The effect of the buffering capacity of the supporting electrolyte on the electrochemical oxidation of dopamine and 4-methylcatechol in aqueous and nonaqueous solvents

Article abstract of DOI:10.1002/asia.201000909

Dopamine was electrochemically oxidized in aqueous solutions and in the organic solvents N,N-dimethyl-formamide and dimethylsulfoxide containing varying amounts of supporting electrolyte and water, to form dopamine ortho-quinone. It was found that the electrochemical oxidation mechanism in water and in organic solvents was strongly influenced by the buffering properties of the supporting electrolyte. In aqueous solutions close to pH 7, where buffers were not used, the protons released during the oxidation process were able to sufficiently change the localized pH at the electrode surface to reduce the deprotonation rate of dopamine ortho-quinone, thereby slowing the conversion into leucoaminochrome. In N,N-dimethylformamide and dimethylsulfoxide solutions, in the absence of buffers, dopamine was oxidized to dopamine ortho-quinone that survived without further reaction for several minutes at 25 °C. The voltammetric data obtained in the organic solvents were made more complicated by the presence of HCl in commercial sources of dopamine, which also underwent an oxidation process. Copyright

Full text of DOI:10.1002/asia.201000909

FULL PAPERS
DOI: 10.1002/asia.201000909
The Effect of the Buffering Capacity of the Supporting Electrolyte on the
Electrochemical Oxidation of Dopamine and 4-Methylcatechol in Aqueous
and Nonaqueous Solvents
Shanshan Chen, Kah Yieng Tai, and Richard D. Webster*[a]
Abstract: Dopamine was electrochemi-
cally oxidized in aqueous solutions and
in the organic solvents N,N-dimethyl-
formamide and dimethylsulfoxide con-
taining varying amounts of supporting
electrolyte and water, to form dopa-
mine ortho-quinone. It was found that
the electrochemical oxidation mecha-
nism in water and in organic solvents
was strongly influenced by the buffer-
ing properties of the supporting elec-
trolyte. In aqueous solutions close to
pH 7, where buffers were not used, the
protons released during the oxidation
process were able to sufficiently
change the localized pH at the elec-
trode surface to reduce the deprotona-
tion rate of dopamine ortho-quinone,
thereby slowing the conversion into
leucoaminochrome. In N,N-dimethyl-
formamide and dimethylsulfoxide solu-
tions, in the absence of buffers, dopa-
mine was oxidized to dopamine ortho-
quinone that survived without further
reaction for several minutes at 258C.
The voltammetric data obtained in the
organic solvents were made more com-
plicated by the presence of HCl in
commercial sources of dopamine,
which also underwent an oxidation
process.
Keywords: chromophores
· cyclic
voltammetry · oxidation · reduction ·
UV/Vis spectroscopy
Introduction
chrome is oxidized at a less positive potential than dopa-
mine; therefore, it reacts immediately with dopamine ortho-
quinone to form aminochrome and dopamine (Scheme 1, re-
action 3).[5,6] Aminochrome is itself not long lived and ulti-
mately reacts to form neuromelanin, a polymeric material.
Detailed electrochemical experiments have established
that the electrochemical behavior of dopamine is greatly in-
fluenced by the 1) electrode material and surface activity,
4-(2-Aminoethyl)benzene-1,2-diol, also known as dopamine
(DA), is produced in the brain and functions as a neuro-
transmitter and neurohormone.[1] Owing to its catechol
(ortho-hydroquinone) structure,[2–4] it can be oxidized in
aqueous solution relatively easily in a two-electron and two-
proton process transforming into the quinine (Q), which sur-
vives for at least several minutes at pH 1 (Scheme 1, reac-
tion 1). The reaction in Scheme 1 is written as a ꢀ2eꢀ/ꢀ2H+
process (concerted mechanism), but the individual electron-
transfer steps could occur separately to the proton-transfer
reactions (consecutive mechanism). At low pH, the amine
group on dopamine (pKa =8.92) and dopamine ortho-qui-
none (DAQ) is protonated; however, as the pH increases
the amine exists in its unprotonated form and dopamine
ortho-quinone undergoes a cyclization reaction to form leu-
coaminochrome (Scheme 1, reaction 2).[5,6] Leucoamino-
[a] S. Chen, K. Y. Tai, Prof. Dr. R. D. Webster
Division of Chemistry and Biological Chemistry
School of Physical and Mathematical Sciences
Nanyang Technological University
Singapore 637371 (Singapore)
Fax : (+65)6791-1961
E-mail: webster@ntu.edu.sg
Scheme 1. Oxidation mechanism for dopamine.
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2) pH, 3) solvent system, and 4) supporting electrolyte.[5–15]
Because of dopamineꢁs key involvement in neural processes,
there is keen interest in in vivo voltammetric monitoring of
dopamine in mammalian brains.[16] The voltammetric mecha-
nism involves an adsorption/desorption process at
carbon[17–24] and metallic electrodes,[21,25–28] which aids in the
voltammetric detection limits. Pure carbon fibers give excel-
lent detection limits,[17–21] and there have been extensive
studies that use surface-modified electrodes to improve the
detection of dopamine and reduce the interference from
other biological molecules that are easily oxidized, such as
ascorbic acid.[29–49] The oxidation of dopamine at the inter-
face between aqueous and nonaqueous solvents has also
been examined using liquid–liquid electrochemical tech-
niques and 4-electrode potentiostats.[50–52]
The majority of electrochemical studies on dopamine
have been performed in buffered aqueous solutions due to
its low solubility in most organic solvents and because it
exists biologically in an aqueous-phase environment rather
than within bilayer lipid membranes. It has recently been
shown that the buffering capacity of the electrolyte has a
large effect on the electrochemical properties of dopamine,
due to the protons released during the oxidation reaction af-
fecting the environment at the electrode surface.[13] In this
study, we were interested in examining the effect that the
supporting electrolyte had on the oxidation of dopamine
and 4-methylcatechol (MC) in water and in the nonaqueous
solvents: N,N-dimethylformamide (DMF) and dimethylsulf-
oxide (DMSO), and whether the organic solvents could sta-
bilize the dopamine ortho-quinone against the cyclization re-
action.
Figure 1. Cyclic voltammograms of dopamine in aqueous buffered solu-
tions at pH 1 and pH 7 recorded at 22(ꢃ2)8C at a 3 mm diameter GC
electrode and with a starting and finishing potential of 0 V vs. Ag/AgCl.
The scan was commenced in the positive potential direction.
Results and Discussion
0.05 Vsꢀ1 at pH 1. The rate constant for the cyclization reac-
tion in pH 7 buffered solutions has been estimated by varia-
ble scan rate CV experiments to be approximately 0.2 sꢀ1 at
258C.[5,6]
It can be seen in Figure 1 that the oxidation process of
dopamine shifts to more negative potentials as the pH in-
creases. The oxidation potential (Eobs) of the process can be
estimated from the midpoint of the oxidative (Eopx) and re-
ductive (Erped) peaks, which vary according to the pH be-
cause protons are involved in the reduction reaction.[53–56]
Hence, the measured Eobs is shifted from the formal poten-
tial (Eof ) according to the Nernst equation:[57]
Electrochemistry in Buffered Aqueous Solutions
Figure 1 shows cyclic voltammetry (CV) data obtained in
buffered aqueous solutions at pH 1 and pH 7 during the oxi-
dation of dopamine. At pH 1, dopamine is oxidized in a
chemically reversible two-electron, two-proton process to
form dopamine ortho-quinone (Scheme 1, reaction 1). As
the pH increases, the lifetime of dopamine ortho-quinone
decreases due to the cyclization reaction of the amine
group. At slow scan rates (n=0.05 Vsꢀ1) at pH 7, dopamine
was found to undergo oxidation to dopamine ortho-quinone
which quickly reacted to form leucoaminochrome
(Scheme 1, reaction 2). Because leucoaminochrome is more
easily oxidized than dopamine, it is immediately oxidized to
aminochrome (AC), which can then be detected as a reduc-
tion process on the reverse scan.[5,6] Aminochromeꢁs struc-
ture is closer to that of a zwitterionic para-quinoneimine,
than an ortho-quinone, which accounts for the large shift in
reduction potential between aminochrome and dopamine
ortho-quinone.[1] At pH 7, as the scan rate was increased to
0.7 Vsꢀ1, the cyclization reaction of dopamine ortho-quinone
was outrun and a chemically reversible two-electron, two-
proton process was observed, similar to that observed at n=
2
RT
nF
½DAQꢂ½Hþꢂ
Eobs ¼ E0f þ
ꢁ ln
ð1Þ
½DAꢂ
where n is the number of electrons transferred (2), R is the
gas constant (8.3143 JKꢀ1 molꢀ1), T is the temperature (in
K) and F is the Faraday constant (96485 Cmolꢀ1). When
[DA]=[DAQ], then Eobs =E1f =2 (the reversible half-wave po-
tential).[57] According to Equation (1), at 258C, the Ef1=2 opti-
mally shifts by ꢀ59.2 mV per unit increase in pH (providing
the Ef1=2 does not significantly vary with the change in pH).
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Equation (1) is valid for pH values between 1–7, whereas in
more basic conditions, the acid dissociation constants of all
the protonated compounds need to be incorporated into the
Nernst equation.[54–56]
duction process at 0 V vs. silver wire was only evident if the
scan was first performed in the positive potential direction,
and was therefore associated with reduction of ortho-qui-
none that forms through the ꢀ2eꢀ/ꢀ2H+ oxidation process
(Scheme 1).
N,N-dimethylformamide and dimethylsulfoxide solutions
containing dopamine displayed an additional oxidation pro-
cess at approximately +1.1 V, which solutions containing 4-
methylcatechol did not show (Figure 2). The process at
+1.1 V was determined to be due to the oxidation of HCl,
by a comparison of a cyclic voltammogram of a solution
containing pure HCl in N,N-dimethylformamide and dime-
thylsulfoxide. Many compounds used for medicinal purposes
are commercially only available combined with HCl because
the hydrochloric acid is used to aid the adsorption of mole-
cules in vivo. HCl is less dissociated in organic solvents
(compared to in water) resulting in a number of complicated
equilibria between the dissociated and nondissociated
forms.[58] A detailed electrochemical study found that the
electrochemical oxidation of HCl in dimethylsulfoxide in-
volves the oxidized form reacting with the solvent.[58] At a
platinum electrode, N,N-dimethylformamide and dimethyl-
sulfoxide solutions containing HCl displayed an additional
reduction process at negative potentials that was not ob-
served on GC and is due to a surface-based process associat-
ed with the initial oxidation reaction. The HCl present in
the dopamine solutions could partially be removed by
adding an equal molar amount of AgSbF6 to the dopamine
solutions, thereby causing the Clꢀ to precipitate as AgCl.
There is a very wide separation between the forward
(Epox =+0.8 V vs. Ag wire) and reverse (Erped =0 V vs. Ag
wire) processes in N,N-dimethylformamide and dimethyl-
sulfoxide associated with dopamine and 4-methylcatechol
converting into their respective ortho-quinones (Figure 2).
There is no evidence, in the case of dopamine, of formation
of the leucoaminochrome reaction product after the initial
oxidation, as it occurs in pH 7 buffered aqueous solutions
(Figure 1). Thus, the protons released during the oxidation
process are sufficiently labile in N,N-dimethylformamide
and dimethylsulfoxide to decrease the pH at the electrode
surface and reprotonate the quinone during the reduction
cycle to regenerate the hydroquinone. The reason for the
large separation between the oxidation and reduction peaks
most likely relates to the individual electron-transfer steps
occurring in a consecutive manner, with each electron-trans-
fer step having a unique redox potential. Scheme 2 shows
the theoretical series of electron-transfer and proton-trans-
fer reactions that are involved in the electrochemically in-
duced conversion of the dihydroquinone (QH2) into the qui-
none, in which the oxidation and reduction reactions occur
through different pathways. In addition to the consecutive
electron-transfer and proton-transfer steps, a concerted
mechanism, in which the electron and proton-transfer steps
simultaneously occur, is also possible and would occur by di-
agonal arrows in Scheme 2. The large potential separation
between the forward and reverse processes in organic sol-
vents has also been reported for the reversible ꢀ2eꢀ/ꢀH+
Electrochemistry in Pure N,N-Dimethylformamide and
Dimethylsulfoxide
Figure 2 shows cyclic voltammograms of dopamine and 4-
methylcatechol in N,N-dimethylformamide and dimethyl-
sulfoxide. 4-Methylcatechol has a structure similar to dopa-
mine but has a methyl group in place of the propylamine
Figure 2. Cyclic voltammograms of 1 mm dopamine and 4-methylcatechol
in DMF and DMSO with 0.2m Bu4NPF6 recorded at 22(ꢃ2)8C at a
1 mm diameter GC electrode with a scan rate of 0.1 Vsꢀ1. The starting
and finishing potential was 0 V vs. Ag wire (0.5m Bu4NPF6 in CH3CN)
with the scan commenced in the positive potential direction.
group, therefore, cannot undergo the cyclization reaction
that dopamine ortho-quinone undergoes. The cyclic voltam-
metric results for dopamine obtained in the two solvents
were similar but showed additional processes to those ob-
served in aqueous solutions and small variations in the data
existed between the platinum and glassy carbon (GC) elec-
trode surfaces. Both dopamine and 4-methylcatechol dis-
played an oxidation process at approximately +0.8 V vs. Ag
wire and a reverse reduction process at approximately 0 V
vs. Ag wire when the scan direction was reversed. The re-
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Electrochemical Oxidation of Dopamine
lieved that trace water has a large effect of the voltammetric
properties of dopamine in N,N-dimethylformamide and di-
methylsulfoxide.
In situ UV/Vis Spectroscopy of Oxidation Reaction
As dopamine ortho-quinone is known to be relatively short
lived in pH 7 buffered solutions (Figure 1), we were interest-
ed in comparing how long lived the quinone was in pure
N,N-dimethylformamide and dimethylsulfoxide solutions
(containing Bu4NPF6), because the timescale of the cyclic
voltammetric experiments shown in Figure 2 only indicate
that the ortho-quinone has a lifetime of at least a few sec-
onds. Figure 3 shows the in situ electrochemical UV/Vis
spectra obtained during the oxidation of dopamine in an op-
tically semitransparent thin layer electrochemical (OSTLE)
cell at pH 1 in aqueous solution and in N,N-dimethylforma-
mide and dimethylsulfoxide. At pH 1, the oxidized product
shows a broad absorbance band at approximately 385 nm,
which is associated with the two-electron oxidation product,
dopamine ortho-quinone.[60] In both dimethylsulfoxide and
N,N-dimethylformamide, the oxidized product also displays
a broad absorbance band at 385 nm, similar to that observed
Scheme 2. Electrochemical square-scheme mechanism showing series of
possible consecutive electron-transfer and proton-transfer reactions asso-
ciated with the reversible transformation between ortho-hydroquinones
and ortho-quinone in acidic conditions. Species QH2 represents any
ortho-hydroquinone, including DA and MC. One resonance structure is
shown for each compound.
electrochemical transformations of the phenolic compound
vitamin E into a diamagnetic cation.[59]
An alternative reason for the wide separation between
the oxidation and reduction processes of DA/DAQ in N,N-
dimethylformamide and dimethylsulfoxide, based on resist-
ance effects of the solutions, is not supported by electro-
chemical experiments on other compounds. For example,
cyclic voltammograms performed on ferrocene in N,N-dime-
thylformamide and dimethylsulfoxide, under the same con-
ditions as the experiments on dopamine, resulted in a mea-
sured potential separation between the forward and reverse
processes of 70 mV, close to the theoretically expected value
for a electrochemically reversible one-electron transfer,[57]
and considerably less than observed for dopamine and 4-
methylcatechol (about 800 mV).
N,N-dimethylformamide and dimethylsulfoxide are both
very hygroscopic solvents, thus the initial water concentra-
tions of both solvents (containing 0.2m Bu4NPF6) were cal-
culated to be 25ꢃ5 mm, based on the published experimen-
tal procedure where values of jE1ꢀE2 j for a quinone were
correlated with the amount of water present (E1 and E2 are
the first and second reduction potentials of vitamin K1).[55,56]
Cyclic voltammetric experiments were also performed on 4-
methylcatechol and dopamine in N,N-dimethylformamide in
the presence of 3 ꢂ molecular sieves, where the water con-
tent could be lowered to 2.5 mm. It was found that the vol-
tammetric behavior was the same as observed at the higher
water concentration shown in Figure 2, thus it is not be-
Figure 3. In situ electrochemical UV/Vis spectra obtained in an OSTLE
cell during the oxidation of dopamine and reduction of the oxidized
product at 22(ꢃ2)8C. Experiments in aqueous solution at pH 1 used sul-
furic acid as the electrolyte, whereas 0.2m Bu4NPF6 was used for experi-
ments in DMSO and DMF.
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in aqueous solutions at low pH. Previous UV/Vis experi-
ments on oxidized products of dopamine in aqueous solu-
tions at pHꢄ7 have indicated that aminochrome has an ab-
sorbance maxima at higher wavelength (475 nm),[60,61] and it
can form as a reaction product after the initial formation of
dopamine ortho-quinone at 385 nm.[60] Therefore, the UV/
Vis spectroscopic experiments in N,N-dimethylformamide
and dimethylsulfoxide are consistent with the formation of
dopamine ortho-quinone as a moderately stable product
over the longer electrolysis timescales.
The UV/Vis data in Figure 3 indicate that the dopamine
ortho-quinone can be reduced back to dopamine under elec-
trolysis conditions, although the reaction is not completely
chemically reversible and the UV/Vis spectra show residual
absorbances above 300 nm possibly due to the formation of
the polymeric reaction compound, neuromelanin.[60,61] Nev-
ertheless, because the UV/Vis spectra shown in Figure 3
were collected over a period of 30 min, the partially reversi-
ble nature of the UV/Vis spectra indicate that the dopamine
ortho-quinone survives for many minutes in dimethylsulfox-
ide and N,N-dimethylformamide, much longer than in pH 7
buffered aqueous solutions.
Chemical oxidation experiments were performed by react-
ing dopamine with 2 and 4 mol equivalents of NOSbF6 in
deuterated dimethylsulfoxide and studying the products by
NMR spectroscopy. The NMR spectra of the reaction prod-
ucts were similar regardless of whether 2 or 4 mol equiva-
lents of NO+ was used as the oxidant, which suggests that
the dopamine was undergoing a two-electron oxidation (and
not being further oxidized on the long timescale synthetic
experiments). NO+ was used as the chemical oxidant be-
cause it is a sufficiently powerful oxidant for phenolic com-
pounds and reacts cleanly to form NO(g) which is easily re-
moved from solution.[62] The 13C NMR spectra of the oxi-
dized solution indicated the presence of several products,
which were possibly caused by the reactions being per-
formed at high concentrations to increase the signal-to-noise
ratio of the NMR spectrum. Strong bands were detected at
178 and 182 ppm and assigned to the carbon atoms in the
C=O bonds, which are expected for a quinone product.
tensity, and, when a ratio of 100:5 was reached, the peak for
the reduction of dopamine ortho-quinone could not be de-
tected. Furthermore, as soon as the aqueous buffer was
added to the N,N-dimethylformamide solution, a new oxida-
tive process was detected between +0.2 to +0.4 V, and the
oxidation process at +0.9 V diminished in intensity. As
more aqueous buffer was added to the solution, the process
at +0.9 V progressively decreased in intensity, so that at a
N,N-dimethylformamide/water (buffer) ratio of 100:30, only
the oxidation process at +0.2 to +0.4 V occurred. The addi-
tion of water to the N,N-dimethylformamide solutions also
resulted in the oxidation process of HCl moving to increas-
ingly more positive potentials, so that at a N,N-dimethylfor-
mamide/water (buffer) ratio of 100:30, the oxidation peak of
HCl was not detected within the potential window exam-
ined.
The results in Figure 4 can be interpreted based on the
presence of the buffer in the aqueous solution neutralizing
(or partially neutralizing) the protons released during the
oxidation process and causing a shift in the Eopx towards
more negative potentials, according to Equation (1). The ob-
servation that both the new oxidation peaks (at +0.2 to
+0.4 V) and initial oxidation process (at +0.9 V) are evident
at intermediate buffer ratios implies that the interactions
with the buffer are complicated and a minimum concentra-
tion of buffer is needed in order to fully stabilize the local-
ized pH at the electrode surface.
As the N,N-dimethylformamide/water (buffer) ratio in-
creased above 100:10, a new reductive peak became evident
at approximately ꢀ0.4 V, which is likely associated with the
formation of aminochrome, due to the buffer decreasing the
acid concentration at the electrode surface to allow the de-
protonation of the dopamine ortho-quinone.
Electrochemistry of Dopamine in Mixed Aqueous/Organic
Solvents Containing Inert Electrolyte, LiClO4
Figure 5 (from bottom to top) shows cyclic voltammograms
of dopamine in N,N-dimethylformamide containing 0.2m
Bu4NPF6 as increasing ratios (v/v) of aqueous solutions of
0.2m LiClO4 were added. As water (containing LiClO4) was
successively added to the N,N-dimethylformamide solution,
the voltammograms in Figure 5 show that the potential gap
between the Eopx and Erped peaks narrowed.
Electrochemistry of Dopamine in Mixed Aqueous/Organic
Solvents Containing Buffers
Figure 4 (bottom to top) shows the results that were ob-
tained when water containing increasing ratios (v/v) of pH 7
buffer was added to N,N-dimethylformamide solutions con-
taining dopamine. The nonaqueous reference electrode was
used throughout the measurements in order to provide a
better comparison with the potentials obtained in N,N-dime-
thylformamide and in N,N-dimethylformamide/water mix-
tures, although a junction potential unavoidably occurs (it
was found that the potential difference between the aqueous
and nonaqueous reference electrodes was <100 mV).
Figure 5 (top) shows voltammograms of dopamine and 4-
methylcatechol in pure water containing LiClO4, which had
a measured pH of approximately 7. Whereas the forward
process consisted of one peak (similar to in pure N,N-dime-
thylformamide or dimethylsulfoxide) in water containing
0.2m LiClO4, the reverse reductive process was split into
two processes, which was likely due to the electron-transfer
steps in the reduction reaction occurring sequentially
(Scheme 2). In addition, the reductive peak at approximate-
ly ꢀ0.2 V associated with the cyclized reaction product, ami-
nochrome, was much smaller than observed in pH 7 buf-
fered aqueous solution (compare Figure 1), which indicated
that the rate of the cyclization reaction was slower in aque-
At
a N,N-dimethylformamide/water (buffer) ratio of
100:1, the reductive peak for the dopamine ortho-quinone
(formed by initial oxidation of dopamine) decreased in in-
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Electrochemical Oxidation of Dopamine
Figure 5. Cyclic voltammograms of 2 mm dopamine (solid lines) or of 4-
methylcatechol (dotted lines) recorded at 22(ꢃ2)8C at a 1 mm diameter
GC electrode with a scan rate of 0.1 Vsꢀ1, and a starting and finishing po-
tential of 0 V vs. Ag wire (0.5m Bu4NPF6 in CH3CN). Voltammograms in
pure DMF contained 0.2m Bu4NPF6 whereas voltammograms in pure
water contained 0.2m LiClO4. Voltammograms of mixed solutions of
DMF and water were obtained by adding water (containing 0.2m LiClO4)
to DMF solutions (containing 0.2m Bu4NPF6) in the given volume ratio.
Figure 4. Cyclic voltammograms of 2 mm dopamine (solid lines) or 4-
methylcatechol (dotted lines) recorded at 22(ꢃ2)8C at a 1 mm diameter
GC electrode with a scan rate of 0.1 Vsꢀ1, and a starting and finishing po-
tential of 0 V vs. Ag wire (0.5m Bu4NPF6 in CH3CN). Voltammograms in
pure DMF contained 0.2m Bu4NPF6 whereas voltammograms in pure
water contained pH 7 buffer. Voltammograms of mixed solutions of
DMF and water were obtained by adding water (containing pH 7 buffer)
to DMF solutions (containing 0.2m Bu4NPF6) in the given volume ratio.
ous solutions containing LiClO4 compared to pH 7 buffered
solutions (although the pH values of the bulk solution were
the same). The reason for the difference between the data
in Figures 4 and 5 is related to the buffering properties of
the electrolytes, because in non-buffered solutions the pro-
tons released during the oxidation reaction are able to de-
crease the pH at the electrode surface, thereby reducing the
rate of the cyclization reaction of dopamine ortho-quinone.
The data in Figure 4 and 5 indicated that the addition of
water to organic solvents has a much smaller effect on the
electrochemistry of dopamine, compared to the buffering
properties of the electrolyte. Figures 4 and 5 show that there
was a substantial shift in the oxidation peak potential (Eopx)
of dopamine in aqueous solutions containing LiClO4
(Figure 5) and aqueous solutions containing the pH 7 buffer
(Figure 4), although the pH values of the bulk solutions
were the same. The cyclic voltammogram of dopamine in
water containing 0.2m LiClO4 had an Eopx =+0.8 V vs. Ag
wire compared to dopamine in pH 7 buffered water with
Eopx =+0.4 V vs. Ag wire. The shift to more positive poten-
tials of dopamine in water containing LiClO4 implied that
dopamine experienced a more acidic environment at the
electrode surface compared to the buffered solution.
Conclusions
Dopamine was electrochemically oxidized in N,N-dimethyl-
formamide or dimethylsulfoxide (containing an inert elec-
trolyte such as Bu4NPF6 or LiClO4) in a chemically reversi-
ble ꢀ2eꢀ/ꢀ2H+ process to form dopamine ortho-quinone.
The separation between the forward (Eopx) and reverse (Erped
)
peaks was large (i.e., 0.8–0.9 V), which was significantly
greater than in aqueous solutions. In situ electrochemical-
UV/Vis spectroscopy confirmed that dopamine ortho-qui-
none survives for at least several minutes in dimethylsulfox-
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R. D. Webster et al.
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ide and N,N-dimethylformamide at 22(ꢃ2)8C (with
Bu4NPF6), compared to in pH 7 buffered aqueous solutions
in which the half-life is several seconds. Whereas the addi-
tion of water (containing LiClO4) to the N,N-dimethylfor-
mamide or dimethylsulfoxide solutions had a relatively
small effect on the voltammetric behavior, the addition of
pH 7 buffered water to the N,N-dimethylformamide or di-
methylsulfoxide solutions resulted in a shift in the oxidation
peak to more negative potentials and a loss in the reduction
process associated with dopamine ortho-quinone.
The electrochemical oxidation of dopamine in unbuffered
aqueous solutions at pH 7 resulted in the formation of dopa-
mine ortho-quinone, which survived longer than in pH 7
buffered solutions. The reason for the increased lifetime of
the dopamine ortho-quinone in pH 7 unbuffered solutions
was most likely due to the protons released during the oxi-
dation process, which changed the pH in the vicinity of the
electrode thereby stabilizing the dopamine ortho-quinone
against deprotonation (and slowing the following reaction to
form leucoaminochrome). Therefore, the lifetime of dopa-
mine ortho-quinone in a biological medium (produced by
oxidation of dopamine) may be significantly longer than in
ideally buffered pH 7 solutions under laboratory conditions.
Acknowledgements
This work was supported by a Singapore Government Ministry of Educa-
tion research grant (T208B1222).
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