Analytical Chemistry
ARTICLE
was much lower using a glassy carbon electrode. This supports
further that this reaction requires a noble metal surface that can
be reactivated through redox cycling (Supporting Information
Figure S1). In addition, a linear increase in the generation of
4-hydroxylation product was observed under pulsed potential
condition over a 30 min period, suggesting that successful surface
reactivation was achieved under pulse condition (Supporting
Information Figure S2).
’ CONCLUSIONS
Constant potential oxidation of lidocaine generates low yields
of N-dealkylation and 4-hydroxylation products. By varying the
cycle time of square-wave potential pulses and the voltage of the
lower potential step, we increased the yield of 4-hydroxylidocaine
by about 50-fold relative to constant potential conditions result-
ing in an overall yield of approximately 10% at a cycle time of 1 s,
whereas N-dealkylation was favored at short cycle times below
0.2 s. Pulsed potentials are thus effective in modulating electro-
chemical oxidation reactions that are initiated by direct electron
transfer. We show that the Pt electrode surface is rapidly
passivated under oxidative conditions and that it is regenerated
during the low-potential step of the square-wave pulse. How
cycle time affects the selectivity of the reaction (N-dealkylation at
short pulse times versus 4-hydroxylation at longer pulse times)
remains to be elucidated. It is conceivable that the lidocaine
molecule reorients itself on the electrode during pulsing and that
different parts of the molecule are in contact with the electron-
transferring surface. Further experiments are needed to study this
in greater detail. Square-wave potential pulses may be applicable
to other drug compounds in order to generate oxidation products
with greater selectively and higher yield based on optimization of
cycle times and potentials. This could widen the scope of direct
electrochemistry-based oxidation reactions for the imitation of
in vivo drug metabolism.
Figure 4. LCÀMS analysis of lidocaine oxidation products in the pre-
sence H218O with square-wave potential pulses alternating between +3.0
and À1.0 V and a cycle time of 2 s under air atmosphere. The extracted
ion chromatograms correspond to the MH+ ions of the 4-hydroxylation
and the N-oxidation products at m/z 251 (solid line, 16O) and 253
(dashed line, 18O).
superoxide anions that may be sufficiently basic for proton ab-
straction to form the iminium intermediate, facilitating generation
of the N-dealkylation product.21 Generation of the N-oxidation
product requires molecular oxygen (see Figure 1, parts e and f),
but it is also dependent on cycle time (Figure 3, parts e and f). A
lower yield of the N-oxidation product was observed at cycle
times between 0.2 and 12 s. It is intriguing that a minimum in
N-oxidation yield coincides with the optimum for the 4-hydroxyla-
tion reaction suggesting a competition between the electrochemical
generation of hydrogen peroxide and oxidation of the aromatic
ring for subsequent reaction with water.
’ ASSOCIATED CONTENT
The anodic substitution mechanism (see Scheme 2) implies
that the oxygen atom in 4-hydroxylated lidocaine originates from
water. To verify this we replaced H216O by H218O during oxida-
tion of lidocaine using square-wave potential pulses. The ex-
tracted ion chromatograms (Figure 4) show that the major part
of the 4-hydroxylation product contained 18O, although about
15À20% of 16O incorporation was observed, which is likely due
to a remaining 3% of H216O in the commercial H218O vial and to
residual water from the atmosphere. This result is in agreement
with the proposed mechanism. Producing the N-oxidation product
using square-wave potential pulses in the presence of H218O
resulted in approximately 30% incorporation of 18O, whereas the
remainder contained 16O in accordance with the proposed me-
chanism through which molecular oxygen is reduced to hydrogen
peroxide. The 30% of 18O incorporation in the N-oxidation
product under pulsed potential conditions might be due to
oxidation of H218O to 18O2 followed by reduction to H218O2.
The only observed hydroxylation product of lidocaine upon
pulsed potential oxidation was 4-hydroxylidocaine, whereas the
Fenton reaction, which leads to generation of hydroxyl radicals,
results additionally in 3-hydroxylation and benzylic hydroxylation.19
The absence of the latter hydroxylation products indicates
strongly that no adsorbed OH radicals are generated on the Pt
electrode during water oxidation. Pulsed potential oxidation of
lidocaine on a gold electrode showed the same response as on the
Pt electrode, whereas the yield of the 4-hydroxylation product
S
Supporting Information. Additional information as noted
b
in text. This material is available free of charge via the Internet at
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: a.p.bruins@rug.nl. Phone: +31-50-3633262. Fax: +31-
503638347.
’ ACKNOWLEDGMENT
The Dutch Technology Foundation (STW) is gratefully
acknowledged for financial support (Grant 07047). Further
financial support was obtained from Astra Zeneca (Mølndal,
Sweden) and Organon (Oss, The Netherlands; currently a
subsidiary of Merck & Co. Inc., Whitehouse Station, NJ, U.S.A.).
LC equipment was provided by LC Packings, Amsterdam, The
Netherlands (now part of Dionex, Sunnyvale, CA, U.S.A.).
’ REFERENCES
(1) Lohmann, W.; Karst, U. Anal. Bioanal. Chem. 2009, 394,
1341–1348.
(2) Lohmann, W.; D€otzer, R.; G€utter, G.; Van Leeuwen, S. M.; Karst,
U. J. Am. Soc. Mass Spectrom. 2009, 20, 138–145.
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dx.doi.org/10.1021/ac200897p |Anal. Chem. 2011, 83, 5519–5525