Adsorption/desorption behaviour of cysteine and cystine in neutral and basic media: electrochemical evidence for differing thiol and disulfide adsorption to a Au(1 1 1) single crystal electrode

The adsorption/desorption behaviour of the thiol/disulfide redox couple, cysteine/cystine, was monitored at a Au(1 1 1) single crystal electrode. The monolayers were formed electrochemically from 0.1 M KClO4 and 0.1 M NaOH solutions containing either the thiol or the disulfide. Distinct features in the adsorption potential were noted. An adsorption peak was observed in the cyclic voltammograms (CVs) from Au(1 1 1) in 0.1 M KClO4 solutions containing cystine at −0.57 V vs. saturated calomel electrode. Under the same conditions, the CVs from solutions containing cysteine showed an adsorption peak at −0.43 V (0.14 V more positive than the corresponding peak from disulfide solutions). This showed that the thiol and disulfide species have different adsorption properties. Similar behaviour was observed in 0.1 M NaOH. Cyclic voltammetric and chronocoulometric data were employed to determine the surface coverage of the different monolayers. Cysteine solutions prepared in 0.1 M KClO4 provided coverages of 3.0×10−10 and 2.5×10−10 mol cm−2 for the l and the d–l species, respectively as evaluated from the desorption peaks. Desorption of cystine in the same medium yielded coverages of 1.2×10−10 mol cm−2 for both l and d–l solutions (or 2.4×10−10 mol cm−2 in cysteine equivalents). Surface coverages obtained from Au(1 1 1) in 0.1 M NaOH corresponded to 3.9×10−10 mol cm−2 for l-cysteine, and 1.2×10−10 mol cm−2 (or 2.4×10−10 mol cm−2 cysteine equivalents) for l and d–l cystine.