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Effect of oxygen on the electrochemical reduction of nitroxyl radical: interpretation of the mechanism for a redox probe in biological systems.

Though this reaction has been widely employed as a redox probe in biological systems,1–4 we have a question as to why the reduced form of the nitroxyl radicals effectively works as a redox probe for oxidative stress in vivo regardless of their slow oxidation at electrodes, and their irreversible electrochemical property.5–7 We cannot help commenting that its reaction mechanism has been sufficiently elucidated. It is only at hanging mercury drop electrodes (HMDE) in a basic aqueous solution that they can be reduced to their hydroxl amine form, >N–OH, (or deprotonated one, >N–O–), in a reversible or a quasi-reversible electrochemical process.8,9 Taking into account these facts, an investigation of the effect of oxygen on the cathodic reduction of the TEMPO derivatives at an HMDE might be a clue to explain the mechanism of a redox spin probe process for oxidative stress. In this note, we report on the preliminary results, and give an explanation of the cathodic reduction of a TEMPO derivative in both the presence and absence of oxygen in a neutral or basic aqueous solution, and suggest the regeneration mechanism of a nitroxyl radical of the active ESR during a spin-probe experiment.