Cysteine function in superosxide dismutase direct electrochemistry and superoxide anion sensing

چکیده مقاله

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•−.