Exchange and shuttling of electrons by nitroxide spin labels

The ability of nitroxide spin labels to act as oxidizers of reduced nitroxides (hydroxylamines) in biological and model systems was demonstrated. All of the nitroxides tested were able to act as oxidizing agents with respect to hydroxylamine derivatives of nitroxides. The rates of these reactions were first order with respect to nitroxide concentration and with respect to hydroxylamine concentration, making the reaction second order overall. The second-order rate constants are reported for a number of these reactions. These reactions proceeded to an equilibrium state and the equilibrium constants for several combinations of reactants are presented. Both the rate constants and the equilibrium constants were found to be dependent on the ring structure of the nitroxide and hydroxylamine, with piperidines being reduced more easily and pyrrolidines and oxazolidines being oxidized more easily. All of the hydroxylamine derivatives were oxidized by air to their respective nitroxides, with the rate of this oxidation greater for pyrrolidines than for piperidines. Furthermore, hydroxylamines that are permeable to lipid bilayers were able to act as shuttles of reducing equivalents to liposome-encapsulated nitroxides that were otherwise inaccessible to reducing agents. This mechanism of shuttling of electrons was able to explain the relatively rapid reduction by cells of a nonpermeable nitroxide in the presence of a permeable nitroxide.