Journal of Electroanalytical Chemistry, 374 (1994) 215-222 215 Electrocatalytic activity of vitamin B,, adsorbed on graphite electrode for the oxidation of cysteine and glutathione and the reduction of cystine J.H. Zagal l , M.J. Aguirre and C.G. Parodi Departamento de Quimica, Facultad de Ciencia, Universidad de Santiago de Chile, Casilla 307, Santiago 2 (Chile) J. Sturm Facultad de Ciencias Q&micas y Farmacolbgicas, Universidad de Chile, Casilla 233, Santiago I (Chile) (Received 16 November 1993; in revised form 4 February 1994) zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Abstract We have investigated the electrocatalytic activity of vitamin B,, adsorbed on ordinary pyrolytic graphite (OPG) for the oxidation of cysteine and glutathione and for the reduction of cystine, using cyclic voltammetry and rotating-disc techniques. The activity of the bare graphite substrate is very low for all reactions studied. The presence of monolayer levels of vitamin Bi2 on OPG substantially decreases the overpotential of these reactions. Tafel plots of slope RT/F are obtained for the oxidation of cysteine, suggesting a mechanism controlled by the Co(II/III) couple in the vitamin. Similar slopes are obtained for the oxidation of glutathione, but they gradually change to 2RT/F for acid pH. No linear Tafel regions are obtained for cysteine reduction and the process occurs on Co(I). 1. Introduction Bioelectrocatalysis is associated with the accelera- tion of electrochemical reactions through the presence of biological catalysts or enzymes on an electrode sur- face. This phenomenon is interesting because of the possibility of using enzymes to promote electrochemi- cal reactions involving compounds related to biological systems. Activity of vitamin B,, for the electrochemical reduction of alkyl halides has been reported [ll. Anion-selective electrodes based on vitamin B,, have also been described [2]. However, vitamin B,, does not participate directly in the promotion of redox reactions in biological systems. In previous work [3,4] we have found that vitamin B,, adsorbs very strongly on ordi- nary, pyrolytic graphite (OPG). Thus it is possible to examine the activity of the vitamin immobilized on the electrode surface with none of it in the electrolyte. Preliminary work carried out in aqueous solutions has l To whom correspondence should be addressed. 0022-0728/94/$7.00 SSDI 0022-0728(94)03365-A shown that aquocobalamine presents catalytic activity for the reduction of oxygen and for the oxidation of cysteine (R-SH) and glutathione (G-SH) [4,5]. Cys- teine and cystine (R-SS-R) are crucial to biological systems. The electrochemical reactivity of these amino acids has been studied on graphite electrodes modified with pre-adsorbed phthalocyanines [6,7] and with cobalt porphyrins [&lo]. Glutathione, which contains a cys- teinyl sulphydril group, is the most important non-pro- tein thiol in living systems. The multifunctional proper- ties of glutathione are reflected by the growing interest shown in this molecule by investigators in a variety of subjects [ll]. The electrochemistry of glutathione is known from polarographic studies [12-151. It under- goes a one-electron oxidation to give the disulphide G-SS-G. However, its behaviour on mercury is com- plicated by the formation of mercurous compounds and more than one oxidation wave is observed [12-141. Better responses are expected with graphite or car- bon-modified electrodes. Prussian Blue films contain- ing cobalt phthalocyanine [17,18] have been used as sensors for detecting sulphur-containing amino acids 0 1994 - Elsevier Science S.A. All rights reserved