Displacement-based, chromogenic calix[4]pyrrole-indicator complex for selective sensing of pyrophosphate anion

Article abstract of DOI:10.1016/j.tetlet.2013.04.129

A supramolecular complex composed of bis-pyridinium picket calix[4]pyrrole and azophenol indicator is a highly visible colorimetric displacement assay and sensor. The system shows significant selectivity and a higher affinity for pyrophosphate anions over other competing anions.

Full text of DOI:10.1016/j.tetlet.2013.04.129

Tetrahedron Letters xxx (2013) xxx–xxx
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Displacement-based, chromogenic calix[4]pyrrole–indicator
complex for selective sensing of pyrophosphate anion
a,
Sandeep Kaur a, Hoon Hwang a, Jeong Tae Lee b, , Chang-Hee Lee
⇑
⇑
a Department of Chemistry, Kangwon National University, Chun Cheon 200-701, Republic of Korea
b Department of Chemistry, Hallym University, Chun Cheon 200-700, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A supramolecular complex composed of bis-pyridinium picket calix[4]pyrrole and azophenol indicator is
a highly visible colorimetric displacement assay and sensor. The system shows significant selectivity and
a higher affinity for pyrophosphate anions over other competing anions.
Received 2 April 2013
Revised 23 April 2013
Accepted 30 April 2013
Available online xxxx
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Calix[4]pyrrole
Indicator displacement assay
Pyrophosphate
Azophenolate
Anion recognition
The design and synthesis of sensitive chemosensors for selec-
tive detection of anions1 have received considerable attention in
the chemical, biological, and environmental sciences.2 The key
components in an effective chemosensor are the recognition do-
main (binding site) and the signaling unit (indicator).3 In most
molecularly constructed sensors, the receptor site and signaling
unit are covalently linked to facilitate observable changes associ-
ated with the binding interaction. Anslyn and co-workers4 have
developed the indicator-displacement assay (IDA), where sensing
of a target analyte is achieved by a binding-induced displacement
within a supramolecular receptor–indicator complex. This ap-
proach relies upon the competition between the analyte and the
indicator for a binding cavity on the receptor; the analyte must
have the higher binding affinity. It has been used for the selective
sensing of anions such as phosphate,5 pyrophosphate,6 nitrate,7
cyanide,8 and citrate.9
Zinc–dipicolylamine (Zn(II)–DPA) and Cu(II)–DPA complexes
have been employed as IDAs for the detection of pyrophosphate
anions.15,16 The metal centers become coordination spheres to
accommodate the oxoanions. Other receptors for pyrophosphate
anion include macrocyclic pyrrole, imidazolium-based macrocy-
cles, and dipyrrolyquinoxalines.17 We reported a bis-pyridinium
calix[4]pyrrole derivative for ‘turn on’ fluorescence detection of
pyrophosphate in an aqueous organic solvent18 that utilizes a
hydrogen bonding interaction and electrostatic interactions com-
bined with a fluorescent dye-displacement assay. We have also
developed a supramolecular receptor–indicator complex19 com-
posed of bis-pyridinium calix[4]pyrrole and an azo dye for selec-
tive recognition of HP2O3À over other competing anions,
7
including FÀ and AcOÀ. The recognition of FÀ in organic media
was achieved with a colorimetric IDA using an octamethyl-
calix[4]pyrrole-(p-nitrophenolate) complex by Sessler and co-
The detection of pyrophosphate anion (HP2O3À) is particularly
workers and a merocyanine dye by Machado and co-workers.21
20
7
important for the analysis of bioenergetic and metabolic pro-
cesses.10 It plays a critical role in energy storage11 and signal trans-
ductions, as well as being a structural component of teeth and
bones. Also, it is a product of ATP hydrolysis and participates in
many enzymatic reactions12 such as the adenylate cyclase-cata-
lyzed synthesis of cyclic AMP, aminoacyl tRNA synthetase-cata-
lyzed attachment of amino acids to tRNA in protein synthesis,
and DNA sequencing/replication13 catalyzed by DNA polymerase.
Here, we report on an IDA-based colorimetric detection of
HP2O3À anion using dicationic calix[4]pyrrole combined with an
7
azo dye indicator. The pyrrole was designed to allow multiple
interactions with the guest anion (hydrogen bonding, anion–
p
interactions, and coulombic interactions). The azo dye, initially
bound to the receptor, is replaced by the target analyte, resulting
in colorimetric detection (Scheme 1). The cis-5,15-(4-pyridyl)-
5,10,10,15,20,20-hexamethylcalix[4]pyrrole was prepared in mod-
erate yield by acid-catalyzed condensation of 5-(4-pyridyl)dipyr-
romethane with acetone. The hexafluorophosphate salt of
bis-pyridinium calix[4]pyrrole 1 was obtained via methylation of
cis-5,15-(4-pyridyl)-5,10,10,15,20,20-hexamethylcalix[4]pyrrole,
High levels of HP2O3À are known to cause several diseases.14
7
⇑
Corresponding authors. Tel.: +82 33 250 8490; fax: +82 33 253 7582 (C.-H.L.).
E-mail address: chhlee@kangwon.ac.kr (C.-H. Lee).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.04.129
Please cite this article in press as: Kaur, S.; et al. Tetrahedron Lett. (2013), http://dx.doi.org/10.1016/j.tetlet.2013.04.129

2
S. Kaur et al. / Tetrahedron Letters xxx (2013) xxx–xxx
NO2
NO2
N
N
H
O
N
O
OH
P
O
O
P
O
O
N+
O
N+
N+
O
N+
N+
N+
3-
HP2O7
N
O
H
N
H
N
2-
+
H
N
OH
H
N
H
N
H
N
+
H
N
H
N
H
N
H
N
N
H
N
H
3-
1
[1 2-]
2-
[1 HP2O7
]
Scheme 1. Formation of the complex [1Á2À] and the recognition of HP2O37À via displacement of dye 2À.
followed by treatment with NH4PF6. Tetrabutylammonium salt of
azo-dye 2À was obtained in good yield by treating 4-(4-nitrophe-
nylazo)resorcinol with tetrabutylammonium hydroxide. The struc-
ture of all the compounds was confirmed with spectroscopic
means (SI).
0.28
0.23
0.18
0.13
0.08
0.03
-0.02
The anionic form of indicator 2À (10
with twin absorption bands at 538 nm (
and 447 nm (
max = 49,500 l-molÀ1cmÀ1) in acetonitrile. Incremen-
tal addition of receptor 1 (0–30 M)
M) to the solution of 2À (10
l
M) has a red/pink color
e
max = 42,800 l-molÀ1cmÀ1
)
e
l
l
results in decreased absorbance at 538 nm and increased absor-
bance at 447 nm, with a 10 nm bathochromic shift, as shown in
Figure 1.
The binding of indicator 2À to receptor 1 is accompanied by a
distinctive color change from pink to yellow. Saturation occurs
when 30 lM of receptor is added (Fig. 2). The isosbestic point at
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
500 nm indicates an equilibrium complexation of receptor 1 and
indicator 2À. A Job plot also supports the 1:1 binding stoichiometry
between receptor 1 and indicator 2À.22 Analysis of the titration
−
−
X = [2 ]/{[2 ]+[1]}
Figure 2. Job plot for the binding between 1 and 2À in CH3CN.
data in Figure 1 with a non-linear least square fit (HypSpec23
)
yields an association constant Ka = (1.80 0.04)Â106 MÀ1. These
observations clearly indicate that formation of the supramolecular
receptor–indicator complex [1Á2À] is favorable and that the hydro-
gen-bonding interactions between the anionic indicator 2À and the
pyrrole N–H bonds are strong. The binding behavior of the anionic
indicator 2À and receptor 1 can be monitored by 1H NMR spectros-
copy (Fig. 3). When receptor 1 (2.28 mM) is titrated with anionic
indicator 2À in CD3CN, the signals corresponding to the ortho-pro-
tons from the anionic center are shifted up-field and the signals
from the rest of the aromatic protons become broad relative to
the spectrum of pure 2À in CD3CN.
In addition, the signal corresponding to NH protons of receptor
1 shows a considerable down-field shift from d 7.90 to d 10.85 ppm
(Fig. 3 (iii)), indicating hydrogen bonding between phenolate anion
and pyrrole N–H bonds, as well as possible
p–p
interactions.24
Only 1.0 equiv of 2À is required for complete binding.
The anion recognition properties of the complex [1Á2À] were
investigated by UV–vis absorption spectroscopy in CH3CN. When
it is titrated with the anions FÀ, CNÀ, AcOÀ, and ClÀ (as their tetra-
butyl ammonium salt, 0–17.5 lM), only small increases in absor-
bance at 538 nm are observed (Fig. 4). However, a significant
change in absorbance, as well as in the visual color, is observed
upon titration with pyrophosphate anion (HP2O3À). Thus the affin-
7
ity of pyrophosphate anion toward receptor 1 is strong and is capa-
ble of complete replacement of the inÀdicator to form the new
complex [1ÁHP2O3À]. In contrast, H2PO , BrÀ, and IÀ anions do
7
4
not produce appreciable changes in the absorption spectra.
However, the initial [1Á2À] absorption spectrum is shifted to a
broad absorption band at 400–430 nm upon titration with HSO4À
(17.5 lM). This spectral change is associated with the ionization
of monobasic HSOÀ4 to dibasic SOÀ4 by indicator anion 2À, which
is sufficiently basic to deprotonate HSOÀ4 . Formation of the result-
ing azophenol derivative 2 can be confirmed by comparing the
Figure 1. Changes in the absorption spectrum of 1.2À (10
pyrophosphate ion (0–30
M) in CH3CN. The inset displays the color of the [1.2À]
and the complex [1H2P2O73À].
lM) upon titration with
l
absorption spectra with 2À. The detection limit25 for HP2O3À is
7
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S. Kaur et al. / Tetrahedron Letters xxx (2013) xxx–xxx
3
Figure 3. Partial 1H NMR spectra of solutions containing (a) 1 only (2.28 mM), (b) 2Àonly (2.28 mM) (c) 1+2À (1.0 equiv), (d) [(c) + HP3O3À (1.5 equiv)], and (e) 1 + HP2O3À (1.5
7
7
equiv).
Figure 4. Concentration-dependent absorbance changes at 538 nm with increasing concentrations of various anions at a fixed concentration of [1Á2À] in CH3CN. [1]/
[2À] = (30
lM)/(10 lM).
5.6
l
M, which is much lower than the intracellular concentration
displacement of azophenolate indicator by pyrophosphate anion
was observed from the shift of the [1Á2À] N–H proton signal from
d 10.85 to d 11.85 ppm, which correlates with that of the recep-
tor–pyrophosphate complex. These results clearly confirm IDA-
based sensing of pyrophosphate anion. The preferable binding of
pyrophosphate over fluoride anion was also verified with 1H
of pyrophosphate (ꢀ50
l
M).26 Thus, the sensing complex could
be employed in biological systems.
We have previously demonstrated a highly selective fluorescent
dye replacement assay sensor for pyrophosphate anion based on a
coumarin-calix[4]pyrrole supramolecular complex.18 Now we
show below that the calix[4]pyrrole-based, chromogenic indicator
displacement assay for the pyrophosphate anion can selectively
recognize anionic analytes as well.
NMR by mixing equimolar amounts of HP2O3À and FÀ to the recep-
7
tor 1 in CD3CN; exclusive formation of the receptor–pyrophos-
phate complex was retained in the presence of FÀ. In summary,
these results indicate that [1Á2À] selectively detects pyrophosphate
anion in the presence of common interfering anions such as fluo-
ride or acetate anion.
The higher affinity for pyrophosphate anion over other anions
may be attributed to several factors: (1) the presence of appropri-
ately spaced cationic charges; (2) the matching size of the binding
In order to assess the selectivity of the complex [1Á2À] for pyro-
phosphate anion, a solution of complex [1Á2À] containing an equi-
molar concentration of a competing anion such as FÀ or AcOÀ was
titrated with HP2O3À (0–12.5
lM) in CH3CN. Constant absorbance
7
throughout the titration indicated no significant competition from
the other anions. The sensing performance of the complex [1Á2À]
was also examined with 1H NMR spectroscopy. When the solution
of the complex [1Á2À] was titrated with 1.5 equiv pyrophosphate
anion (as its tetrabutyl ammonium salt) in CD3CN, exclusive
domain; and (3) anion–p interactions and the presence of multiple
hydrogen-bonding donors. In an ideal IDA system, the binding of
the complex formed by the target analyte with the receptor must
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4
S. Kaur et al. / Tetrahedron Letters xxx (2013) xxx–xxx
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be larger than that of the complex of the receptor and indicator. To
be more quantitative, the binding constants were calculated from
the titration data obtained from UV–vis and 1H NMR spectroscopy
using HypSpec.23 All the titration data were fit with a 1:1 binding
stoichiometry. The affinity constant for the direct formation of
[1Á2À] (Ka = (1.80 0.04) Â 106 MÀ1) is 1/13.6 compared with the
formation of [1ÁHP2O3À] (Ka = 2.45 0.13) Â 107 MÀ1); thus there
7
is a significant preference for [1ÁHP2O3À]. The affinity constant for
7
the formation of [1ÁHP2O3À
]
from [1Á2À] by displacement
7
(Ka = (7.7 0.11) Â 106 MÀ1) is less than the value for direct bind-
ing. This difference is due to pre-dissociation of [1Á2À] to 1 and 2
followed by complexation of 1 with HP2O3À. The dissociation con-
7
stant of the complex [1Á2À] is 3.18 Â 106 MÀ1, as calculated by the
equilibrium equation.
In conclusion, we have developed a sensitive calix[4]pyrrole-
based indicator displacement assay chemosensor displaying signif-
icant selectivity for pyrophosphate anion. The dicationic
calix[4]pyrrole, armed with bis-pyridinium groups, forms a strong
supramolecular receptor–indicator complex with azophenol dye.
The system is
a highly visible colorimetric IDA sensor for
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Acknowledgments
This work was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF) funded
by the Ministry of Education, Science and Technology (No. 2009-
0087013) to CHL and is partially supported by Basic Science Re-
search Program through the National Research Foundation of Kor-
ea (NRF) funded by the Ministry of Education, Science and
Technology (No. 2012-013930) to JTL.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.04.
129.
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25. Detection limit (DL) is given as DL = (0.03 Â RSDB)/(
vA/co), where RSDB
(relative standard deviation of the back ground expressed as a percentile) is
the sensitivity (the slope of the calibration curve of intensity versus
composition), vA is the net analyte signal (i.e., signal above background) and
co is the composition of the element in the sample.
´
´
´
~
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Please cite this article in press as: Kaur, S.; et al. Tetrahedron Lett. (2013), http://dx.doi.org/10.1016/j.tetlet.2013.04.129