reacts with itself or with most organic molecules at diffusion-controlled rates. Its rate of reaction is limited mainly by the frequency with which it collides with other species. Thus, the direct detection of hydroxyl radicals by EPR in a biologic system is impossible. For short-lived radicals of lesser reactivity compared to the hydroxyl radical, there are various means of detection using EPR. A simple method is to slow the rate of disappearance of the radical by rapidly freezing the sample. This has the disadvantage that the radical is no longer in a fluid environment, and the resultant anisotropic effects can obscure the identification of the radical. In theory, spin trapping can overcome many of these difficulties. This technique consists of using a spin trap––that is, a compound that forms a stable free radical by reacting covalently with an unstable free radical. This chapter discusses the spin trapping of the biologically important free radicals: superoxide and hydroxyl.