Journal of Electroanalytrcal Chemutry, 369 (1994) 53-70 53 The reactions of cystine at mercury electrodes M Heyrovslj J Heyrovsky’ Institute of Physical Chemutry, Academy of Sciences of the Czech Republzc, 182 23 Prague 8 (Czech Republic) P Mader Faculty of Agronomy, Untverslty of Agrtculture, 165 21 Prague 6 (Czech Repubbc) V. Veselii and M. Fedurco Institute of Experimental Phyacs, Slovak Academy of Scrences, 043 53 KoGce (Slovak Repubhc) (Received 5 August 1991, m revised form 24 June 1993) zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Abstract Cystme 1x1aqueous solutions reacts chemically with mercury, forming the surface-bound cysteme mercuric thlolate When the potential of mercury IS made less positive this compound IS transformed (at a partially free mercury surface m a currentless process and at an occupied electrode under the passage of current) mto cysteme mercurous ttuolate The latter compound IS also formed m a reversible interaction of cystme with mercury m the potential range where cystme IS adsorbed At the negative end of this range the electroreductlon of cystme occurs by the reduction of cysteme mercurous thlolate At negatively charged mercury, where cystme IS not adsorbed, two electrons are transferred directly to the cystme -S-S- bond m an overall irreversible process On the positive side the dissolution of mercury mto solutions containing cystme starts by anodlc formation of slightly soluble mercuric cystmate near the electrode surface 1. Introduction When the importance of cystme m blologlcal pro- cesses was recognized, attention was directed towards the problems of determining cystme analytically and investigating its redox reactivity Electrolysis by means of mercury electrodes appeared to be a promlsmg approach to the solution of these two problems Conse- quently, the electrochemistry of cystme on mercury electrodes has been studied by many authors [l-38] who have gathered a large number of experimental results over the period of several decades Thus Kolthoff and Barnum [7,8,10] confirmed by polarogra- phy what Mlchaehs and coworkers [l-3] had found by potentlometry with mercury electrodes, that cystme and cysteme do not behave as a reversible redox system on mercury and the reduction of cystme IS accompa- nied by an interaction of cysteme with the electrode metal Kalousek et al [ll] explained the results of their systematic polarographlc study m terms of the adsorp- tion of cystme at the dropping electrode and its cheml- 0022-0728/94/$7 00 SSDI 0022-0728(93)03122-6 cal reaction with mercury, leading to a cysteme-mercury compound which IS reduced m the first kmetlcally controlled reduction step of cystme, the second step 1s a direct irreversible reduction This interpretation was further corroborated by Blegler and coworkers [16,171 using ac polarographlc experiments Biezma [151 demonstrated that when the adsorptlvlty of cystme 1s increased by complex-forming ions, the first reduction step of cystme IS accelerated up to the dlffuwon-con- trolled level Nyg%rd provided substantial experimental evidence [18,19] for the occurrence of chemical reac- tions with mercury m processes preceding electron transfer m polarographlc reductions of various organic dlsulphldes and dlselemdes mcludmg cystme and se- lenocystme Miller and Teva [251 found that com- pounds of cysteme with both monovalent and dlvalent mercury are formed on the mercury surface m contact with cystme solutions at potentials more positive than that of cystme reduction, and that the rates of these reactions depend on pH Aware of the importance of attaining the adsorption equlhbrlum for the study of 0 1994 - Elsevler Sequoia All rights reserved