Voltammetric Investigation of Interactions Between Superoxide Ion and Some Sulfur Amino Acids

In this paper we analyze a group of aminoacids containing sulfur, either as thiol or as disulfides and sulfide (L-Cysteine, L-Cystine, S-Carboxymethyl-L-Cysteine, N-Acetyl-L-Cysteine, reduced and oxidized L-Glutathione) for their interaction with superoxide ion (O2-) in protic and aprotic media. Superoxide ion was electrochemically generated by reduction of oxygen molecule dissolved inside the solutions. O2-concentration changes for 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 of the medium electrochemical techniques largely 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 O2- ion giving disulfides and can be considered antioxidants for the superoxide ion in the following decreasing order: N-acetylcysteine > 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.