Electrochemical study of cystine in the presence of cadmium and folic acid

Abstract

The cyclic voltammetric behaviour of cystine (RSSR) was studied on a static mercury drop electrode (SMDE) in the hanging mercury drop elecrode (HMDE) mode at pH 7.4. RSSR gave two peaks at −0.106 V (mercuric cysteine thiolate, RS–Hg–SR) and −0.552 V (mercurous cysteine thiolate, RS–HgHg–SR), respectively. The electrochemical behaviour of complexes of RSSR or folic acid with cadmium(II), and cadmium(II)–cysteinate (Cd(SR) 2) complex in the presence of folic acid was also investigated. Experimental results show that the Cd(SR) 2 complex forms a peak at −0.808 V on the mercury electrode surface. Also, the electroreduction of the cadmium(II)–folic acid complex occurs at −0.722 V. These complexes are reduced at a more negative potential than that of the free cadmium ion. When folic acid is added to RSSR solution, it causes shifts of the voltammetric reduction peaks of RS–Hg–SR and RS–HgHg–SR towards positive potentials. The shifts are invariably accompanied by a decrease of peak currents. This is explained as due to the formation of adducts between folic acid and cysteine thiolate (reduction product RS −) at the electrode surface. Analogous results were observed between folic acid and Cd(SR) 2 complex, and were ascribed to the same cause.