Lifetime of veratryl alcohol radical cation electrogenerated in acetonitrile

Article abstract of DOI:10.1246/cl.2004.408

The electrochemical oxidation of veratryl alcohol at a carbon fiber microdisk electrode with relatively high sweep rates in acetonitrile shows a chemically reversible response. Analysis of the cathodic and anodic current ratio allows one to estimate the lifetime of the short-lived radical cation in the solution.

Full text of DOI:10.1246/cl.2004.408

4
08
Chemistry Letters Vol.33, No.4 (2004)
Lifetime of Veratryl Alcohol Radical Cation Electrogenerated in Acetonitrile
ꢀ
Shin-ya Kishioka and Akifumi Yamada
Nagaoka University of Technology, Kamitomioka, Nagaoka, Niigata 940-2188
(Received December 18, 2003; CL-031253)
The electrochemical oxidation of veratryl alcohol at a car-
bon fiber microdisk electrode with relatively high sweep rates
in acetonitrile shows a chemically reversible response. Analysis
of the cathodic and anodic current ratio allows one to estimate
the lifetime of the short-lived radical cation in the solution.
0.5
µ
A
The electrochemical oxidation of benzyl alcohols has re-
1
,2
ceived attention to date. It was reported that a methoxy group
substituted on an aromatic ring allows oxidation at less positive
potentials because it concentrates the charge density on the ring
3
,4
0
+1
and stabilizes the corresponding radical cation. A further me-
thoxy substituted compound, veratryl alcohol (3,4-dimethoxy-
benzyl alcohol, VA) is a secondary metabolite produced by P.
chrysosporium and it is thought to be a one-electron redox me-
diator from the enzyme to an insoluble and hydrophobic lignin
E / V vs. Ag/Ag
Figure 1. Cyclic voltammogram for 5 mM veratryl alcohol
(solid line) in 0.6 M NaClO4/CH3CN solution at a CF electrode.
Dashed line is the background. The sweep rate was 200 V s
ꢂ1
.
5
–7
polymer during biodegradation processes. However, the ques-
tion has been raised why such a short-lived species acts as an ef-
ꢁ
þ
ficient mediator. Though initial studies suggested that VA/VA
works as a diffusible mediator and a later correction indicated
Figure 1 shows the cyclic voltammogram for the VA oxida-
ꢂ1
tion at 200 V s . Since the current should exhibit the time de-
8
that the VA-enzyme complex stabilizes the radical cation, a
complete mechanism has not been formally accepted. Thus the
pendence expected of a planar electrode for the stated conditions
of electrode size and time scale and VA undergoes a reversible
electron transfer to form the radical cation as the initial stage,3
the absence of a reverse current response indicates the following
chemical reaction. At higher sweep rates (>500 V s ), a reverse
cathodic current appeared. After subtraction of the background
current, the ratio of the cathodic and anodic peak currents
ꢁ
estimation of the lifetime of VA is an important subject, and
þ
enzymatic and radiation chemistry techniques have been em-
ployed to evaluate the lifetime in connection with ESR and con-
ꢂ1
9
–12
ductivity measuerments.
On the other hand, according to the
electrochemical measurements on a conventional time scale, VA
shows irreversible oxidation properties either in aqueous or
aprotic solvents.1 In this study, we attempted to carry out cy-
clic voltammety of VA in acetonitrile at a relatively fast sweep
rate.15 Aprotic solvent can be used as a hydrophobic model at-
mosphere of enzymatic reactions. The reverse cathodic peak cur-
ꢂ1
(ipc=ipa) of the voltammogram at 2 kVs approaches unity. Vol-
tammograms obtained at 1 kV s are shown in Figure 2. Deaer-
3,14
ꢂ1
ation with argon gas did not affect the shape of the voltammo-
grams at any sweep rates. For the monomethoxy substituted,
anisyl alcohol, the reverse current could not be observed in the
time scale we employed. These results suggest that two methoxy
groups in VA as electron-donating substituents would stabilize
the positive charge and extend the lifetime of the radical cation.
In order to explain the formation of the corresponding aldehydes
as a result of the benzyl alcohols oxidation, the following mech-
anism has been proposed: VA undergoes a reversible electron
rent proves the existence of the radical cation intermediate and it
ꢁ
enables us to approximately estimate the lifetime of VA .
þ
Veratryl alcohol (Aldrich), anisyl alcohol (4-methxybenzyl
alcohol, Wako Pure Chemical), ferrocene (Fc, Tokyo Kasei), so-
dium perchlorate (Wako Pure Chemical) and acetonitrile (1
pure grade, Wako Pure Chemical) were used as received. The
electrochemical measurements were carried out using the con-
ventional three-electrode cell configuration. The electrode po-
tential was controlled by a potentiostat (Hokuto Denko, HA-
ꢁ
þ
transfer to form the radical cation (VA ) followed by proton
elimination, and the resulting neutral radical (VA ) should be
ꢁ
þ
oxidized to an intermediate (I ) and complete the reaction via
3
,4,14
501G) and a function generator (Kenwood, FG-281), and the
further deprotonation.
voltammograms were displayed and stored using a digital stor-
age oscilloscope (Tektronics, TDS-1012). A carbon fiber (CF)
microdisk electrode (diameter 33 mm, BAS) was used as the
ꢁ
þ
ꢂ
ꢁ
þ
þ
ꢂ
þ
VA!VA + e !VA +H !I +e !(aldehyde)+H
ꢁ
Reduction current of I to VA was not observed, which indi-
cates that deprotonation of I should be facile. The plot of the
anodic peak currents normalized by the square root of the sweep
rate (v) vs logv in Figure 3 guarantees that the redox potential of
VA is less positive to that of VA. These results allow us to as-
þ
þ
ꢂ3
þ
working electrode. An Ag/Ag (0.01 M (mol dm ) AgNO3 in
acetonitrile) electrode and a Pt spiral electrode were the refer-
ence and auxiliary electrodes, respectively. The potential-cur-
ꢁ
ꢂ1
rent curves at fast sweep rates up to 5 kV s were confirmed
þ
16
for measuring the Fc/Fc redox couple in 0.6 M NaClO4/
CH3CN. All experiments were performed at laboratory temper-
sume an ECE mechanism. A first-order rate constant of
4
ꢂ1
ð5:5 ꢃ 2Þ ꢅ 10 s
is estimated from the working curve of
ipc=ipa vs logꢀ, where ꢀ ¼ ðRT=FvÞk is the dimensionless pa-
ꢄ
ature (22 ꢃ 1 C).
Copyright ꢀ 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.4 (2004)
409
0
.015
a
µ
0
.5
µ
A
0
.01
2
.5
3
3.5
−
1
log(v / V s )
Figure 3. Variation in normalized peak current for the oxida-
tion of 5 mM veratryl alcohol at a CF electrode with sweep rate.
References
0
+1
E / V vs. Ag/Ag
1
2
H. Lund, Acta Chem. Scand., 11, 491 (1957).
E. A. Mayeda, L. L. Miller, and J. F. Wolf, J. Am. Chem.
Soc., 94, 6812 (1972).
3
4
5
O. R. Brown, S. Chandra, and J. A. Harrison, J. Electroanal.
Chem., 34, 505 (1972).
O. R. Brown, S. Chandra, and J. A. Harrison, J. Electroanal.
Chem., 38, 185 (1972).
T. K. Kirk, in ‘‘Lignin Biodegradation: Microbiology,
Chemistry and Potentials Application,’’ ed. by T. K. Kirk,
T. Higuchi, and H. Chang, CRC Press, Boca Raton, FL
b
0
.5
µA
(
1980), Vol. 2, p 51.
6
7
8
9
1
1
P. J. Harvey, H. E. Schoemaker, and J. M. Palmer, FEBS
Lett., 195, 242 (1986).
D. C. Goodwin, S. D. Aust, and T. A. Grover, Biochemistry,
3
4, 5060 (1995).
A. Khindaria, G. Nie, and S. D. Aust, Biochemistry, 36,
5181 (1997).
1
E. Baciocchi, M. Bietti, M. F. Gerini, and O. Lanzalunga,
Biochem. Biophys. Res. Commun., 293, 832 (2002).
0 L. P. Candeias and P. J. Harvey, J. Biol. Chem., 28, 16745
0
1
+
(
1 A. Khindaria, I. Yamazaki, and S. D. Aust, Biochemistry, 35,
1995).
E / V vs. Ag/Ag
Figure 2. (a) Cyclic voltammogram for 5 mM veratryl alcohol
6418 (1996).
12 M. Bietti, E. Baciocchi, and S. Steenken, J. Phys. Chem. A,
02, 7337 (1998).
(
Dashed line is the background. The sweep rate was 1 kV s
solid line) in 0.6 M NaClO4/CH3CN solution at a CF electrode.
ꢂ1
.
b) Cyclic voltammogram obtained by background subtraction
1
(
from (a).
1
3 E. Baciocchi, M. F. Gerini, O. Lanzalunga, and S.
Mancinelli, Tetrahedron, 58, 8087 (2002).
1
4 D. T. Sawyer, A. Sobkowiak, and J. L. Roberts, Jr., ‘‘Electro-
chemistry for Chemists,’’ 2nd ed., John Wiley & Sons, NY
(1995), p 461.
rameter of interest and k is a first-order rate constant.17,18 This
rate constant corresponds to a half-life of approximately 13 ms.
It was reported that the rate of decay of VA generated with
ꢁ
þ
15 R. M. Wightman and D. O. Wipf, Acc. Chem. Res., 23, 64
(1990).
16 R. S. Nicholson and I. Shain, Anal. Chem., 37, 178 (1965).
17 A. J. Bard and L. R. Faulkner, ‘‘Electrochemical Methods
Fundamental and Applications,’’ 2nd ed., John Wiley &
Sons, NY (2001), p 512.
18 R. S. Nicholson, Anal. Chem., 38, 1407 (1966).
19 A. Khindaria, I. Yamazaki, and S. D. Aust, Biochemistry, 34,
16860 (1995).
ꢂ1
19
the enzyme is k ¼ 1:85 s (t1=2 ¼ 375 ms), and k ¼ 1:2 ꢅ
3
ꢂ1
1
0 s (t1=2 ¼ 0:58 ms) when generated with Ce(IV) both in
1
1
ꢂ1
acidic aqueous solutions. In another case, k ¼ 17:1 s
9
(
t1=2 ¼ 40:5 ms) in acidic aqueous acetonitrile. Acidic condition
is necessary for peroxidase activity. The reason why the lifetime
estimated in this study is much shorter than those would proba-
bly be due to the basicity of acetonitrile as the solvent compared
to that of acidic aqueous solutions.
Published on the web (Advance View) March 6, 2004; DOI 10.1246/cl.2004.408