The Mechanism of Reaction of Ebselen with Superoxide in Aprotic Solvents as Examined by Cyclic Voltammetry and ESR.

Abstract

Ebselen, 2-phenyl-1,2-benzisoselenazol-3(2H)-one ( Fig. 1) has been developed for use as an antioxidative agent. It is a lipophilic compound with varied antioxidant activities similar to those of glutathione (GSH) peroxidase.1-3) These activities are thought to prevent against disease states by scavenging reactive free radicals, but the mechanisms of these redox reactions have not yet been clarified.One of the most active oxygen species is the superoxide anion radical (O 2 · Ϫ ), which is produced by the one-electron reduction of molecular oxygen. This radical species is implicated in several harmful biological processes, such as lipid peroxidation and protein denaturation. The reactivity of O 2 · Ϫ in an aqueous media is quite different from that in aprotic media. In an aqueous system, O 2 · Ϫ spontaneously disproportionates into hydrogen peroxide and molecular oxygen, whereas in an aprotic media O 2 · Ϫ is stable and can have several reactivities, e.g. electrogenerated base, nucleophile, reductant or oxidant.To evaluate the antioxidative mechanism of certain lipophilic compounds, it is important to understand the reactivity of these antioxidants with O 2 · Ϫ in aprotic media in order to mimic the lipophilic domain of the lipid bilayer. In this report we describe the mechanism of redox reactions of ebselen with O 2 · Ϫ in aprotic media using both electrochemical and ESR techniques. Electrochemical measurements provide useful information on the mechanisms of electron transfer and free radical reactions for redox active compounds. The voltammetric behavior of ebselen, primarily of its anodic oxidation, 4) has already been studied. The primary oxidation step has been identified as the formation of selenoxide. However, no cathodic reduction behavior of ebselen has been reported. Therefore, in the present study we used cyclic voltammetry to determine the interaction between superoxide anion radical and ebselen in both cathodic and anodic currents. As ESR spin-trapping is useful in discriminating trapped radical species and in estimating various short-lived radicals, 5,6) we employed this technique to evaluate the antioxidative activity of ebselen. Further, rapid mix, continuous-flow ESR was used to examine a short-lived free radical species which is generated during the reaction of ebselen with superoxide. ExperimentalMaterials Ebselen was synthesized at Aventis Pharma AG (Frankfurt, Germany). 5,5-Dimethyl-1-pyrroline N-oxide (DMPO), potassium superoxide (KO 2 ) and 18-crown-6 ether were purchased from Sigma Co., Ltd., (St. Louis, MO, U.S.A). Tetraethylammonium perchlorate (TEAP) was purchased from Aldrich Chemical Co., Ltd., (Milwaukee, WI, U.S.A).ESR Spin-Trapping ESR measurements were recorded on a JEOL JES-FE2XG spectrometer (JEOL, Tokyo, Japan) with 100 kHz field modulation operating at 9.44 GHz and at room temperature. The following instrument parameters were employed: modulation amplitude, 0.063 mT; microwave power, 8.0 mW; scan time, 2 min.Five milliliters of a dry toluene-DMSO (3/2, v/v) solution containin...

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