CATHODIC STRIPPING VOLTAMMETRY OF SULFUR-CONTAINING AMINO-ACIDS AND PEPTIDES IN THE PRESENCE OF NICKEL ION - CATALYTIC AND INHIBITING EFFECTS

The paper reviews recent studies on the effect of addition of Ni(II) i n the cathodic stripping voltammetry of the following compounds: cyste ine, penicillamine, cystine, glutathione (either reduced or oxidised) and N-acetylcysteine. With the exception of N-acetylcysteine, the abov e compounds give a cathodic stripping peak at -0.6 V (vs. Ag/AgCl, 3 M KCl electrode) which is due to the catalytic reduction of nickel ion. Even in the case of the disulphides the actual catalyst is the thiol produced by the cleavage of the -S-S- bond during the accumulation ste p. The catalytic peak enables the detection of the analyte with a bett er selectivity than is obtained with the stripping peak due to the red uction of mercury thiolates. In addition, NI(II) suppresses the mercur y thiolate peak of ligands such as cysteine or penicillamine, but does not modify the behaviour of thiols with low complexing properties (su ch as N-acetylcysteine). Consequently, compounds such as cysteine and its N-acyl derivatives can be determined simultaneously by means of th e catalytic peak and the mercury thiolate peak (at -0.4 V) respectivel y.