Cysteine Function in Superosxide Dismutase Direct Electrochemistry and Superoxide Anion Sensing

Cysteine self assembled monolayer-modified gold (cys/au) electrode is used toimmobilize superoxide dismutase (sod) and establish a direct electron transfer betweenenzyme and electrode surface. however, due to the redox activity of copper ion on cysmonolayer, there would be an ambiguity in electrochemical studies of immobilized sod oncys/au electrode. we designed a series of experiments to clarify the role of cys in thisprocess. comparison between voltammograms of different electrodes revealed that thecurrent intensity was increasing by the order of cu+2/cys/au > sod/cys/au > cys/au, whiletheir electrochemical working windows were overlapping. furthermore, for these electrodesthe electron transfer rate constant were 0.77, 0.73, and 0.29 s-1 and the surface concentrationof electroactive species were 1.05×10-10, 1.51×10-11, 1.50×10-11 mol cm-2, respectively. whenphosphate 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 electrodeshowed a redox response. edta as chelating agent could pick up cu2+ from pbs andconsequently no electrochemical response was observed for cys/au electrode. comparingthese results indicated that the source of the inherent electrochemical activity of cys/auelectrode is cu+2. finally, the cys/au electrode was also examined as a sensing system fordetermination of o2•?.