Electroreduction of nitrite on gold electrode modified with Cu-containing nitrite reductase model complex

Article abstract of DOI:10.1039/b507932b

A gold electrode modified with copper complexes containing a tridentate aromatic amine compound (bis(6-methyl-2-pyridyl-methyl)amine ethyl sulfide, which is a model for the nitrite reduction centre of copper-containing nitrite reductase, catalyzed electrochemically the reduction of nitrite to nitrogen monoxide under acidic conditions. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b507932b

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Electroreduction of nitrite on gold electrode modified with
Cu-containing nitrite reductase model complex{
Takamitsu Hiratsu, Shinnichiro Suzuki and Kazuya Yamaguchi*
Received (in Cambridge, UK) 7th June 2005, Accepted 22nd July 2005
First published as an Advance Article on the web 9th August 2005
DOI: 10.1039/b507932b
A gold electrode modified with copper complexes containing a
tridentate aromatic amine compound (bis(6-methyl-2-pyridyl-
methyl)amine ethyl sulfide, which is a model for the nitrite
reduction centre of copper-containing nitrite reductase, cata-
lyzed electrochemically the reduction of nitrite to nitrogen
monoxide under acidic conditions.
To prepare the Me
Scheme 1) a well-polished and ultrasonically cleaned Au electrode
od 1.6 mm, BAS, USA) was immersed in a 50 mM solution of
2
bpaEtS monolayer on the gold surface (1 in
(
5
2 2 2
thiol-derivatized ligand (Me bpaEtS) in 1 : 3 H O–methanol for
10 min at room temperature, and then the electrode was
sufficiently rinsed with the mixed solvent, distilled water, and
methanol to remove the unbound ligand molecules. The insertion
of copper ion was performed by soaking the ligand monolayer on
The nitrogen cycle has received much attention in recent years
because of its ecological importance concerning the widespread
agricultural use of potentially polluting nitrate and nitrite.
Inorganic nitrogen is introduced into the biosphere by biological
fixation of dinitrogen and removed from there again by
denitrification. Denitrification is the dissimilatory reduction of
4 2
the gold surface in 10 mM Cu(ClO ) aqueous solution for 30 min
at room temperature. The surface was thoroughly washed with
distilled water to remove excess free Cu ion. In the X-ray
photoelectron spectrum (XPS, SHIMADZU ESCA-850) of the
Cu complex on Au electrode (2 in Scheme 1) the signals of the
2
2
nitrate (NO
3
) or nitrite (NO
2
) to produce dinitrogen by
sulfur 2p and copper 2p3/2 photoelectrons were observed at 163
6
prokaryotic organisms. It is part of the bioenergetic apparatus of
the cell of denitrifying bacteria which occupy a wide range of
natural habitats including soil and water. Nitrite reductase, a key
enzyme of denitrification, catalyzes the reduction of nitrite to
and 932 eV, respectively.{ The results suggest that CuMe
2
bpaEtS
is bound to the Au surface by sulfur. The copper : sulfur ratio in 2
of 1:1.0 ¡ 0.1 indicated that copper ion was quantitatively
coordinated to 1.
1
nitrogen monoxide. Copper-containing nitrite reductase (CuNIR)
The cyclic voltammograms of 2 as working electrode in 100 mM
has two kinds of Cu centres per subunit. The type 1 Cu accepts an
electron from an external electron donor protein, and the type 2
Cu, which accepts an electron from the reduced type 1 Cu site, is
the reduction centre of nitrite. The type 2 Cu site showing a
distorted tetrahedral geometry is bound by three His residues and
one solvent water. Uptake of nitrite at the type 2 copper appears to
involve displacement of water. Subsequently, the reduction of
nitrite to NO proceeds with attack of protons on the nitrite bound
to Cu site and a concomitant electron transfer reaction from the
NaClO
presented in Fig. 1. The surface coverage of 2 on the gold surface
4
aqueous solution (E1/2: +294 mV vs. Ag/AgCl) are
211
22
mol cm by integrating the
was determined to be 9.0 6 10
charge under the redox wave at E1/2. This is consistent with the
theoretical value expected from close-packed monolayers, 1.2 6
2
10
22
2
10 mol cm , assuming a molecular area of ca. 132 A based on
˚
7
+
X-ray crystallographic data of [Cu(II)Me
2 4
bpa(ClO )] . After the
addition of NaNO to the NaClO solution, the half-peak
2
4
potential was shifted to +140 mV (b in Fig. 1). The result implies
that the fourth ligand of the Cu complex bound on the gold
electrode was replaced by nitrite. As the redox potentials of
2
type 1 copper. Several Cu(II) complexes have been so far reported
3,4
as models of the CuNIR active site. Recently, we have reported
+
2 2
that [Cu(II)Me bpa] (Me bpa: bis(6-methyl-2-pyridylmethyl)-
2
CuMe bpaEt in the same condition are +41 and 235 mV in the
amine) catalyzes the reduction of nitrite with acid to produce
4
NO effectively. The coordination modes of the nitrite ligands in
absence and presence of nitrite, respectively, the gold matrix was
found to affect the redox potentials of the Cu complexes. By the
CuMe
ion; nitrite is coordinated to Cu(II) through two oxygen atoms
O,O9-coordination mode) and to Cu(I) through one nitrogen
2
bpa complexes depend on the oxidation state of copper
4 4
further addition of acid (HClO ) to the NaClO solution
containing nitrite (final pH 5.3), the shape of the voltammogram
is dramatically changed and an enhanced sigmoidal cathodic
current–potential curve (catalytic current) was observed (c in
(
atom (N-coordination mode). In this study, we prepared a gold
electrode modified with CuMe bpa complex, and investigated
2
the electrochemical reduction of nitrite to NO on the modified
electrode.
Department of Chemistry, Graduate School of Science, Osaka
University, Toyonaka, Osaka, 560-0043, Japan.
E-mail: kazu@ch.wani.osaka-u.ac.jp; Fax: +81-6-6850-5785;
Tel: +81-6-6850-5768
{
Electronic supplementary information (ESI) available: XPS core level
spectra for CuME bpaEt and 2 on Au surface. See http://dx.doi.org/
2
10.1039/b507932b
Scheme 1
4
534 | Chem. Commun., 2005, 4534–4535
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to 50% for 6 min. When the bare and ligand only (1) modified gold
electrodes were used as working electrodes, no NO generation was
detected at all.
In conclusion, the electroreduction of nitrite to NO on the gold
electrode modified with Cu complex, which is a model for the
nitrite reduction centre of CuNIR, was observed under acidic
conditions. The results suggest that the modification of an
electrode by metal complexes would be useful for the catalytic
redox reaction of substrate, such as nitrite. Furthermore, the
electrode might be applicable to detection of nitrite in some
environments and electrochemically controlled generation of NO
as a vasodilator, an anticancer agent, and a neurotransmitter in
living bodies. The substrate selectivity is being investigated and the
improvements of the modified electrode are in progress.
We thank Dr Keita Kobayashi and Dr Hiroshi Nakao (Sakai
Chemical Industry Co., LTD.) for XPS measurement. This work
was supported by Mitsubishi Chemical Corporation Fund, The
Association for the Progress of New Chemistry, and Grant-in Aid
for Scientific Research on Priority Areas (No. 17036037
Fig. 1 Voltammetric behaviour of 2 as working electrode (a) in 100 mM
21
NaClO
electrode: Au wire, reference electrode: Ag/AgCl), (b) after addition of
00 mM NaNO to the electrolyte solution a, and (c) after addition of
HClO to b for adjustment of pH to 5.3.
4
aqueous solution at 25.0 uC and scan rate of 10 mV s (counter
‘‘Chemistry of Coordination Space’’ to K.Y.) from Ministry of
5
2
Education, Science, Sports and Culture, Japan.
4
Notes and references
1
2
3
(a) W. G. Zumft, Microbiol. Mol. Biol. Rev., 1997, 61, 533; (b) S. Suzuki,
K. Kataoka, K. Yamaguchi, T. Inoue and Y. Kai, Coord. Chem. Rev.,
1999, 190–192, 245; (c) S. Suzuki, K. Kataoka and K. Yamaguchi, Acc.
Chem. Res., 2000, 33, 728.
(a) K. Kataoka, H. Furusawa, K. Takagi, K. Yamaguchi and S. Suzuki,
J. Biochem., 2000, 345; (b) M. J. Boulanger, M. Kukimoto,
M. Nishiyama, S. Horinouchi and E. P. Murphy, J. Biol. Chem., 2000,
275, 23957.
(a) W. B. Tolman, Inorg. Chem, 1991, 30, 4880; (b) P. P. Paul and
K. D. Karlin, J. Am. Chem. Soc., 1991, 113, 6331; (c) J. A. Halfen,
S. Mahapatra, M. M. Olmstead and W. B. Tolman, J. Am. Chem. Soc.,
1
994, 116, 2173; (d) N. Komeda, H. Nagao, Y. Kushi, G. Adachi,
M. Suzuki, A. Uehara and K. Tanaka, Bull. Chem. Soc. Jpn., 1995, 68,
81; (e) L. Casella, O. Carugo, M. Gullotti, S. Doldi and M. Frassoni,
5
Inorg. Chem., 1996, 35, 1101; (f) J. A. Halfen, S. Mahapatra,
E. C. Wilkinson, A. J. Gengenbach, V. G. Young, Jr., L. Que, Jr. and
W. B. Tolman, J. Am. Chem. Soc., 1996, 118, 763; (g) E. Monzani,
G. J. Anthony, A. Koolhaas, A. Spandre, E. Leggieri, L. Casella,
M. Gullotti, G. Nardin, L. Randaccio, M. Fontani, P. Zanello and
J. Reedijk, J. Biol. Inorg. Chem., 2000, 5, 251; (h) R. L. Richards and
M. C. Durrant, J. Chem. Res. (S), 2002, 95.
Fig. 2 NO generated upon electron supply on the modified electrode 2 at
200 mV in the solution (2 ml) of 100 mM NaClO and 500 mM NaNO
2
4
2
at pH 5.3 and 25.0 uC. Solid line: electron supply, broken line: no electron
supply.
4 H. Yokoyama, K. Yamaguchi, M. Sugimoto and S. Suzuki, Eur. J.
Inorg. Chem., 2005, 1435.
5
S. Itoh, M. Nagagawa and S. Fukuzumi, J. Am. Chem. Soc., 2001, 123,
087.
Fig. 1). The appearance of the catalytic current indicates the
regeneration of the oxidized Cu complex on the gold surface by the
4
6 (a) W. Yang, J. J. Gooding and D. B. Hibbert, J. Electroanal. Chem.,
2001, 516, 10–16; (b) T. L. Brower, J. C. Garno, A. Ulman, G.-y. Liu,
C. Yan, A. Golzhauser and M. Grunze, Langmuir, 2002, 18, 6207.
4
reduction of nitrite ligand.
The generated NO was measured directly in real time with an
NO electrode (World Precision Instruments, USA). Fig. 2 shows
7
A. Sato, M. Abe, T. Inomata, T. Kondo, S. Ye, K. Uosaki and Y. Sasaki,
Phys. Chem. Chem. Phys., 2001, 3, 3420.
that NO was not generated in NaClO solution at pH 5.3 with
4
8 When nitrite was reduced electrochemically at 2200 mV on the gold
electrode modified with CuM bpaEtS (2) in 2 ml solution containing
00 mM NaClO and 500 mM NaNO at pH 5.3 and 25.0 uC for 187 s,
cathodic charge flow and generated NO concentration were observed to
be 1.61 6 10 C and 7.82 6 10 M, respectively. Accordingly, the
amounts of supplied electron and generated NO were calculated to be
500 mM NaNO as indicated by the line with zero slope prior
2
2
1
4
2
supply of electron. On supply at 2200 mV vs. Ag/AgCl on the
24
27
2
gold electrode modified with CuM bpaEtS (2), linear generation of
NO was observed, which stopped when the electron supply was
turned off. The subsequent electron supply on the electrode at
2
9
24
21
1
10
.66 6 10 mol (51.61 6 10 C/96 485 mol C ) and 1.56 6
29 27
mol (57.82 6 10 M 6 0.002 L), respectively. The current
2
200 mV triggered further NO generation until the electron
21
29
efficiency (ca. 94%) was estimated from the calculation (1.56 6 10 mol/
29
supply was turned off. The rate of NO generation was 4.2 nM s
.
1.66 6 10 mol). Meanwhile, as the surface coverage of 2 on the gold
2
11
22
The current efficiency and the turnover number of Cu complex on
the gold electrode for the reduction of nitrite to NO were estimated
surface (od1.6 mm) was 9.0 6 10
complex was estimated to be 1.8 6 10
number of Cu complex on the gold electrode for the reduction of
nitrite to NO was calculated to be 2.3 6 10 s (54.2 nM s /1.8 6
mol).
mol cm , the amount of Cu
212
mol. Therefore, turnover
3
21
8
to be ca. 94% and 2.3 6 10 s , respectively. A continuous
supply of electrons, however, causes a decrease in current efficiency
3
21
21
212
10
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Chem. Commun., 2005, 4534–4535 | 4535