The redox potential of ergothioneine Revisited: Determination by spectroscopic and electrochemical methods

The reducing strength of the antioxidant ergothioneine (EGT) at physiological pH has been probed by spectroscopy and electrochemistry. Optical spectroscopy, used to follow the reductions of methylene blue, Fe(III) cytochrome c, and ferricyanide by EGT, showed that the traditionally accepted value of its redox potential (-0.06 V vs NHE) is not correct, and has led to EGT being classified as a much stronger reducing agent than is warranted. To obtain a more reasonable number for the redox potential, cyclic voltammetry (CV) experiments were carried out on EGT in pH 7.2 phosphate buffer solutions. Glassy carbon and edge-plane pyrolytic graphite working electrodes required either careful mechanical polishing or plasma activation of their surfaces to give reliable voltametric data. Repeatable results were obtained for CVs taken at scan rates at or below 0.2 V/s. They showed that the anodic oxidation of EGT is a diffusion-controlled, totally irreversible, one-electron process. Anodic peak potentials of 0.42 V to 0.47 V were observed within a group of different carbon-based electrodes, allowing a redox potential of 0.45 ± 0.02 V vs NHE to be assigned to EGT. This value is slightly positive of the accepted value for ascorbate anion (0.35 V), and some 500 mV positive of the previously employed value for EGT. Based on what appear to be the most reliable present and literature redox potentials, a ranking of the reducing strengths of several important antioxidants at the physiological pH is glutathione ≈ cysteine ≫ ascorbate > α-tocopherol ≈ ergothioneine > tyrosine.