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Electrocatalytic effects of adsorbed cobalt phthalocyanine tetrasulfonate in the anodic oxidation of cysteine
J. Electroanal. Chem., 119 (1981) 403--408
403
Elsevier Sequoia S.A., Lausanne --Printed in The Netherlands Preliminary n o t e E L E C T R O C A T A L Y T I C E F F E C T S OF A D S O R B E D COBALT P H T H A L O C Y A N I N E T E T R A S U L F O N A T E IN THE ANODIC OXIDATION OF CYSTEINE
JOSE ZAGAL*, CRISTIAN FIERRO and ROBERTO ROZAS Departamento de Qufmica, Facultad de Ciencia, Universidad T~cnica del Estado, Casilla 5659, Santiago 2 (Chile)
(Received 24th October 1980, in revised form 5th January 1981)
INTRODUCTION Chemical modification of an electrode surface, which will exhibit certain pre-determined properties, has attracted interest in the literature because of the possible applications to electrocatalysis [1--8]. One w a y of modifying the electrode surface is by irreversible adsorption of an electroactive molecule which possesses catalytic activity for a specific reaction [2,9]. Cobalt phthalocyanine tetrasulfonate (Co-TSP) readily adsorbs on graphite substrates forming very stable layers which show electrocatalytic activity for 02 reduction [2, 9 - - 1 1 ] , and for N2H4 electro-oxidation [12]. These properties correlate well with the capability of Co-TSP to bind oxygen reversibly [13,14] and to bind and oxidize hydrazine in aerated aqueous solutions [15,16]. Co-TSP is also an efficient homogeneous catalyst for the one-electron oxidation of cysteine to cystine by air [ 1 6 , 1 7 ] . Stankovich and Bard [18] have proposed that cysteine oxidation on mercury involves the formation of a strongly adsorbed organomercury species, i.e., Hg(R--S)2 which can be reduced back to cysteine. This is in contrast to the irreversibility observed with Pt and Au electrodes [ 1 9 - - 2 1 ] . Koryta and P r a d ~ [ 21] have found that in acid, cysteine is oxidized at both platinum and gold electrodes to cystine. Tafel slopes of a b o u t 0.11 V for Pt and 0.18 V for Au are found. At more positive potentials oxidation of cysteine to cysteinic acid (R--SO3H) occurs. The oxide layers on the metals seem to play an important role. However, the mechanistic features of the process are n o t clear. Studies of the electrocatalytic activity of adsorbed Co-TSP in cysteine oxidation are interesting because of the similarity of the structure of Co-TSP with those of natural enzymes like vitamin B12 and its derivatives. Vitamin B12 takes part in many oxidation--reduction reactions activating --SH and S--S groups [ 2 2 , 2 3 ] . Our report concentrates on the electrocatalytic activity of Co-TSP adsorbed on graphite in the anodic oxidation of cysteine. Co-TSP has been found to *To whom correspondence should be addressed. 0022-0728/81/0000--0000/$ 02.50, © 1981, Elsevier Sequoia S.A.
404
have catalytic activity in both acid and alkaline media. The one-electron oxidation of cysteine to give cystine is irreversible. EXPERIMENTAL
L-Cysteine m o n o h y d r o c h l o r i d e and L-cystine were used as obtained (Merck Chemical Co.). The chemicals NaOH, Na~CO3, Na~SO4, NaH2PO4, and H2 SO4, used in the preparation of buffer and supporting electrolyte solutions, were all A.R. grade (Merck). All solutions were freshly prepared prior to each experiment. Demineralized water used as solvent was distilled twice and stored in a Pyrex glass container. Voltammetric studies were carried out using a stationary ordinary pyrolytic graphite electrode (OPG) (Union Carbide Corp.) m o u n t e d in Kel F (projected area = 0.51 cm 2 ). Polarization curves were obtained using an OPG rotating disc electrode m o u n t e d in Teflon (area = 0.46 cm 2 ). The r o t a t o r was a model ASR manufactured by Pine Instrument Company. Before each experiment the surface of the electrode was polished with filter paper. The electrode was activated by dipping it into a fresh aqueous solution (ca. 10 -s M) of Co-TSP for 20 min and was then thoroughly washed with purified water. Co-TSP was prepared [24] and purified [25] according to literature methods. All electrochemical investigations were conducted using a three electrode system. The cell design was similar to one described previously [10,12]. Solutions in the glass cell were all deaerated with purified nitrogen. A saturated calomel electrode (SCE) was used as the reference electrode for all experimental work. Cyclic voltammograms and polarization curves were obtained using a Princeton Applied Research (PARC) type 174 potentiostat coupled to a PARC Model 175 Universal Programmer. The data was registered on a Houston Omnigraphic 2000 X-Y recorder. Measurements of pH were made using a Tacussel PHN 78 pH meter fitted with a glass electrode and a saturated calomel electrode. RESULTS AND DISCUSSION
Typical voltammetric i--E curves of 10 -3 M cysteine in alkaline and acid media are shown in Fig. I in solid lines. The pyrolytic graphite electrode was pre-treated with Co-TSP as described in the experimental section. The pair of current peaks at ca. - 9 . 5 0 V are due to the adsorbed Co-TSP and probably involve a faradaic process taking place on the metal center and n o t on the macrocyclic ligand [26]. This process is likely to be the reversible change: C o I TSP~COII TSP + e- [12]. The current peaks vary linearly with potential sweep rate and the peak potential shows little dependence on pH in the range from 14 to 1. The current peak at more anodic potentials appears only when cysteine is present in the electrolyte and varies linearly with the square root of the scan rate, as expected for a diffusing reactant. It corresponds to the oxidation of cysteine. The reaction is irreversible as shown by the absence of a cathodic peak current. In the absence of adsorbed Co-TSP on graphite the oxidation of cysteine is greatly inhibited. This is illustrated with dashed lines
405
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Fig. 1. Cyclic v o l t a m m o g r a m s in a N 2 - s a t u r a t e d s o l u t i o n o f 0 . 0 0 1 M c y s t e i n e o n a g r a p h i t e electrode without (..... ) and with ( ) C o - T S P c o a t i n g . E l e c t r o l y t e ( A ) 0.1 M N a 2 C O 3 a n d (B) 0.2 M N a H 2 P O + . E l e c t r o d e a r e a 0.51 c m 2 ; s c a n r a t e 0.2 V s - ' . Fig. 2. R o t a t i n g disc d a t a f o r c y s t e i n e e l e c t r o - o x i d a t i o n in a d e a e r a t e d s o l u t i o n o f 0 . 0 1 M cysteine with ( ) and without (..... ) Co-TSP adsorbed on the graphite surface. Rotation r a t e s i n d i c a t e d o n c u r v e s . E l e c t r o l y t e 0.6 M N a 2 S O + + 0 . 0 6 6 M N a ~ C O 3 ; S c a n r a t e : 1 m V s -1 ; e l e c t r o d e a r e a = 0 . 4 6 c m 2 ; T 27 ° C.
in Fig. 1. Electro-oxidation of cysteine involves a strong interaction of the sulfur atom with an active site on the electrode surface for noble metals [20,27]. This interaction is probably very weak on graphite. The catalytic effect of Co-TSP is also illustrated in the polarization curves of Fig. 2 obtained with a rotating disc electrode. Plots of 1 / / v s . 1 / x / - f ( f = rotation rate) give parallel straight lines which suggest that the reaction is first order with respect to the diffusing reactant [ 2 8 ] . The low apparent diffusion limiting current for the graphite surface in the absence of Co-TSP and the low sensitivity of the currents to rotation rate can be explained if the reaction takes place on sites separated by distances which are large compared to the Nernst diffusion layer thickness. This behavior of a rotating disc graphite electrode has also been observed in 02 reduction in the absence of adsorbed catalysts [9,10]. A series of controlled-potentiai experiments were c o n d u c t e d with 0.1 M cysteine solutions in alkaline (0.2 M Na2CO3 ) and acid (0.2 M NaH2PO+ ) media, using a large area (3.2 cm 2 ) pyrolytic graphite electrode activated with an adsorbed layer of Co-TSP. The potential was set at the diffusion limiting current. The adsorbed catalyst was found to be stable over the time involved in the experiment (ca. 10 h), since the activity of the electrode remained the same when used for a new electrolysis. The products of the reaction were separated using analytical paper chromatography (phenol + water as eluent) and compared with authentic samples of cysteine and cystine. In acid, cystine was found to be the major product of the oxidation reaction with traces of cysteinic 'acid (R--SO3H) present. In alkaline media there were other products apart from cystine which are probably due to the chemical decomposition of cystine that takes place in alkaline media [29].
406
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-I/., -12 -I0 -8 -6 4, -2 Log [H"]
Fig. 3. P l o t s o f E vs. log I k f o r t h e o x i d a t i o n o f c y s t e i n e ( c u r r e n t s c o r r e c t e d f o r diffusion). E l e c t r o l y t e 0.01 M c y s t e i n e c l o r h y d r a t e + 0.6 M N a 2 S O 4 in: 0.1 M H2SO 4 ( p H = 1.2); 0.2 M NaH2PO 4 ( p H = 3 . 2 ) ; 0 . 1 8 M N a H 2 P O 4 + 0 . 0 2 M N a H C O 3 ( p H = 5.3); 0.14 M NaH~PO 4 + 0.06 M N a H C O 3 ( p H = 6.2); 0.2 M NaHCO~ ( p H = 7.6), 0.2 M N a O H (pH ~ 1 3 ) a n d 2 M N a O H ( w i t h o u t Na~SO 4 ) (pH ~ 14). T 27 ° C. Fig. 4. E f f e c t o f p r o t o n c o n c e n t r a t i o n o n t h e c y s t e i n e o x i d a t i o n k i n e t i c c u r r e n t s ( c o r r e c t e d f o r d i f f u s i o n ; d a t a t a k e n f r o m Fig. 3). T 27 ° C.
For a process which is first order in a diffusing reactant, the disc current I is related to the kinetic current I k and to the diffusion limiting current IL by the following expression [28] :
Ik = I [IL/(IL - - / ) ]
(1)
Thus, the disc currents can be corrected for diffusion by multiplying by the factor IL/(IL --I). Figure 3 shows the Tafel plots for different pH values using currents corrected for diffusion. The Tafel slopes range from 0.140 V in alkali to 0.120 V in acid. These values are close to 2 R T / F (0.118 V per decade) which is the slope for a first one-electron transfer rate determining step with a symmetrical energy barrier. Figure 4 illustrates the pH dependence of the currents extrapolated in some cases from the Tafel lines in Fig. 3. The chemical order in protons is --1 at pH values lower than ca. 8.5. At higher pH values the reaction becomes independent of pH. This correlates well with a dissociation of a proton from the --SH group in the cysteine. The reaction R--SH ~ RS- + H ÷ has a PKA value of 8.66 [30]. According to our results and those reported in the literature [17, 21--23,27], it is likely t h a t dissociation of a proton from the --SH group takes place before the rate determining step, the latter involving a one-electron transfer. A possible mechanism is the following: R--SH ~ RS- + H ÷
(2)
ItS-
(3)
, RS- + er.d.s. RS. + R S - -+ R--SS--R where R--SH = cysteine and R--SS--R = cystine. The way the Co-TSP catalyst interacts with the cysteine molecule is n o t clear at the m o m e n t . A d d u c t formation of cysteine with Co-TSP probably
(4)
407
involves a sulfur atom that interacts with the metal center. According to the kinetic parameters, reduction of the cobalt center from Co II to Co I does n o t occur; if it did, a redox t y p e of mechanism w o u l d be observed, with slopes dE/dlog i near 0.060 V or lower, as found for N2 H4 electro-oxidation on Co-TSP [ 1 2 ] . However, a region of small slope is observed at more alkaline pH values, near the region where the reversible transition Co I ~ Co l / + etakes place (see Fig. 3). This could be due to the approach of the system to an equilibrium or better to a redox-type of catalysis operating at those potentials, with the process being controlled by the concentration of CoIITSP on the surface which is under Nernstian control. In solution cysteine reduces the Coil to Co! in Co-TSP as shown by e.p.r, studies [16] with CoI being oxidized back to Co II by 0 2 . The same authors [16] have shown that in the pH range from 9.5 to 12 Co-TSP catalyzes the reduction of cystine to cysteine. This is n o t observed in the present studies. There are obviously some differences between the behavior o f Co-TSP when adsorbed and when in the solution phase. It is likely that the homogeneous reduction of cystine b y Co-TSP involves the simultaneous interaction of two metal centers with the sulfur atoms of the cystine leading to the dissociation of the disulfide bond. This type of interaction of the cystine molecule is observed on a mercury surface [ 1 8 ] . However, when Co-TSP is immobilized on the electrode, assuming that the molecule lies flat with the macrocyclic rings parallel to the surface, it is sterically impossible for the t w o sulfur atoms of the cystine to interact with t w o cobalt centers at the same time. From this w o r k we conclude that Co-TSP is an effective catalyst for the electro-oxidation of cysteine. The active species seems to involve Co(II). The catalytic activity of Co-TSP appears to be strongly related to its ability to bind extraplanar ligands such as 0 2 , N2H4 and cysteine [16]. ACKNOWLEDGMENTS This work has been supported b y the Direcci6n de Investigaciones (DICYT) of the Universidad T~cnica del Estado. Thanks are due to Professor E. Yeager for providing the graphite, to Dr. J.A. Costamagna for facilities relating to instrumentation and to Miss Fanny Cavieres for her technical assistance. Some interesting discussions with Dr. A.J. Appleby are gratefully acknowledged. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12
J.F. Evans, T. Kuwana, M.Y. Henne and G.P. Royer, J. Electroanal. Chem., 80 (1977) 409. J. Zagal, R.K. Sen and E. Yeager, J. Electroanal. Chem., 83 (1977) 207. H.L. Land~tm, R.T. Salom and F.M. Hawkridge, J. Am. Chem. Soc., 99 (1977) 3154. J.F. Staxgartt, F.M. Hawkrldge and H.L. Landrum, Anal. Chem., 50 (1978) 930. D.C.S. Tse and T. Kuwana, Anal. Chem., 50 (1978) 1315. A. Bettelheim, R.J.H. Chan and T. Kuwana, J. Electroanal. Chem., 99 (1979) 391. J.B. Kerr and L.L. Miller, J. Electroanal. Chem., 101 (1979) 263. J.P. Conman, M. Marrocco, P. Denisevich, C. Koval and F.C. Anson, J. Electroanal. Chem., 101 (1979) 117. R.K. Sen, J. Zagal and E. Yeager, Inorg. Chem., 16 (1977) 3379. J. Zagal, P. Bindra and E. Yeager, J. Electrochem. Soc., 127 (1980) 1506. E. Yeager, J. Zagal, B.~. Nikoh~ and R.R. Ad~i~ m E. Yeager, J.D.E. Mc Intyre, B. Miller and S. Bruckenstein (Eds.), Electrode Processes, The Electrochemical Society, PlqJnceton, N.J., 1980. J. Zagal, J. Electroanal. Chem., 109 (1980) 389.
408
13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
vv
D.M. Wagnerov~, E. Schwertnerov~ and J. Veprek-Siska, Collect. Czech. Chem. Commun., 39 (1974) 1980. D.M. Wagnerov~, E. Schwertnerov~ and J. V e p r e k ~ i s k a , Collect. Czech. Chem. Commun., 38 (1973) 756. E. Schwertnerovtl, D.M. Wagnerov~ and J. Veprek-Slska, Z. Chem., 14 (1974) 311. F.J. Cookson, T.D. Smith, J.F. Boas, P.R. Hicks and J.R. Pilbrow, J. Chem. Soc. Dalton, (1977) 109. N.N. Ku ndo and N.P. Keier, J. Phys. Chem., 42 (1968) 707. M.T. Stankovich and A.J. Bard, J. Electroanal. Chem., 75 (1977) 487, and references therein. I.M. Kolthoff and C. Barnum, J. Am. Chem. Soc., 62 (1940) 3061. D.G. Davies and E. Bianco, J. Electroanal. Chem., 12 (1966) 254. J. K o r y t a and J. P r a d ~ , J. Electroanal. Chem., 17 (1968) 185. J.L. Peel, J. Biol. Chem., 237 (1962) PC 264. J. Ar o novitch and N. Grossowicz, Biochem. Blophys. Res. Commum, 8 (1962) 416. J.H. Weber and D.M. Busch, Inorg. Chem., 4 (1965) 469. G. Mc L e n d o n and A.E. Martell, Inorg. Chem., 16 (1977) 1812. L.D. R o l l m a n n and R.T. I w a m o t o , J. Am. Chem. Soc., 90 (1968) 1455. Z. Samec, Zh. Malycheva, J. Ko~Yta and J. Prada~, J. Electroanal. Chem., 65 (1975) 573. V.G. Levich, Physiochemical H y d r o d y n a m m s , Prentice Hall, Engl e w ood Cliffs, N.J., 1962. H.T. Clarke and E.E. Dreger, Organic Synthesis Vol. I., Wiley, New York, 1941, p. 195. W. Stricks and I.M. Kolthoff, J. Am. Chem. Soe., 75 (1953) 5673.
403
Elsevier Sequoia S.A., Lausanne --Printed in The Netherlands Preliminary n o t e E L E C T R O C A T A L Y T I C E F F E C T S OF A D S O R B E D COBALT P H T H A L O C Y A N I N E T E T R A S U L F O N A T E IN THE ANODIC OXIDATION OF CYSTEINE
JOSE ZAGAL*, CRISTIAN FIERRO and ROBERTO ROZAS Departamento de Qufmica, Facultad de Ciencia, Universidad T~cnica del Estado, Casilla 5659, Santiago 2 (Chile)
(Received 24th October 1980, in revised form 5th January 1981)
INTRODUCTION Chemical modification of an electrode surface, which will exhibit certain pre-determined properties, has attracted interest in the literature because of the possible applications to electrocatalysis [1--8]. One w a y of modifying the electrode surface is by irreversible adsorption of an electroactive molecule which possesses catalytic activity for a specific reaction [2,9]. Cobalt phthalocyanine tetrasulfonate (Co-TSP) readily adsorbs on graphite substrates forming very stable layers which show electrocatalytic activity for 02 reduction [2, 9 - - 1 1 ] , and for N2H4 electro-oxidation [12]. These properties correlate well with the capability of Co-TSP to bind oxygen reversibly [13,14] and to bind and oxidize hydrazine in aerated aqueous solutions [15,16]. Co-TSP is also an efficient homogeneous catalyst for the one-electron oxidation of cysteine to cystine by air [ 1 6 , 1 7 ] . Stankovich and Bard [18] have proposed that cysteine oxidation on mercury involves the formation of a strongly adsorbed organomercury species, i.e., Hg(R--S)2 which can be reduced back to cysteine. This is in contrast to the irreversibility observed with Pt and Au electrodes [ 1 9 - - 2 1 ] . Koryta and P r a d ~ [ 21] have found that in acid, cysteine is oxidized at both platinum and gold electrodes to cystine. Tafel slopes of a b o u t 0.11 V for Pt and 0.18 V for Au are found. At more positive potentials oxidation of cysteine to cysteinic acid (R--SO3H) occurs. The oxide layers on the metals seem to play an important role. However, the mechanistic features of the process are n o t clear. Studies of the electrocatalytic activity of adsorbed Co-TSP in cysteine oxidation are interesting because of the similarity of the structure of Co-TSP with those of natural enzymes like vitamin B12 and its derivatives. Vitamin B12 takes part in many oxidation--reduction reactions activating --SH and S--S groups [ 2 2 , 2 3 ] . Our report concentrates on the electrocatalytic activity of Co-TSP adsorbed on graphite in the anodic oxidation of cysteine. Co-TSP has been found to *To whom correspondence should be addressed. 0022-0728/81/0000--0000/$ 02.50, © 1981, Elsevier Sequoia S.A.
404
have catalytic activity in both acid and alkaline media. The one-electron oxidation of cysteine to give cystine is irreversible. EXPERIMENTAL
L-Cysteine m o n o h y d r o c h l o r i d e and L-cystine were used as obtained (Merck Chemical Co.). The chemicals NaOH, Na~CO3, Na~SO4, NaH2PO4, and H2 SO4, used in the preparation of buffer and supporting electrolyte solutions, were all A.R. grade (Merck). All solutions were freshly prepared prior to each experiment. Demineralized water used as solvent was distilled twice and stored in a Pyrex glass container. Voltammetric studies were carried out using a stationary ordinary pyrolytic graphite electrode (OPG) (Union Carbide Corp.) m o u n t e d in Kel F (projected area = 0.51 cm 2 ). Polarization curves were obtained using an OPG rotating disc electrode m o u n t e d in Teflon (area = 0.46 cm 2 ). The r o t a t o r was a model ASR manufactured by Pine Instrument Company. Before each experiment the surface of the electrode was polished with filter paper. The electrode was activated by dipping it into a fresh aqueous solution (ca. 10 -s M) of Co-TSP for 20 min and was then thoroughly washed with purified water. Co-TSP was prepared [24] and purified [25] according to literature methods. All electrochemical investigations were conducted using a three electrode system. The cell design was similar to one described previously [10,12]. Solutions in the glass cell were all deaerated with purified nitrogen. A saturated calomel electrode (SCE) was used as the reference electrode for all experimental work. Cyclic voltammograms and polarization curves were obtained using a Princeton Applied Research (PARC) type 174 potentiostat coupled to a PARC Model 175 Universal Programmer. The data was registered on a Houston Omnigraphic 2000 X-Y recorder. Measurements of pH were made using a Tacussel PHN 78 pH meter fitted with a glass electrode and a saturated calomel electrode. RESULTS AND DISCUSSION
Typical voltammetric i--E curves of 10 -3 M cysteine in alkaline and acid media are shown in Fig. I in solid lines. The pyrolytic graphite electrode was pre-treated with Co-TSP as described in the experimental section. The pair of current peaks at ca. - 9 . 5 0 V are due to the adsorbed Co-TSP and probably involve a faradaic process taking place on the metal center and n o t on the macrocyclic ligand [26]. This process is likely to be the reversible change: C o I TSP~COII TSP + e- [12]. The current peaks vary linearly with potential sweep rate and the peak potential shows little dependence on pH in the range from 14 to 1. The current peak at more anodic potentials appears only when cysteine is present in the electrolyte and varies linearly with the square root of the scan rate, as expected for a diffusing reactant. It corresponds to the oxidation of cysteine. The reaction is irreversible as shown by the absence of a cathodic peak current. In the absence of adsorbed Co-TSP on graphite the oxidation of cysteine is greatly inhibited. This is illustrated with dashed lines
405
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24
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/~-~-
1500 _,ooo
250 20(X -08 +oo,,,'1ooc /." O4 J " . . . . . . . . . . . ~." , 0 +06+04*02 0 -02-04 -06-08 E/V vs SCE
Fig. 1. Cyclic v o l t a m m o g r a m s in a N 2 - s a t u r a t e d s o l u t i o n o f 0 . 0 0 1 M c y s t e i n e o n a g r a p h i t e electrode without (..... ) and with ( ) C o - T S P c o a t i n g . E l e c t r o l y t e ( A ) 0.1 M N a 2 C O 3 a n d (B) 0.2 M N a H 2 P O + . E l e c t r o d e a r e a 0.51 c m 2 ; s c a n r a t e 0.2 V s - ' . Fig. 2. R o t a t i n g disc d a t a f o r c y s t e i n e e l e c t r o - o x i d a t i o n in a d e a e r a t e d s o l u t i o n o f 0 . 0 1 M cysteine with ( ) and without (..... ) Co-TSP adsorbed on the graphite surface. Rotation r a t e s i n d i c a t e d o n c u r v e s . E l e c t r o l y t e 0.6 M N a 2 S O + + 0 . 0 6 6 M N a ~ C O 3 ; S c a n r a t e : 1 m V s -1 ; e l e c t r o d e a r e a = 0 . 4 6 c m 2 ; T 27 ° C.
in Fig. 1. Electro-oxidation of cysteine involves a strong interaction of the sulfur atom with an active site on the electrode surface for noble metals [20,27]. This interaction is probably very weak on graphite. The catalytic effect of Co-TSP is also illustrated in the polarization curves of Fig. 2 obtained with a rotating disc electrode. Plots of 1 / / v s . 1 / x / - f ( f = rotation rate) give parallel straight lines which suggest that the reaction is first order with respect to the diffusing reactant [ 2 8 ] . The low apparent diffusion limiting current for the graphite surface in the absence of Co-TSP and the low sensitivity of the currents to rotation rate can be explained if the reaction takes place on sites separated by distances which are large compared to the Nernst diffusion layer thickness. This behavior of a rotating disc graphite electrode has also been observed in 02 reduction in the absence of adsorbed catalysts [9,10]. A series of controlled-potentiai experiments were c o n d u c t e d with 0.1 M cysteine solutions in alkaline (0.2 M Na2CO3 ) and acid (0.2 M NaH2PO+ ) media, using a large area (3.2 cm 2 ) pyrolytic graphite electrode activated with an adsorbed layer of Co-TSP. The potential was set at the diffusion limiting current. The adsorbed catalyst was found to be stable over the time involved in the experiment (ca. 10 h), since the activity of the electrode remained the same when used for a new electrolysis. The products of the reaction were separated using analytical paper chromatography (phenol + water as eluent) and compared with authentic samples of cysteine and cystine. In acid, cystine was found to be the major product of the oxidation reaction with traces of cysteinic 'acid (R--SO3H) present. In alkaline media there were other products apart from cystine which are probably due to the chemical decomposition of cystine that takes place in alkaline media [29].
406
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"3~O91 .'2 0
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E=-OO5V
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~
-I/., -12 -I0 -8 -6 4, -2 Log [H"]
Fig. 3. P l o t s o f E vs. log I k f o r t h e o x i d a t i o n o f c y s t e i n e ( c u r r e n t s c o r r e c t e d f o r diffusion). E l e c t r o l y t e 0.01 M c y s t e i n e c l o r h y d r a t e + 0.6 M N a 2 S O 4 in: 0.1 M H2SO 4 ( p H = 1.2); 0.2 M NaH2PO 4 ( p H = 3 . 2 ) ; 0 . 1 8 M N a H 2 P O 4 + 0 . 0 2 M N a H C O 3 ( p H = 5.3); 0.14 M NaH~PO 4 + 0.06 M N a H C O 3 ( p H = 6.2); 0.2 M NaHCO~ ( p H = 7.6), 0.2 M N a O H (pH ~ 1 3 ) a n d 2 M N a O H ( w i t h o u t Na~SO 4 ) (pH ~ 14). T 27 ° C. Fig. 4. E f f e c t o f p r o t o n c o n c e n t r a t i o n o n t h e c y s t e i n e o x i d a t i o n k i n e t i c c u r r e n t s ( c o r r e c t e d f o r d i f f u s i o n ; d a t a t a k e n f r o m Fig. 3). T 27 ° C.
For a process which is first order in a diffusing reactant, the disc current I is related to the kinetic current I k and to the diffusion limiting current IL by the following expression [28] :
Ik = I [IL/(IL - - / ) ]
(1)
Thus, the disc currents can be corrected for diffusion by multiplying by the factor IL/(IL --I). Figure 3 shows the Tafel plots for different pH values using currents corrected for diffusion. The Tafel slopes range from 0.140 V in alkali to 0.120 V in acid. These values are close to 2 R T / F (0.118 V per decade) which is the slope for a first one-electron transfer rate determining step with a symmetrical energy barrier. Figure 4 illustrates the pH dependence of the currents extrapolated in some cases from the Tafel lines in Fig. 3. The chemical order in protons is --1 at pH values lower than ca. 8.5. At higher pH values the reaction becomes independent of pH. This correlates well with a dissociation of a proton from the --SH group in the cysteine. The reaction R--SH ~ RS- + H ÷ has a PKA value of 8.66 [30]. According to our results and those reported in the literature [17, 21--23,27], it is likely t h a t dissociation of a proton from the --SH group takes place before the rate determining step, the latter involving a one-electron transfer. A possible mechanism is the following: R--SH ~ RS- + H ÷
(2)
ItS-
(3)
, RS- + er.d.s. RS. + R S - -+ R--SS--R where R--SH = cysteine and R--SS--R = cystine. The way the Co-TSP catalyst interacts with the cysteine molecule is n o t clear at the m o m e n t . A d d u c t formation of cysteine with Co-TSP probably
(4)
407
involves a sulfur atom that interacts with the metal center. According to the kinetic parameters, reduction of the cobalt center from Co II to Co I does n o t occur; if it did, a redox t y p e of mechanism w o u l d be observed, with slopes dE/dlog i near 0.060 V or lower, as found for N2 H4 electro-oxidation on Co-TSP [ 1 2 ] . However, a region of small slope is observed at more alkaline pH values, near the region where the reversible transition Co I ~ Co l / + etakes place (see Fig. 3). This could be due to the approach of the system to an equilibrium or better to a redox-type of catalysis operating at those potentials, with the process being controlled by the concentration of CoIITSP on the surface which is under Nernstian control. In solution cysteine reduces the Coil to Co! in Co-TSP as shown by e.p.r, studies [16] with CoI being oxidized back to Co II by 0 2 . The same authors [16] have shown that in the pH range from 9.5 to 12 Co-TSP catalyzes the reduction of cystine to cysteine. This is n o t observed in the present studies. There are obviously some differences between the behavior o f Co-TSP when adsorbed and when in the solution phase. It is likely that the homogeneous reduction of cystine b y Co-TSP involves the simultaneous interaction of two metal centers with the sulfur atoms of the cystine leading to the dissociation of the disulfide bond. This type of interaction of the cystine molecule is observed on a mercury surface [ 1 8 ] . However, when Co-TSP is immobilized on the electrode, assuming that the molecule lies flat with the macrocyclic rings parallel to the surface, it is sterically impossible for the t w o sulfur atoms of the cystine to interact with t w o cobalt centers at the same time. From this w o r k we conclude that Co-TSP is an effective catalyst for the electro-oxidation of cysteine. The active species seems to involve Co(II). The catalytic activity of Co-TSP appears to be strongly related to its ability to bind extraplanar ligands such as 0 2 , N2H4 and cysteine [16]. ACKNOWLEDGMENTS This work has been supported b y the Direcci6n de Investigaciones (DICYT) of the Universidad T~cnica del Estado. Thanks are due to Professor E. Yeager for providing the graphite, to Dr. J.A. Costamagna for facilities relating to instrumentation and to Miss Fanny Cavieres for her technical assistance. Some interesting discussions with Dr. A.J. Appleby are gratefully acknowledged. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12
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