Electrode processes of the sulfhydryl-disulfide system

ELECTROANALYTICAL CHEMISTRY AND INTERFACIAL ELECTROCHEMISTRY Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

177

E L E C T R O D E PROCESSES OF T H E S U L F H Y D R Y L - D I S U L F I D E SYSTEM II. CYSTINE AT A GOLD ELECTRODE

j i i ~ f KORYTA

J. Heyrovsk~ Institute of Polarography, Czechoslovak Academy of Sciences, Prague(Czechoslovakia) JIl~f P R A D / ~

Institute of Biophysics, Faculty of General Medicine, Charles University, Prague (Czechoslovakia) Receive:l A u g u s t 8th, I967)

INTRODUCTION

In comparison with a platinum electrode in solutions containing cystine and cysteine (see preceding articlel), the potential of the gold electrode is less reproducible 2,3 and is independent of stirring of the solution 2. The adsorption of hydrogen at the gold electrode is much smaller, and the surface oxide formation starts at considerably more positive potentials, at gold than at platinum electrodes 4. In the present paper, the electrode processes of cystine at the gold electrode are compared with those at the platinum electrode, particularly with respect to the different adsorption properties of the gold surface. Investigations of the electrode processes of cystine at a gold electrode have not so far been reported. EXPERIMENTAL

The experimental methods used were cyclic voltammetry, recording of i-t curves, and radiometric measurements of adsorption of cystine. The experimental procedure and conditions have already been describedk The potential interval in cyclic voltammetry was usually from - o . 18 V to 1.9 V. All electrode potentials have been recalculated to N.H.E. RESULTS

Cyclic voltammetry The voltammetric curve in ! N H2SO4 obtained b y cyclic polarization is shown in Fig. I. Figure 2 shows the voltammetric curve of lO -3 M cystine in I N H2SO4. The oxidation peak of adsorbed hydrogen is completely suppressed by the adsorption of cystine. The oxidation of cystine is observed in the potential region of surface oxide formation and the current peak, al, has the same potential as the peak of surface oxide formation ( E = I . 4 5 V). The second oxidation wave, a2, is observed at more positive potentials (E ~ 1.8 V) than the potential of oxygen evolution. After reversal of voltage pulse, a slightly different cathodic peak of surface J. Electroanal. Chem., 17 (1968) 177-183

178

J. KORYTA, J. PRADA~

A/cm 2 5 . 1 0 ..6

a2

2,10 -5

-5.10 ~ i

\gik~ Fig. I. V o l t a m m e t r i c c u r v e w i t h gold electrode in I N H~SO4. A f t e r pre-electrolysis b y f a s t pulses, t h e electrode was s u b j e c t e d to slow p o l a r i z a t i o n cycles ( n u m b e r d e n o t e s t h e order of each cycle) Fig. 2. V o l t a m m e t r i c c u r v e w i t h gold electrode in I0 -s M c y s t i n e a n d i N H~SO4. (at, a2), anodic w a v e s of c y s t i n e o x i d a t i o n ; (kl), p e a k of surface oxide r e d u c t i o n ; (ks), c a t h o d i c w a v e of c y s t i n e r e d u c t i o n ; ( . . . . . ), v o l t a m m e t r i c c u r v e w i t h gold electrode in I N H~SO4.

5.1o-5

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0:5

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4J 5-

1 0- 5

I

I

i

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i

.......

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Fig. 3. V o l t a m m e t r i c c u r v e s w i t h gold electrode in I N HzSO4; c y s t i n e concns.: (I), lO-4; (2), 5"1o-4; (3), lO-3; (4), 5"IO-S; (5), Io-2 M . (as, a4), anodic w a v e s o b s e r v e d a t h i g h e r c o n c n s . ; (ks), c a t h o d i c w a v e o b s e r v e d a t h i g h e r conchs. ; (---), v o l t a m m e t r i c c u r v e s recorded w i t h stirring of t h e s o l u t i o n ; (. . . . . ), v o l t a m m e t r i c c u r v e w i t h gold electrode in I N HzSO4. Fig. 4. D e p e n d e n c e of p e a k c u r r e n t of c y s t i n e o x i d a t i o n , al, on c y s t i n e conch. (cf. Fig. 3).

j . Eleclroanal. Chem., 17 (1968) 177-183

CYSTINE AT A u ELECTRODE

179

oxide reduction kl, and another cathodic wave, ks, is observed in the potential region o.i V to - o . I V. Figure 3 demonstrates the dependence of the voltammetric curves on cystine concentration. The dependence of the current in the main anodic peak of cystine (E = 1.45 V) on cystine concentration is shown in Fig. 4. At cystine concentrations above 5" lO-3 M, a new wave (a3) is observed with a peak at about 0. 9 V. If the solution is stirred during cyclic polarization in the potential region of the most positive wave of cystine (Em 1.8 V), this wave disappears (dotted line in Fig. 3). A new anodic wave (a4) is observed in the potential region 1.o-1.2 V. The cathodic peak of reduction of the surface oxide decreases with increase of cystine concentration. At higher cystine concentrations, a new cathodic wave is formed at 0.2 V (k2). Like peak a3, this wave also disappears if the solution is stirred in the potential region of the most positive wave. The concentration dependence of the main cathodic wave of cystine is shown in Fig. 5-

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0

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y

0.2

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Fig. 5. Dependence of limiting current of cystine reduction, ks, on cystine concn. (cf. Fig. 3). Fig. 6. Voltammetric curves in polarization region --o.18 V to 0. 4 V vs. N.H.E. (i), polarization cycle 1.9 V -~ --o.18 ~ 0. 4 V; (2), subsequent polarization cycle 0. 4 -+ --o.18 --->0. 4 V.

Change of polarization region I n the first experiment, at first a steady voltammetric curve was obtained b y repetitive scanning in the potential region - o . 18 to 1.9 V, then cyclic voltammetric curves were recorded in the potential region 0.4 V to - o . 1 8 V (Fig. 6). After the first cycle, wave k2 (corresponding to a product formed at 1.8 V) has already completely disappeared. T h e height of the main cathodic wave (ks) slowly decreased during cyclization (Fig. 7). The electrode was then polarized in the potential region - o . 1 8 V to 1.5 V. A steady v o l t a m m e t r i c curve is shown in Fig. 8. W a v e ka and the cathodic peak of surface oxide reduction disappeared.

Extrapolated polarization curves Polarization curves obtained b y extrapolation of i-t curves at constant potential to t = o and t-+oo are shown in Fig. 9.

j. Electroanal. Chem., 17 (1968) 177-183

180

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2

3

4

5

6

7

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Fig. 7. D e p e n d e n c e of l i m i t i n g c u r r e n t of c y s t i n e r e d u c t i o n , ks, on n u m b e r of polarization cycles o. 4 - ~ --o.18 --> o. 4 V (horizontal coordinate). Fig. 8. V o l t a m m e t r i c c u r v e in p o l a r i z a t i o n region - - o . 1 8 V to 1. 5 V. C y s t i n e concn., 5" 1°-8 M.

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Fig. 9, E x t r a p o l a t e d v o l t a m m e t r i c c u r v e s w i t h gold electrode in 5" lO-3 M c y s t i n e a n d I N H2SO4. (1), t =

o ; (2), t ~

oo.

Fig. IO. T i m e - d e p e n d e n c e of coverage of a p r e - h e a t e d gold electrode b y c y s t i n e in lO -4 M c y s t i n e a n d I N H~SO4.

Adsorption measurements without external polarization Adsorption of 3~S-cystine on a preheated gold electrode is much faster than on the platinum electrode (Fig. IO).

Adsorption measurements after external polarization As with the platinum electrode 1, the gold electrode was first polarized by j . Electroanal. Chem., 17 (1968) 177-183

CYSTINE AT A u ELECTRODE

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cyclic pulses in I N I-I2SO4, the sweep was stopped at 1.6 V and 3sS-cystine was dissolved in the electrolyte (i.e., the electrode was partly covered b y surface oxide). The radiometric activity of the electrode after 15 min corresponds to a surface concentration of cystine, of F = 2 . o x lO -1° mole cm-~. /--

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\/."

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Fig. 1I. D e p e n d e n c e of coverage of gold electrode b y c y s t i n e on p o t e n t i a l d u r i n g cyclic s c a n n i n g in lO .4 M c y s t i n e a n d 1 N H2SO4 ( × ) . Full p o i n t s w i t h arrow: c o v e r a g e of t h e electrode a f t e r 15-min electrolysis a t c o n s t a n t p o t e n t i a l . ( . . . . . ), v o l t a m m e t r i c c u r v e w i t h gold electrode in lO -4 M cystine a n d 1 N" H2SO4.

/

/x

10-4

10 -3

L 6.10 -3

J 10-2M

Fig. 12. Dependence of coverage of gold electrode by cystine on cystine conch, after cyclic scanning a t 0. 4 V.

Adsorption measurement during cyclic polarization The electrode was subjected to cyclic polarization in a solution containing 35S-cystine until a steady voltammetric curve was obtained (cf. ref. I). The adsorbed quantity of sulphur-containing substances on the electrode at various potentials is shown in Fig. I I . After I5-min polarization at the potential of anodic current peak, the decrease of surface concentration is a m a x i m u m while at the potential of anodic current minimum (E =1.55 V) there is an increase of the adsorbed quantity. In the "double-layer" region (Emo.4 V) the change in quantity adsorbed is very small. Figure 12 shows the dependence of adsorbed quantity on cystine concentrat;_on at 0. 4 V when the electrode is under conditions of a steady voltammetric curve. j . Electroanal. Chem., 17 (1968) 177-183

182

j. KORYTA, J. PRADA~

Desorption in cystine-free electrolyte The procedure has been described in the preceding paper 1. The amount adsorbed remains constant in the potential range 0.4-0.8 V. After a scan to 1.55 V, the quantity adsorbed decreases and after the electrode has remained at this potential for 15 min a complete desorption is observed; the same effect was achieved b y the second cycle (Fig. 13). Similar results are obtained with voltammetrie curves in cystine-free electrolyte (Fig. 14). Here, the polarization curve of the second cycle is identical with the curve measured in the absence of cystine. After oxidative desorption of cystine, the surface of the electrode becomes more active and a new peak of surface oxide formation is observed at 1.2 V (cf. Fig. I). In the third cycle it decreases owing to the diminishing activity of the electrode. A/cm ~

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2,3

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Fig. 13. D e p e n d e n c e of coverage of gold electrode b y c y s t i n e in cystine-free 1 N H2SO4 on electrode p o t e n t i a l ( x ). T h e n u m b e r a t full p o i n t s d e n o t e s t h e order of t h e cycle. F u l l p o i n t w i t h arrow : coverage after I 5 - m i n electrolysis a t c o n s t a n t potential. ( . . . . . ), v o l t a m m e t r i c c u r v e w i t h gold electrode in lO-4 M c y s t i n e a n d I N H2SO4. Fig. 14. Successive v o l t a m m e t r i c c u r v e s of p r e v i o u s l y a d s o r b e d c y s t i n e in cystine-free I N H2SO4. N u m b e r s d e n o t e t h e order of t h e cycle.

Coulombic yields in cystine-free solution The total amount of charge consumed in the main anodic peak of cystine was determined and compared with the amount of cystine adsorbed as in the preceding paperk The adsorbed quantity at E =0.4 V was 2.3 × lO -l° mole cm -2 and the total amount of charge, 2.2 x 10 -4 C cm-2. Therefore cystine releases approximately ten electrons on oxidation at the gold electrode.

Cysteic acid, alanine and serine As with the platinum electrode, these substances are neither adsorbed appreciably nor participate in electrode reactions at the gold electrode in I N H2S04. DISCUSSION

As with the platinum electrode the electrochemical inactivity of the model substances, alanine and serine, at the gold electrode supports the assumption that J. Electroanal. Chem., 17 (1968) 177-183

183

CYSTINE AT A u ELECTRODE

the presence of the-S-S-group in the cystine molecule is the only cause of its ability to adsorb at the electrode and to participate in the electrode reaction. The rate of adsorption of cystine at a gold electrode is larger, and the adsorbed quantity measured during cyclic polarization at 0.4 V in a solution of lO -4 M cystine smaller, than at the platinum electrode. This indicates that weaker chemisorption bonding of cystine to the gold electrode m a y be expected. On the other hand, these phenomena are influenced b y more extensive chemisorption of oxygen at the platinum than at the gold electrode. From the coulombic yields, it follows that also at the gold electrode the principal product is cysteic acid. In the potential range of the most positive wave another, probably less oxidized, product is formed which reduces in the cathodic wave, k2. I t is only weakly adsorbed so that it can be removed from the electrode b y stirring of the solution. The potential range of the main anodic peak is practically identical with the potential range of the anodic peak of surface oxide formation and is shifted to more positive potentials b y o.15 V, compared with the platinum electrode. The relation between the oxidation of cystine and of the electrode m a y be caused b y the oxygen transfer mechanism of the oxidation of cystine 5-7. RSaas. + 5 0 H a a s . -+ RS03- + 2 H~O + H + The main cathodic wave of cystine (k3) is situated in the potential range of adsorption of hydrogen at the gold electrode. The reciprocal value of the slope of the voltammetric curve in semi-logarithmical coordinates at current values lower than the current plateau is 8 E / 3 l o g i ~ o . ! I V. The reduction product is probably cysteine which is oxidized in wave a4 (this will be reported on in a following paper). SUMMARY

In I N H2S04, cystine is less adsorbed at a gold electrode than at a platinum electrode, but more quickly. The principal electrode oxidation product is cysteic acid. The potential range of oxidation of cystine (o.85-1.6 V vs. NHE) is almost the same as the potential range of surface oxide formation. A mechanism of oxidation of cystine with participation of the surface oxide has been proposed. Cystine is also reduced (probably to cysteine) in the potential range o.I V to - o . I V vs. N.H.E. REFERENCES i 2 3 4 5 6 7

J. PRAD~ AND J. KORYTA,J. Electroanal. Chem., 17 (1968) 167. M. DIxoN AND J. H. QUASTELL, f. Chem. Soc., 123 (1923) 2943L. MICHAELISAND J. NORD, J. Biol. Chem., 69 (1926) 295. F. G. WILL AND C. A. •NORR, Z. Elektrochem., 64 (196o) 258. S. GILMAN, General Electric Research Laboratory Rept. No. 63-RL-34oo C. A. KUTSCHKERAND W. VIELSTICH, Electrochim. Acta, 8 (1963) 985. A. N. FRUMKINAND B. I. PODLOVCHENKO, Dokl. Akad. Nauk SSSR, 15o (1963) 349. j . Electroanal. Chem., 17 (1968) 177-183