Square-wave adsorptive stripping voltammetric behaviour of fresh and aged cystine solutions at physiological pH

Different cathodic square-wave stripping voltammetric behaviour for fresh a nd aged cystine solutions at slightly alkaline buffered medium (pH 7.4) is observed at distinct pH values. The fresh solutions of cystine at pH 7.4 ex hibited three reduction peaks at -0.29 (mercuric cysteine thiolate), -0.64 (mercurous cysteine thiolate) and -0.75 V (free cystine). The peak at -0.75 V is assigned as the reduction of the -S-S- bond of free cystine which is not detected for aged cystine solution (a few hours) but a new peak at -0.6 8 V is seen. The peak at -0.68 V is attributed to the reduction of the disu lfidic anion S-2(2-), the decomposition product of cystine at physiological pH (7.4). (C) 1999 Elsevier Science S.A. All rights reserved.