Electrochemical oxidation of dopamine in the presence of sulfhydryl compounds: Application to the square-wave voltammetric detection of penicillamine and cysteine

Electro-oxidation of dopamine at a glassy carbon electrode was investigated in the presence of some biologically important thiols (R-SH), e.g. cysteine and penicillamine. Results of cyclic voltammetric studies together with the spectrophotometric foundations via Ellman's test during the controlled-potential coulometry show a nucleophilic addition/reduction of thiol to the electrochemically generated dopaminoquinone by a 1 + 4 Michael addition. The resulting ring substituted substrate (as RS-form) is more easily oxidized leading to an increase in the anodic current of dopamine, which is proportional to the concentration of thiol. The square-wave voltammetry (SWV) were applied as a very sensitive voltammetric detection method for the detection of cysteine and penicillamine. A linear response from 0.10 to 2.5 μM with a detection limit of 0.08 μM is resulted by the voltammetric determinations for penicillamine. The effect of other biological thiols, such as N-acetyl cysteine, glutathione, methionine, captopril and mercaptoglycine were also assessed and found to present no appreciable change in the voltammetric response of dopamine. The proposed method can be considered as a relatively selective method for the voltammetric detection of penicillamine and cysteine. The practical utility of the method was investigated in the determination of thiol species in pharmaceutical preparations.