Voltammetric investigation of the interactions between superoxide ion and some sulfur amino acids

In this paper, we analyze a group of amino acids containing sulfur, either as thiols or as disulfides and sulfides (l-cysteine, l-cystine, S-carboxymethyl-l-cysteine, N-acetyl-l-cysteine, reduced and oxidized l-glutathione) for their interaction with superoxide ion in protic and aprotic media. Superoxide ion was electrochemically generated by reduction of oxygen molecules dissolved in the solutions. concentration changes due to the presence of interacting species and was easily monitored under mild and controlled experimental conditions, in the presence or in the absence of the selected substrate, in protic and aprotic media. The reactions were followed by cyclic (CV) or linear (LSV) voltammetry, depending on the medium and the electrochemical techniques widely used to study the antioxidant capacity of biological and chemical systems. According to the experimental results, it was concluded that substrates having a free thiol group react with ion giving disulfides and can be considered as antioxidants for the superoxide ion in the following decreasing order: N-acetyl-l-cysteine > l-cysteine > reduced glutathione. Compounds having protected sulfur (l-cystine, S-carboxymethyl-l-cysteine, oxidized glutathione) do not react with the superoxide ion and do not form compounds with a superior S oxidation number, such as sulfoxide and sulfone.