The oxidation of cysteine by electrogenerated octacyanomolybdate (V)

Article abstract of DOI:10.1039/b400141a

We report on the response of solid carbon electrodes (glassy carbon and boron doped diamond to the one electron oxidation of octacyanomolybdate(IV) in a wide pH range (1-11). Under alkaline conditions the oxidation of octacyanomolybdate(IV) was found to proceed via a EC reaction mechanism, where one electron oxidation to octacyanomolybdate(V) was followed by inner sphere decomposition of the oxidised species. Further, the electrochemical oxidation of octacyanomolybdate(IV) was studied in the presence of thiols (cysteine, homocysteine and glutathione); an electrocatalytic process occurs, where electrochemically generated octacyanomolybdate(V) species is reduced by cysteine back to the parent octacyanomolybdate(IV) species via a E(C)C′ reaction mechanism. DIGISIM modelling was utilised in order to confirm the suggested mechanism and obtain relevant kinetic data.

Full text of DOI:10.1039/b400141a

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R E S E A R C H P A P E R
The oxidation of cysteine by electrogenerated
octacyanomolybdate (V)
Olga Nekrassova, Jessica Kershaw, Jay D. Wadhawan, Nathan S. Lawrence and
Richard G. Compton*
Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road,
Oxford OX1 3QZ, UK. E-mail: Richard.compton@chemistry.ox.ac.uk; Fax: 01865 275410;
Tel: 01865 275413
Received 6th January 2004, Accepted 26th January 2004
F|rst published as an AdvanceArticle on the web 23rd February 2004
We report on the response of solid carbon electrodes (glassy carbon and boron doped diamond to the one
electron oxidation of octacyanomolybdate(IV) in a wide pH range (1–11). Under alkaline conditions the
oxidation of octacyanomolybdate(IV) was found to proceed via a EC reaction mechanism, where one electron
oxidation to octacyanomolybdate(V) was followed by inner sphere decomposition of the oxidised species.
Further, the electrochemical oxidation of octacyanomolybdate(IV) was studied in the presence of thiols
(cysteine, homocysteine and glutathione); an electrocatalytic process occurs, where electrochemically generated
octacyanomolybdate(V) species is reduced by cysteine back to the parent octacyanomolybdate(IV) species via a
E(C)C0 reaction mechanism. DIGISIM2 modelling was utilised in order to confirm the suggested mechanism
and obtain relevant kinetic data.
however, to date no data has been reported on the electro-
1. Introduction
chemical behaviour of the system. In contrast, the reaction
of ferrocyanide with cysteine is well established,11–13 with the
reaction process being utilised as a means of detecting these
important molecules and a commercially available sulfide
sensor having been produced.14,15
Thiols play an important role in biochemical processes. The
ubiquity of thiols in physiological fluids provides scientists
with the opportunity to diagnose diseases more accurately.
The most common targets are cysteine, homocysteine and glu-
tathione (see Scheme 1). Cysteine and glutathione are recog-
nised as antioxidants due to their ability to scavenge free
radicals; thus a depletion of these species within the biological
sample can accompany leukaemia and several types of cancer.
However, abnormally high concentrations of cysteine and
glutathione can be associated with brain disorders.1 Homo-
cysteine plays an important role in the trans-sulfonation
pathway and some other recycling pathways including the
folate cycle. Therefore the elevated levels of homocysteine in
body fluids can indicate possible deficiencies of the various
vitamins (vitamin B6 , cobalamin and folic acid) and as a result
lead to pregnancy complications, cardiovascular diseases,
depression and cancer. Finally, it was recently discovered that
by screening the concentration of homocysteine rather than
cholesterol the risk of heart attacks can be monitored more
accurately.2 Thus, there is an urgent need for developing rapid
and reliable sensing techniques for detection of thiols in
biological samples.
In this report the electrooxidation of octacyanomolybda-
te(IV) at solid carbon electrodes over a wide pH range was first
examined, after which the electrochemically initiated oxidation
of cysteine with electrochemically generated octacyanomolyb-
date(V) was investigated in alkaline solutions. This reaction
was found to be similar to that demonstrated for ferrocyanide
and cysteine,16 whereby ferrocyanide is first oxidised to
ferricyanide at the electrode surface. The ferricyanide then
undergoes a catalytic reduction by the cysteine back to
ferrocyanide, which can then be electrochemically reoxidised
to produce an enhancement in the oxidation current. However,
it was found that in case of octacyanomolybdate the reaction
occurs via an electrocatalysed E(C)C0 reaction, where the inner
coordination sphere of the oxidised octacyanomolybdate(V)
complex undergoes chemical reaction in parallel with electro-
catalysis.
Reactions of metal complexes with sulfur containing com-
pounds have been investigated as possible models for biologi-
cal sensors. Octacyanomolybdate has been used in the
oxidation of organic compounds and thiol species,3–10
2. Experimental
All reagents were obtained from Aldrich; these were of the
highest grade available and used without further purification.
All solutions and subsequent dilutions were prepared using
deionised water from Vivendi (Vivendi, UK) UHQ grade
water system with a resistivity of not less than 18 MO cm.
All experiments were carried out in a solution volume of 10
cm3 and at 20 ꢀ 2 ꢁC.
The electrochemical measurements were recorded using a
m-Autolab computer controlled potentiostat (Eco-Chemie,
Netherlands) with a standard three electrode configuration.
Boron doped diamond (BDD) (Element Six Ltd, UK) and glassy
carbon (GC) (BAS Technocol, UK) served as the working
electrode with the active area of electrode being 0.25 cm2 and
0.07 cm2, respectively. Platinum wire provided the counter
Scheme 1 Common thiol-biomarkers.
T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4
1316
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 1 3 1 6 – 1 3 2 0

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electrode with a saturated calomel reference electrode (SCE,
Radiometer, Denmark) completing the cell assembly. The BDD
electrode was housed in a Teflon2 mounting using araldite and
consisted of a polished, boron doped diamond film (5 ꢂ 5 ꢂ 0.6
mm3) that had undergone no surface pre-treatment. The GC
electrode was polished between experiments with diamond
pastes (Kemet, UK) of decreasing particle size (6 to 1 mm).
where n is the number of electrons transferred, F is the Fara-
day constant, t is the time elapsing after the potential was
stepped and D is diffusion coefficient. The obtained values of
1
2
nD were then compared to n3=2D1=2 obtained from the cyclic
ˇ
voltammetry data and calculated using the Randles–Sevcik
equation. Reassuringly, the value of n was found to be equal
to one. Examination of the voltammetric data provided an
experimentally determined diffusion coefficient of 3.7 ꢂ 10ꢃ6
cm2 sꢃ1. This compares favourably with the previously pub-
lished values, which range from 1.5 ꢂ 10ꢃ6 18 to 5.2 ꢂ 10ꢃ6 19
.
3. Results and discussion
Both oxidation and reduction processes were found to be
pH independent. The slight difference in the peak potentials
as well as lower magnitude of oxidation peak currents
observed in acidic solutions might be rationalised by the rapid
decomposition of the octacyanomolybdate(IV) complex to an
electrochemically inactive species. It was suggested that the
substitution of the CN ligand in the inner sphere takes place
analogous to that shown previously for the Mo(V) complexes20
(eqn. (1)).
Initial examination of the oxidation of octacyanomolyb-
date(IV) was carried using cyclic voltammetry at both BDD and
GC electrode substrates over a wide pH range (1–10) in
order to determine optimal conditions for the electrochemi-
cal reaction. The cyclic voltammetric responses obtained at
a BDD electrode to 200 mM octacyanomolybdate at different
pH’s (pH 1, 0.1 M hydrochloric acid, pH 4.2, 0.1 M acetate
and pH 10, 0.1 borax, 0.5 M KCl) are shown in Fig. 1a.
These reveal a well-defined oxidation peak at þ0.53 (pH 1
and 4.2) and þ0.59 V (pH 10) (vs. SCE), which may be
attributed to the one electron oxidation of Mo(IV) to
Mo(V) at the electrode surface.17 Upon reversal of the scan
direction a corresponding reduction peak was observed at
þ0.44 (pH 1 and 4.2) and þ0.51 V (pH 10), consistent with
the reduction of octacyanomolybdate(V) species back to
octacyanomolybdate(IV). To confirm that the redox process
3ꢃ
½MoðCNÞ8ꢄ4ꢃ þ H2O ! ½MoðCNÞ H2Oꢄ þ CNꢃ ð1Þ
7
It should be noted that commencing the scan immediately after
the addition of the octacyanomolybdate(IV) gave a voltam-
metric response similar to that obtained at the higher pH. It
was found that over time the peak currents decreased, suggest-
ing the loss of parent material via a CE reaction mechanism.
In order to corroborate the result obtained at the BDD elec-
trode, the electrochemical response of glassy carbon electrode
to 200 mM octacyanomolybdate(IV) was next recorded. Fig. 1b
illustrates the responses for the various systems studied
obtained at glassy carbon electrode. These show an oxidative
peak at þ0.53 (pH 1 and 4.3) and þ0.59 V (pH 10), with a cor-
responding reduction peak at þ0.46 (pH 1 and 4.3) to þ0.51 V
(pH 10) analogous to that obtained at the BDD electrode.
involved
a single electron transfer, chronoamperometric
experiments were next conducted. In this case the electrode
potential was paused at þ0.85 V (enough to oxidise octacya-
nomolybdate(IV)) and the amperometric response for various
concentrations of octacyanomolybdate (100–400 mM) was
recorded. These responses (not shown) were then analysed
using the Cottrell equation for transient voltammetry to cal-
culate the number of electrons transferred in the reaction:
ˇ
Again, analysis of these waves using the Randles–Sevcik
nFAD1=2
C
equation produced a value of one for the number of electrons
transferred. Taking into consideration the voltammetric
waveshapes for the oxidation of octacyanomolybdate and
the voltammetric profiles for direct oxidation of thiols16 all
further experiments were carried out at the BDD electrode.
Ilim
¼
p
1=2t1=2
3.1. Oxidation of the octacyanomolybdate(IV) complex at
boron doped diamond
Fig. 2 details the voltammetric response to increasing additions
of octacyanomolybdate(IV) (100–1000 mM, pH 10, 0.1 M
borax, 0.5 M KCl, scan rate ¼ 50 mV sꢃ1) at a BDD electrode.
It can be seen that upon the introduction of octacyanomolyb-
date(IV) to the solution an oxidation wave was observed at
þ0.59 V with the corresponding reduction wave at þ0.51 V
analogous to that shown in Fig. 1a. Upon further additions
Fig. 1 Response (scan rate 50 mV sꢃ1) of BDD (A) and GC (B)
electrodes to 200 mM octacyanomolybdate(IV) in pH 1 (dashed), pH
4.2 (solid) and pH 10 (dotted) solutions.
Fig. 2 Cyclic voltammograms (scan rate ¼ 50 mV sꢃ1) detailing the
response of increasing additions of aqueous octacyanomolybdate(IV)
(100 to 1000 mM) in pH 10 (0.1 M borax, 0.5 M KCl) at BDD.
T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 1 3 1 6 – 1 3 2 0
1317

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of octacyanomolybdate (200–1000 mM) to the solution, an
increase in both the oxidation peak and reduction peak pro-
cesses was observed. It was further found that analogous vol-
tammetric profiles were obtained for both pH 9 and 11 with an
increase observed in both the oxidative and reductive peak cur-
rents (at þ0.59 V and þ0.51, respectively). Analysis of this
enhancement as a function of the concentration of octacyano-
molybdate(IV) added, produced the analogous sensitivities of
2.37 ꢂ 10ꢃ2 mA mMꢃ1 (R2 ¼ 1) in three pHs. This indicates
that similar electrode reactions are occurring at each pH
studied (9–11).
The voltammetric response of octacyanomolybdate(IV) to
varying scan rates (25–400 V sꢃ1) and pH’s (9–11) was next
conducted, to obtain further information about the nature of
the electrochemical process. The corresponding voltammo-
grams recorded in pH 10 for 500 mM octacyanomolybdate(IV)
are outlined in Fig. 3a. These show a single oxidation process
at þ0.59 V with a corresponding reduction process at þ0.51 V,
analogous to those shown in Figs. 1a and 2. Fig. 3b illustrates
the plots of oxidation peak currents against square root of scan
rate for the three pH values studied. The linearity of the plots
in the scan range examined indicates a diffusion controlled pro-
cess. The fact that the magnitude of the oxidation peak current
was independent of the hydroxyl ion concentration, suggests
that the previously reported20,21 catalytic reaction (eqn. (2))
could be neglected as was assumed in ref. 3
Fig. 4 Cyclic voltammograms detailing the oxidation of 500 mM
octacyanomolybdate(IV) during consecutive scans (1, 2, 5, 10 and 20)
in pH 10 (0.1 M borax, 0.5 M KCl).
again, similar voltammetric profiles were registered upon each
scan, with the oxidation and corresponding reduction pro-
cesses, however the magnitude of the oxidation peak current
was found to decay upon repetitive scanning. In order to
ensure that no surface fouling of the electrode had occurred
during this test, the boron doped diamond electrode was sub-
sequently placed into an octacyanomolybdate free-buffer solu-
tion and cyclic voltammetry was applied. No characteristic
wave was registered in the potential range applied (0–1 V),
indicating no species had adsorbed to the electrode surface.
Thus, the decrease in the magnitude of the peak current could
be explained by the decomposition of the inner sphere of the
freshly electrogenerated complex, in which one cyano group
is substituted by one molecule of water20 via the reaction
mechanism outlined in Scheme 2b. Such a substitution reac-
tion has been confirmed spectroscopically.20 In order to con-
firm this reaction process and to obtain an electron transfer
rate constant DIGISIM2 (BAS Technicol)22 modelling was
performed next.
3ꢃ
4ꢃ
½MoðCNÞ8ꢄ þ 2OHꢃ ! ½MoðCNÞ8ꢄ þ O2 þ H2O ð2Þ
1
2
The stability of the system to repetitive voltammetric scan-
ning was next examined. Fig. 4 depicts cyclic voltammograms
from twenty repetitive cyclic scans obtained at the BDD elec-
trode in a 500 mM octacyanomolybdate (pH 10) solution. Once
3.2. Elucidation of the reaction mechanism for the oxidation
of octacyanomolybdate(IV) under alkaline conditions
Initial modelling was focused on the determination of the het-
erogeneous rate constant for the reaction detailed in Scheme
2a. This was conducted using DIGISIM2 as a means of model-
ling the voltammetric profiles detailed in Fig. 2 and 3 for the
oxidation of octacyanomolybdate under alkaline conditions. It
was found that a heterogeneous rate coefficient for the electron
transfer of 0.004 cm sꢃ1, and a transfer coefficient, aa of 0.5 pro-
duced excellent agreement between the experimentally obtained
and theoretically predicted oxidation peak currents and peak
potentials. However, a comparison of the theoretically pre-
dicted and experimentally obtained reduction peak currents
revealed that the modelled values were greater than those deter-
mined experimentally, suggesting that the oxidised octacyano-
molybdate undergoes a chemically irreversible reaction.
This chemical reaction is detailed in Scheme 2b in which one
cyano group of the complex is substituted by a water molecule
Fig. 3 (A) Cyclic voltammograms detailing the response to differing
scan rates (25–400 mV sꢃ1) for solution containing 500 mM octacyano-
molybdate(IV) (pH 10) at BDD. (B) Corresponding plots of oxidation
peak current against the square root of the scan rate obtained in pH
studied: pH 9 (L), pH 10 (ꢂ) and pH 11(S).
Scheme 2 Proposed reaction mechanism for the electrooxidation of
octacyanomolybdate(IV).
T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4
1318
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 1 3 1 6 – 1 3 2 0

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whilst there is concomitant reduction of the octacyanomolyb-
date(V) species. It was found that a homogeneous rate constant
of 0.03 ꢀ 0.003 sꢃ1 provided excellent agreement between the
respective peak currents over a wide range of scan rates for
all three pH values studied. This is outlined in Fig. 5, which
compares the experimentally determined (L) and theoretically
predicted (ꢂ) oxidation peak current as a function of the scan
rate (a) and concentration (b).
After understanding the voltammetric signal for the oxi-
dation of the octacyanomolybdate species the response to
increasing concentrations of cysteine was next examined.
3.3. Thiol induced electrocatalytic reduction of
electrogenerated octacyanomolybdate(V)
The voltammetric profiles detailing the response of 500 mM
octacyanomolybdate to increasing additions of cysteine (50–
500 mM) in a pH 10 borate buffer are shown in Fig. 6a. It
can be seen that upon introduction of cysteine to the solution
an increase in the oxidation peak current at þ0.59 V was
observed along with a corresponding decrease in the reduction
peak current. The enhancement in the oxidation peak current
was found to be linear over the concentration range studied
(50–500 mM). This is consistent with the freshly electrogener-
ated Mo(V) being chemically reduced by cysteine to form
cystine and parent octacyanomolybdate(IV) species, which is
subsequently re-oxidised at the electrode surface to produce
the enhancement in the oxidation peak current. The reaction
is detailed in Scheme 3 and is usually termed an EC0 reaction.
However, in this case, due to the octacyanomolybdate under-
going a parallel chemical reaction with the solvent, the reaction
process is more accurately termed an E(C)C0 reaction. It
should be noted that direct oxidation of cysteine produced
no significant voltammetric wave at þ0.59 V as shown in
Fig. 6a (dashed line).
Finally, the effect of varying the pH of the solution from 9 to
11 on the voltammetric signal was examined. Fig. 6b details
the corresponding plot of oxidation peak current against con-
centration of cysteine for the three different pH values studied.
This reveals that the enhancement in the oxidative peak cur-
rent is independent of the solution pH. This behaviour can
be rationalised by examination of the pKa of the thiol moiety
of cysteine; this has been found to be 10.37.23 The fact that
the enhancement is independent of pH suggests that both
protonated and de-protonated forms of cysteine can undergo
oxidation by octacyanomolybdate analogous to that the
behaviour demonstrated for ferrocyanide.16
Fig. 6 (A) Cyclic voltammograms (scan rate ¼ 50 mV sꢃ1) detailing
the response of aqueous octacyanomolybdate(IV) (500 mM, pH 10) to
increasing additions of cysteine (50–500 mM) at BDD. Dashed line:
cyclic voltammetric response (scan rate ¼ 50 mV sꢃ1) detailing the
direct oxidation of 100 mM cysteine (pH 10) at BDD. (B) Correspond-
ing plots of oxidation peak current against concentration of cysteine
obtained in pH studied: pH 9 (L), pH 10 (ꢂ) and pH 11(S).
the oxidative and reductive peak currents enhanced linearly
with respect to square root of the scan rate, confirming a
diffusion controlled process occurring.
3.4. Modelling of the electrocatalytic reduction of
octacyanomolybdate(V)
In order to confirm the E(C)C0 reaction mechanism deduced
above and to obtain kinetic information for the reaction pro-
cess the cyclic voltammetric responses were modelled using
DIGISIM2. First, the diffusion coefficient of cysteine was
estimated using the Wilke–Chang24 relationship:
Further experimental data for the reaction mechanism was
obtained by examining the effect of the scan rate (25–400 V
s
ꢃ1) on the voltammetric response of a solution containing
500 mM octacyanomolybdate(IV) and 250 mM cysteine in pH
10. The corresponding voltammograms (not shown) revealed
that as the scan rate was increased both the magnitudes of
ðwMÞ1=2T
D ¼ 7:4 ꢂ 10ꢃ8
nV0:6
Fig. 5 (A) Comparison of the corresponding plots of oxidation peak
current against square root of scan rate based on experimentally
obtained (L) with modelled (ꢂ) data for solution containing 500 mM
octacyanomolybdate(IV). (B) Comparison of the corresponding plots
of oxidation peak current against concentration of octacyanomolyb-
date(IV) added based on experimentally obtained (L) and modelled
(ꢂ) data for a pH 10 solution.
Scheme 3 Proposed E(C)C0 reaction mechanism.
T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 1 3 1 6 – 1 3 2 0
1319

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where T is the temperature of the solution in Kelvin, M is the
molecular weight of the solvent, w is the association parameter
equal to 2.6 in this case, n is the viscosity: 0.89 cP in the case of
water at 25 ꢁC and V is the molar volume of the solute at the
normal boiling point estimated for complex molecules from
atomic contributions as defined by La Bas.25 This produced
diffusion coefficient value of 9.3 ꢀ 0.9 ꢂ 10ꢃ6 cm2 sꢃ1 for
cysteine. Modelling of the reaction process with DIGISIM2
for a planar electrode suggested that a forward rate constant
of 9.0 ꢀ 1.0 ꢂ 104 molꢃ1 cm3 sꢃ1 for the catalytic reduction
of octacyanomolybdate(V) with cysteine produced an excellent
fit between the experimentally determined and modelled data.
The corresponding plots of oxidation peak current against
scan rate comparing the theoretically (ꢂ) predicted and experi-
mentally obtained (L) data for a solution containing 250 mM
cysteine and 500 mM Mo(IV) in pH 10 (0.1 M borax, 0.5 M
KCl) and shown in Fig. 7a. Fig. 7b compares the theoretically
modelled (ꢂ) and experimentally obtained (L) plots of peak
current against concentration of cysteine. These plots reveal
that excellent and consistent agreement was observed over a
wide range of scan rates (25 to 400 mV sꢃ1) and concentrations
of thiol added (0–500 mM).
4. Conclusions
We have examined the electrochemical oxidation of octacy-
anomolybdate at solid carbon electrodes (GC and BDD).
The response was found to produce a one electron electroche-
mically reversible signal. Under alkaline conditions the oxi-
dised octacyanomolybdate species was found to react with
the solvent in an EC reaction process. This mechanism has
been successfully modelled using DIGISIM2 modelling with
rate constants deduced for both the heterogeneous and homo-
geneous processes.
Furthermore, the electrochemical oxidation of octacyano-
molybdate has been studied in solutions containing thiols
(cysteine, homocysteine and glutathione). It was shown that
in the presence of thiols a second electrocatalytic reaction
occurred involving the oxidation of the thiol species to the cor-
responding disulfide product and the concomitant reduction of
Mo(V) to Mo(IV). The response was found to be independent of
pH (9–11) suggesting that the octacyanomolybdate(V) species
could oxidise both protonated and de-protonated thiol species.
This overall E(C)C’ reaction mechanism was subsequently
modelled with a suitable rate constant deduced for the homo-
geneous oxidation of cysteine.
3.5. Analytical performance
Acknowledgements
In order to prove that the reaction mechanism of the octacy-
anomolybdate with thiols is generic in nature, analogous experi-
ments were conducted using other thiols: homocysteine and
glutathione. It was found that upon addition of homocysteine
or glutathione to the solution of octacyanomolybdate(IV) an
increase in the oxidation peak current was observed with a cor-
responding decrease in the reduction peak current analogous to
that shown for cysteine.
ON expresses her gratitude to the Analytical Division of
the RSC for the award of a studentship. NSL thanks Schlum-
berger Cambridge Research for financial support.
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Fig. 7 (A) Comparison of the corresponding plots of oxidation peak
current against the square root of the scan rate based on experimen-
tally obtained (L) and modelled (ꢂ) data for solution containing 500
mM octacyanomolybdate and 250 mM cysteine (pH 10). (B) Compari-
son of the corresponding plots of oxidation peak current against con-
centration of cysteine added based on experimentally obtained (L) and
modelled (ꢂ) data for solution containing 500 mM octacyanomolyb-
date (pH 10).
T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4
1320
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 1 3 1 6 – 1 3 2 0