Cysteine Function in Superosxide Dismutase Direct Electrochemistry and Superoxide Anion Sensing

Cysteine self assembled monolayer-modified gold (Cys/Au) electrode is used to immobilize superoxide dismutase (SOD) and establish a direct electron transfer between enzyme and electrode surface. However, due to the redox activity of copper ion on Cys monolayer, there would be an ambiguity in electrochemical studies of immobilized SOD on Cys/Au electrode. We designed a series of experiments to clarify the role of Cys in this process. Comparison between voltammograms of different electrodes revealed that the current intensity was increasing by the order of Cu+2/Cys/Au > SOD/Cys/Au > Cys/Au, while their electrochemical working windows were overlapping. Furthermore, for these electrodes the electron transfer rate constant were 0.77, 0.73, and 0.29 s-1 and the surface concentration of electroactive species were 1.05×10-10, 1.51×10-11, 1.50×10-11 mol cm-2, respectively. When phosphate buffer solution (PBS) was prepared by ultrapure phosphate salts (copper ion free) no redox response was observed while, by deliberately addition of Cu2+ the Cys/Au electrode showed a redox response. EDTA as chelating agent could pick up Cu2+ from PBS and consequently no electrochemical response was observed for Cys/Au electrode. Comparing these results indicated that the source of the inherent electrochemical activity of Cys/Au electrode is Cu+2. Finally, the Cys/Au electrode was also examined as a sensing system for determination of O2 •?.