A new boronic acid based fluorescent reporter for catechol

Article abstract of DOI:10.1016/j.bmcl.2012.09.060

Catechol skeleton widely exists in natural products and bioactive substances. Fluorescent reporters which could recognize catechol are very promising for the construction of chemosensors to detect catechol and its derivatives in biological environment. Herein, we reported a novel catechol reporter, 2-(4-boronophenyl)quinoline-4-carboxylic acid, which exhibits significant fluorescent property changes upon binding catechol containing molecules in an aqueous solution.

Full text of DOI:10.1016/j.bmcl.2012.09.060

Bioorganic & Medicinal Chemistry Letters 22 (2012) 7179–7182
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
A new boronic acid based fluorescent reporter for catechol
Zhongyu Wu a, Minyong Li a, Hao Fang a, , Binghe Wang b
⇑
a Department of Medicinal Chemistry, Key Laboratory of Chemical Biology (Ministry of Education), School of Pharmacy, Shandong University,44 West Wenhua Rd., Ji’nan
250012, China
b Department of Chemistry, Gerogia State University, Atlanta 30303, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Catechol skeleton widely exists in natural products and bioactive substances. Fluorescent reporters
which could recognize catechol are very promising for the construction of chemosensors to detect
catechol and its derivatives in biological environment. Herein, we reported a novel catechol reporter,
2-(4-boronophenyl)quinoline-4-carboxylic acid, which exhibits significant fluorescent property changes
upon binding catechol containing molecules in an aqueous solution.
Received 25 April 2012
Revised 22 August 2012
Accepted 18 September 2012
Available online 27 September 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Boronic acid
Catechol detection
Chemosensor
Fluorescent reporter
Catechol structures exist in a variety of natural substances such
as urushiols, gossypol, catechin, as well as hormones/neurotrans-
mitters (including dopamine and adrenaline).1–4 Many of these
molecules are involved in physiological and pathological processes,
including inhibiting the release of prolactin from the anterior lobe
of the pituitary, acting on the sympathetic nervous system, produc-
ing effects such as increased heart rate and blood pressure and
therapies of Parkinson’s disease and Alzheimer’ disease. Moreover,
considering their anti-microbial, anti-oxidative and anti-
inflammatory activities,5–7 many efforts have been conducted to
establish a sensitive, rapid and convenient method to detect the
catechol structure moiety.8–23 Common methods of determination
of catechols include spectrophotometry, chemiluminescence and
gas chromatography-mass spectrometry (GC–MS). Chemosensors
have also been developed but their selective detection on catechol-
amine is very limited.8–23 Therefore, looking for a simple, effective
and reliable tool for detecting catechols is pretty meaningful.
As a Lewis acid, boronic acid can bind with 1,2- or 1,3-diols in
aqueous a solution reversibly covalently to form five or six cyclic
ester.24 In general, the generation of the boronic acid ester can lead
(A)
(B)
5000
0 mM
4000
3000
1 mM
2000
1000
0
360 380 400 420 440 460 480 500
Wavelength (nm)
Figure 1. Fluorescent spectral changes of compound 1 (3 Â 10À5 M) upon addition of various concentration of catechol in 0.1 M phosphate buffer at pH 7.4, kex = 337 nm,
kem = 382 nm. (A) Fluorescence spectra of 1 upon addition of catechol; (B) Relative fluorescent intensity changes versus concentration of catechol.
⇑
Corresponding author.
E-mail address: haofangcn@sdu.edu.cn (H. Fang).
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2012.09.060

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Z. Wu et al. / Bioorg. Med. Chem. Lett. 22 (2012) 7179–7182
OH
O
O
OH
HO
HO
N
+
N
O
B
O
OH
B
1
PET
OH
Scheme 1. The PET quenching mechanism of compound 1 after adding catechol.
Table 1
Apparent association constants (Ka) of the complexes of various catechol derivatives
and compound 1 (3 Â 10À5 M in 0.1 M phosphate buffer, pH 7.4)
Target
Ka (MÀ1
)
Catechol
Dopamine
Pyrogallol
4-Nitrocatechol
3-Methoxycatechol
1910 193
2338 87
2186 262
3655 183
780 96
to the significant fluorescence change. Based on this phenomenon,
boronic acids have been well developed as sensors to recognize the
carbohydrates.
Figure 2. Relative fluorescense changes versus concentration of various targents
(catechol, hydroquinone, resorcinol and phenol) in phosphate buffer (0.1 M, pH 7.4)
added to 1 (3 Â 10À5 M), kex = 337 nm, kem = 382 nm.
Recently, the Wang lab reported 2-(4-boronophenyl)quinoline-
4-carboxylic acid (Compound 1) as a fluorescent reporter for
carbohydrates, which demonstrates large intensity changes upon
binding with carbohydrates in aqueous solution at physiological
pH.25 For example, the fluorescence intensity increased 25-fold
after addition of 0.025 M of fructose. In our recent studies, we
came across that this molecule exhibits opposite appearance after
addition of catechol, which diminished fluorescence intensity sig-
nificantly with the Ka value about 1910 MÀ1 (Fig. 1). The possible
reason may be due to photoinduced electron transfer (PET) from
the catechol into the excited state of quinoline in compound 1
(Scheme 1). In the case of carbohydrate diols, the inexistence of
such a PET quenching leads to an increasing fluorescence intensity.
Similar results have been obtained by Yoon and Czarnik.26This dif-
ferent fluorescence changing properties not only provide a novel
specific fluorescent reporter for catechol derivatives, but also avoid
the identification interference with the carbohydrate. Furthermore,
it could also be helpful to construct brand new selective chemosen-
sors for the catechol derivatives.
In further study, various phenol derivatives, such as phenol, res-
orcinol and hydroquinone, were used to test the fluorescence pro-
file after addition of compound 1 (Fig. 2). The result clearly reveals
that fluorescence intensity did not alter after interaction with these
mono-phenol derivatives, which propose that the additional hydro-
xyl group in catechol scaffold may lead to the fluorescence quench
of 1. In order to determine the role of boronic acid group in this
fluorescent system, compound 1 was converted to its phenol deriv-
atives 2 through the reaction with hydrogen peroxide27 (Scheme 2).
In result, the fluorescence intensity of compound 2 did not provide
fluorescent property change after addition of catechol (Fig. 3).
O
OH
O
OH
H2O2
N
N
OH
OH
B
1
2
OH
Scheme 2. Compound 1 was treated with hydrogen peroxide to remove the boronic
acid group.
900
800
700
600
500
400
300
200
100
380
400
420
440
460
480
500
Wavelength (nm)
Figure 3. Fluorescence spectra of compound 2 upon addition of various concen-
tration (0.1 ꢀ 1 mM) of catechol in 0.1 M phosphate buffer at pH 7.4, kex = 337 nm.
H
N
O
O
OH
COOCH3
1) SOCl2,reflux
2) Glycine methyl ester hydrochloride
N
N
OH
B
OH
B
OH
3
OH
1
Scheme 3. Derivatization of compound 1 by forming amide.

Z. Wu et al. / Bioorg. Med. Chem. Lett. 22 (2012) 7179–7182
7181
160
140
120
100
80
(A)
(B)
0 mM
1 mM
60
rabbit plasma
40
20
0
380 400 420 440 460 480 500
Wavelength (nm)
Figure 4. Fluorescent spectral changes of compound 1 (3 Â 10À5 M) upon addition of various concentration of catechol in rabbit plasma, kex = 337 nm, kem = 401 nm.
(A) Fluorescence spectra of 1 upon addition of catechol; (B) Relative fluorescent intensity changes versus concentration of catechol. All experiments were duplicated.
The detection limit of compound 1 in rabbit plasma was
resolved as the concentration of catechol, which resulted in a
statistically significant decreasing in fluorescence intensity with a
p-value <0.01 when compared with a blank control (Fig. 5).
In summary, compound 1 was found to be a new selective fluo-
rescent reporter for catechol and its derivatives under physiologi-
cal conditions compared with the sugars and some phenol
derivatives. Further structural modifications on carboxyl group
based on such chemical scaffold will be helpful to develop new
selective chemosensors for bioactive catechol derivatives.
Acknowledgments
This work was supported by National Natural Science Founda-
tion of China (Grant No. 20602023 and No. 21172133), Natural Sci-
ence Foundation for Young Scholars of Shandong Province
(2006BS03021) and Scientific Research Foundation for the Re-
Figure 5. Fluorescence responses of 3 Â 10À5 M compound 1 to 0, 50, and 100
lM
catechol in rabbit plasma (kex = 337 nm, kem = 401 nm). Statistical analyses were
turned Overseas Chinese Scholars, State Education Ministry.
performed with
deviation.
a two-tailed Student’s t-test (n = 3). Error bars are standard
Supplementary data
In addition, we determined whether the carboxyl group can
influence on the fluorescence intensity change and converted the
carboxyl group to amide derivatives 3 (Scheme 3).28 The interest-
ing result suggests that the binding profile of compound 3 is sim-
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2012.
09.060. These data include MOL files and InChiKeys of the most
important compounds described in this article.
ilar to that of compound
1 when interacts with catechol
(Ka = 2092 MÀ1), which indicates the carboxyl group of 1 has no ef-
fect on its fluorescence properties. According to these outputs, car-
boxylic acid could be facilitated as the functional group for further
modification. Therefore, compound 1 could be used as not only cat-
echol fluorescent reporter, but also as the building block to con-
struct the selective chemosensors for biological substance
containing the catechol scaffold. For example, dopamine and pyro-
gallol have been well examined by using compound 1 as fluores-
cent sensor (Table 1). The binding studies propose that
compound 1 demonstrated similar apparent association constants
with dopamine and pyrogallol. Other catechol compounds, such as
4-nitrocatechol and 3-methoxycatechol, have also been tested
(Table 1) and the results suggest that electronic effect of the substi-
tuent in catechol has a significant impact on the binding abilities to
compound 1.
In order to expand the implementation of this probe, we deter-
mined if such a probe could detect the catechol quantity in rabbit
plasma using a micro-plate reader. The results substantiate that
the fluorescence intensity formed a linear relationship to the cate-
chol concentration in the range of 0.2 ꢀ 1 mM (Ka = 1132 MÀ1). In
the rabbit plasma, the fluorescence emission wavelength of com-
pound 1 was red shifted to 401 nm and thus the fluorescence inter-
ference of rabbit plasma is subtle (Fig. 4).
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compound 2 (92 mg, 65%) as a yellow powder. The specture data are consistent
with that reported in literature(Bull. Korean Chem. Soc. 2003, 24, 677–680).
mp = 304–306 °C(lit. mp P300 °C). 1H NMR (600 MHz, DMSO-d6): d 9.94 (br s,
1H), 8.61 (d, 1H, J = 7.80), 8.36 (s, 1H), 8.16 (d, 2H, J = 7.20), 8.09 (d, 1H,
J = 8.40), 7.81 (m, 1H), 7.65 (m, 1H), 6.95 (m, 2H). 13C NMR (150 MHz, DMSO-
d6): d 167.40, 159.94, 155.67, 148.46, 136.69, 129.45, 129.32, 128.88, 128.41,
126.50, 125.27, 123.06, 118.62, 115.59, 115.54. MS: m/z (%) = 266(M+1100).
28. The synthesis and characterization of compound 3 are as follows. Compound 1
(0.29 g, 1 mmol) was refluxed in thionyl chloride (15 mL) for 2 h and the
thionyl chloride was removed in vichuo to give the acyl chloride. Then using
triethylamine (0.61 g, 6 mmol) as the acid neutralizer, the acyl chloride reacted
with glycine methyl ester chloride (0.38 g, 3 mmol) in anhydrous methylene
dichloride to yield the amide 3 (0.26 g). The product was purified by column
chromatography. Yield: 71%. 1H NMR (600 Mz, CD3OD) d 8.31–8.33 (m, 1H),
8.12–8.20 (m, 4H), 7.81–7.84 (m, 3H), 7.63–7.66 (m, 1H), 4.25 (s, 2H), 3.83 (s,
3H); 13C NMR (150 Mz, DMSO-d6) d 52, 52.42, 117.11, 123.84, 125.83, 126.59,
127.70, 129.99, 130.76, 135.14, 139.87, 142.99, 148.39, 156.22, 167.82, 170.57;
MS (ESI) m/z(%) 365 (M+1, 100).
27. Compound
2 could be prepared with following procedure. Compound 1
(150 mg, 0.51 mmol) was dissolved in 50 mL methanol and 30% H2O2 solution
(0.3 mL, 3.0 mmol) was added. The mixed solution was stirred for 1 h at room
temperature. Removed the solvent in vacuo to give brown solid. The crude
product was washed with cold ethyl ester and ethanol and dried to give