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FLUORESCENT SENSOR FOR SACCHARIDES BASED ON N-PHENYLBORONIC
ACID-1,8-NAPHTHALIMIDE
Suchada Nawongsri, Boosayarat Tomapatanaget*
Supramolecular Chemistry Research Unit, Department of Chemistry, Faculty of Science,
ChulahongKorn University, Bangkok 10330, Thailand
*e-mail: [email protected]
Abstract: Compound NBM containing napthalamide and boronic acid was synthesized as a
fluorescence probe for sugar sensing. The sensor was synthesized from dehydration reaction
between 4-bromo-1,8-naphthalenedicarboxylic acid anhydride and 3-aminophenyl boronic
acid to afford compound 1 and then the methoxy substitution of compound 1 gave compound
NBM in 40 %yield. NBM was characterized by 1H-NMR spectroscopy. The binding
affinities between NBM and sugar were investigated by fluorescence spectrophotometry. The
preliminary result showed the binding ability of NBM with diol group of sugar at the boronic
acid based sensor in forms of five- or six-member cyclic ester based sensor which exhibited
the remarkably fluorescence enhancement.
Introduction: Saccharides are biologically significant molecule which is involved in many
cellular processes. The multifunction of them is directly related to pathological processes,
cancer1, immune responses2, and viral3. Therefore, the identification and detection of the
sugar in bloodstream is growing importance for diagnosis of human diseases. Recent years, a
wide variety of sensors for biologically important sugar such as glucose, ribose and fructose
has been reported as colorimetric4, optical rotation5, near infrared spectroscopy6,
electrochemistry7 and fluorescence detection8. In the design and synthesis of saccharide
sensor, the most common uses of boronic acid group interacted with 1,2- or 1,3-diols moiety
in disaccharides are to form five- or six-membered cyclic boronic ester, respectively9.
However, the previous fluorescence chemosensors illustrated the selective probe for fructose
offered the highly
and glucose in aqueous solution10-11. Since 1,8-naphthalamide
fluorescence and photostable molecule, many researchers paid attention on the modification
of 1,8-naphthalamide derivatives for sensing application. Heagy and coworkers12 studied the
photophysical properties of the 1,8-naphthalenedicarboximides. They found that the methoxy
substitution on 4′ position of 1,8-naphthalamide containing N-phenylboronic acid displays
much higher quantum yield in 0.407 than those unsubstituted derivatives (φF = 0.010).
Additionally, methoxy substituted show an excellent solubility in water. Thus, in the present
work, we have focused on a synthesized fluorescent sensor containing the boronic acid as a
binding site for disaccharides and naphtalimide as a fluorophore. The binding abilities of the
synthesized molecule NBM with saccharides have been investigated by fluorescence
spectrophotometry revealing that fructose induced the significant fluorescence change of
NBM over other saccharides. However, both monosaccharides and disaccharides affected on
the fluorescence enhancement of NBM in buffer solution at pH 8.5. Interestingly, NBM
allowed the different recognition patterns for various saccharides observing by the different
fluorescence changes.
Methodology: All chemicals were purchased from Aldrich, Fluka, Merck, and Lab Scan and
used for the systhesis of boronic compound and complexation studies. Nuclear magnetic
resonance (NMR) spectra were recorded on Varian 400 MHz spectrometer. Fluorescence
spectra were measured by a Varian eclipse spectrofluorometer with xenon arc lamp. UV-Vis
measurements were carried out on a Varian Cary 50 UV-Vis spectrofluorometer.
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Experimental Section
Figure 1. Synthesis partway of compound NBM
N-(4′′-bromo-1′′,8′′-naphthaloyl)-3-aminophenyl boronic acid (1)12: Into a 50 ml
two-neck bottom flask equipped with a Dean-Stark apparatus , 3-amino phenyl boronic acid
(1.63 mmol), 4-bromo-1,8-naphthalenedicarboxylic acid anhydride (1.19 mmol) and zinc
acetate (0.046 mmol) in 15 ml pyridine were refluxed for 12 h. After removal of the solvent,
the resulting residue was purified by column chromatography with silica gel using acetone
and dichloromethane (1:4) to give a yellow solid of compound 1 in 50% yield: 1H NMR
(DMSO) δ 8.59 (t, J = 8.8 Hz, 2H), 8.30 (dd, J = 34.8, 34.8 Hz, 1H), 8.17 (s, 2H), 8.03 (t, J =
8.0 Hz, 1H), 7.87 (d, J = 7.6 Hz, 1H), 7.69 (s, 1H), 7.49 (t, J = 7.6 Hz, 1H), 7.41 (d, J = 8.0
Hz, 1H).
N-(4′′-Methoxy-1′′,8′′-naphthaloyl)-3-aminophenyl boronic acid The solution of
compound 1 (0.25 mmol) and CuSO4.5H2O (0.04 mmol) in 10 ml methanol was added to the
sodium methoxide solution. The reaction mixture was stirred at room temperature under N2
atmosphere for 18 h. The solvent was removed by rotary evaporator and the yellow crude was
purified by 20% ethyl acetate and dichloromethane to afford a green powder of NBM in 40%
yield : 1H NMR (CDCl3) δ 8.54 (q, J = 8.0 Hz, 3H), 7.66 (t, J = 8.0 Hz, 1H), 7.48 (t, J = 7.6
Hz, 2H), 7.23 (d, J = 7.6 Hz, 2H), 7.36 (d, J = 8.0 Hz, 1H), 4.08 (s,3H).
Results and Discussion:
Compound 1 was prepared in a single-step reaction of 4-bromo-1,8-naphthalenedicarboxylic acid anhydride and 3-aminophenyl boronic acid, 1H-NMR spectrum of compound
1 showed the additional aromatic protons belonging to boronic acid group. NBM was
achieved by the introduction of methoxy substitution of compound 1 to give the desired
product in 40% yield. The characteristic protons of NBM showed the methoxy protons at
4.08 ppm indication that the bromo group was replaced by methoxy group.
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Scheme 1. Analytes used in this study.
Scheme 2. The sample mechanism of boronic acid complexation with diol molecule13.
Notably, 1,2- or 1,3-diols of disaccharides are capable of forming five- or six-member
cyclic boronic ester with boronic acid moiety, respectively (Scheme 2)13. The binding
properties of NBM toward disaccharides including galactose, ribose, lactose, maltose,
fructose, glucose and sucrose were investigated in borate buffer at pH 8.5. The fluorescence
spectrum of NBM showed a weak fluorescence intensity at 467 nm upon excitation at 378
nm. As shown in Figure 1, sensor NBM displayed an increase of fluorescence intensity upon
the addition of various disaccharides.
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Figure 1. Fluorescence spectral changes of sensor NBM (1.0x10-6M) upon the addition of
various saccharides in borate buffer (0.1M) at pH 8.5.
The fluorescence responses in figure 1 suggested that sensor NBM preferentially
bound to diol of monosaccharides to form the five-membered cyclic boronic ester of fructose
and ribose over those of monosaccharide forming six-membered cyclic boronic ester.
Moreover, the tendency of fluorescence changes by addition various saccharide is in the order
of fructose > ribose > glucose > galactose ~ maltose ~ sucrose ~lactose which are the
disaccharide. From the preliminary results, NBM can be utilized for saccharide sensing.
However, the selectivity of sensor have been a subject of debate. Therefore, the
discrimination of various saccharide is extremely challenging. The use of NBM in sensor
array applied with principal component analysis (PCA) for differentiating various saccharide
is further developed.
Conclusion: In summary, NBM have been successfully synthesized as a fluorescence sensor
for saccharide sensing. The sensor contains napthalamide and boronic acid as a sensory and
receptor unit, respectively. The sensor binds to 1,2- or 1,3-diols based saccharides by forming
a five- or six-member cyclic boronic ester, respectively. From fluorescence spectra,
compound NBM showed an increase of emission band at 467 nm upon the addition of
saccharides. However, the spectral changes cannot clearly identify the type of saccharides.
Therefore, the efforts to increase the selectivity of the sensor are highlight in our works. We
are also aware of the need for a sensing to be able to detect saccharides under biological
conditions. Currently, we are working on these aspects.
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Acknowledgements: We gratefully acknowledge the Thailand Research Fund, Commission
on Higher Education and Chulalongkorn University (RMU5380003 and RTA5300083) and
the Higher Education Research Promotion and National Research University Project of
Thailand, Office of the Higher Education Commission and the Ratchadaphisaksomphot
Endowment Fund (AM1009I-55).
Keywords: boronic acid, 1,8-napthalic, fluorescent sensor, glucose, fructose
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