Highly efficient fluorescent sensing for α-hydroxy acids with C3-symmetric boronic acid-based receptors

Article abstract of DOI:10.1016/j.tetlet.2008.05.139

Two boronic acid-based fluorescent chemosensors in C₃ symmetry have been prepared with a facial method. These compounds show remarkable ability to recognize α-hydroxycarboxy acids and sugar acids over most saccharides. The fluorescence intensit

Full text of DOI:10.1016/j.tetlet.2008.05.139

Tetrahedron Letters 49 (2008) 4918–4921
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Tetrahedron Letters
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Highly efficient fluorescent sensing for a-hydroxy acids with C3-symmetric
boronic acid-based receptors
*
Wen-Zhi Xu, Zhi-Tang Huang, Qi-Yu Zheng
Beijing National Laboratory for Molecular Sciences, Research Center for Chemical Biology, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two boronic acid-based fluorescent chemosensors in C3 symmetry have been prepared with a facial
Received 11 March 2008
Revised 16 May 2008
Accepted 30 May 2008
Available online 4 June 2008
method. These compounds show remarkable ability to recognize
over most saccharides. The fluorescence intensity of the receptors decreased obviously upon adding the
-hydroxy acids in a pH 8.71 buffer of methanol–water, which can be explained with the internal charge-
transfer (ICT) mechanism.
a-hydroxycarboxy acids and sugar acids
a
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Boronic acids
Fluorescence
a
-Hydroxy acids
Sugar acids
Molecular recognition
The molecular recognition for the saccharides has attracted
much attention due to its potential application in biomedicine
chemistry. Different from the hydrogen-bonding-interactions-
based recognition between saccharides and their natural receptors
(proteins) in the body, the main artificial hosts for saccharides’ rec-
ognizing and sensing are boronic acid-based small molecules,
which could react with 1,2- or 1,3-diols reversibly to form stable
five- or six-membered cyclic esters.1 In addition to cis-diols, boro-
PET system, most of ICT systems are due to the change of electron
density of anilinic nitrogen. James and co-workers have reported
several ICT-type sensors for saccharides, which showed good selec-
tivity on recognition.6 However, there are still very few efficient
fluorescent ICT systems for a-hydroxy acids or sugar acids. Herein,
we describe the synthesis and fluorescent spectral studies of two
new receptors 3a and 3b, which are C3-symmetric with three bind-
ing sites. The experiment results have demonstrated that 3a and 3b
nic acid can also react with
a
-hydroxy acids,2 which are present in
are efficient sensors for a-hydroxy acids.
the nature with large scale. The detection and quantification of tar-
trate and malic acid could be used in the beverage industry. The
lactate in the body is the unique marker indicative of the capability
Starting from the commercial available materials, o- or m-nitro-
acetophenone, the compounds 3a and 3b were synthesized with a
simple method, respectively7 (Scheme 1). The condensation prod-
ucts 2a/b reacted with 2-formylphenylboronic acid to form Schiff
bases, which were reduced with NaBH4 to afford the final products.
The total yields for 3a/b are 19% and 23%, respectively.8
Similar as what was reported in the previous publications, the
fluorescent intensity of boronic acid 3a/b was strongly pH-
depended, which was mainly due to different boron ionization
states1 (Fig. 1). For example, 3a displayed minimum fluorescent
intensity at pH 8 and maximum changes with tartaric acid at pH
4.0 and 9.0. In this work, the fluorescence titration of 3a/b
with all guests was carried out in a pH 8.71 buffer (68.1 wt % meth-
anol in water with KCl, 0.01000 mol dmÀ3; KH2PO4, 0.002869
mol dmÀ3; Na2HPO4, 0.002869 mol dmÀ3).9
of the muscle for athletic performance. D-Galacturonic acid is a
component of plant gums and bacterial cell walls, and plays an
equivalent role to the core oligosaccharide phosphate residues in
establishing outer membrane integrity in Escherichia coli and Sal-
monella.3 Therefore, the study of molecular recognition of
a-hydro-
xy acids and sugar acids is important.
Fluorescence technique has high sensitivity, and boronic acid-
based fluorescence molecules have been heavily investigated for
sensing saccharides.4 There are two main sensing mechanisms:
the photo induced electron-transfer (PET) and the internal
charge-transfer (ICT). Recently, the ground-state charge-transfer
complex was also employed in the fluorescence sensing system.5
Compared with the boronic acid-tertiary amine interaction in
To our surprise neither 3a nor 3b showed obvious fluorescent
changes for most saccharides under our conditions although some
similar sensing molecules with ICT mechanism have shown good
fluorescent responses in the presence of monosaccharides.6 We
* Corresponding author. Tel.: +86 10 62652811; fax: +86 10 62554449.
E-mail address: zhengqy@iccas.ac.cn (Q.-Y. Zheng).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.05.139

W.-Z. Xu et al. / Tetrahedron Letters 49 (2008) 4918–4921
4919
HN
NH2
B(OH)2
NH
O
iii) 2-formylphenylboronic acid,
MgSO4, EtOH, reflux, 24 h,
i) CF3SO3H, 140 oC, 3 h,
ii) NH2NH2, FeCl3/C
B(OH)2
iv) NaBH4, CH3OH, r.t.
H2N
N
H
reflux, 24 h,
NO2
NH2
(HO)2B
1
2
3
a: para-
b: meta-
Scheme 1.
magnitude at 358 nm (kex = 257 nm). Different fluorescent changes
of 3a and 3b may be due to their different conjugated states or con-
formation properties. Here, we used the buffer to control pH value
of the solution during the titration. Moreover, adding the same or
even larger amount of acetic acid upon the same conditions did not
induce the fluorescent changes of 3a/b. So this fluorescent sensing
procedure did not come from the change of the pH of the solution
but by forming new complexes.
The fluorescent sensors 3a and 3b not only remarkably differen-
tiate
tivity of sensing different common
displays the fluorescent intensity of 3a and 3b upon addition of dif-
ferent -hydroxy acids with the same concentration. The order of
a-hydroxy acids from saccharides, but also show good selec-
a-hydroxy acids. Figure 3
a
their selectivity was the same: tartaric acid > malic acid > lactic
acid > galacturonic acid. From the Stern–Volmer plot for fluores-
cent quenching in a special concentration range,10 we deduced that
this plot fitted approximately the 1:1 stoichiometry of the complex
Figure 1. pH–fluorescence profiles of 3a: (N free; j in the presence of tartaric acid;
CH3OH:H2O = 52:48 wt/wt, using aqueous HCl and NaOH to control pH).
and calculated the binding constants between the a-hydroxy acids
and 3a/b (Table 1). We find that there is an obvious cooperation
between the three boronic acid moieties during binding. Both sen-
sors 3a and 3b have the largest stability constant for binding tar-
taric acid. The selectivity may be attributed to the fact that
have tried
-mannose,
maltose and
D
-arabinose,
D
-xylose,
-mannitol, methyl-
-lactose. Even adding large excess amount of these
D
-ribose,
D
-glucose,
D-galactose,
D
D
-fructose,
D
D
-glucopyranoside,
a
substrates, there are a few changes in the fluorescent density.
However, the fluorescence of 3a/b showed different properties on
tartaric acid containing two
bidentate complex with the sensors. The malic acid, lactic acid
and galacturonic acid have only one -hydroxy carboxyl, and their
a-hydroxy carboxyls could form a
addition of
a-hydroxy acids or sugar acids (Fig. 2). In the fluores-
a
cent spectra of 3a, maximum emission wavelength was observed
at 460 nm (kex = 297 nm), and the intensity was stepwise
decreased following the addition of malic acid. At the same time,
the maximum emission wavelength has a little red-shift. The fluo-
rescent spectra of 3b were little different with 3a. Upon adding
interaction with the receptor is weaker. Furthermore, the binding
difference between 3a and 3b with same substrates was due to
the distance and/or angle between two boronic acids located at
two phenyl branches. So the precise placement of the recognition
elements of the receptor in the proper position and orientation
for optimal complementarity to the guest are very important to
design sensors.
a-hydroxy acids or sugar acids, there was a significant decrease
of fluorescence intensity at 430 nm and an increase with a less
Figure 2. Fluorescence emission spectra of 3a (1.4 Â 10À2 mM) in the presence of malic acid (0, 0.93, 2.75, 3.65, 4.55, 5.43, 6.30, 7.17, 8.03, 9.72, 11.38, 13.81 mM),
kex = 297 nm, left; and fluorescence emission spectra of 3b (1.6 Â 10À2 mM) in the presence of malic acid (0, 0.74, 2.21, 2.94, 3.65, 4.37, 5.08, 5.78, 7.17, 8.54, 10.55,
12.52 mM), kex = 257 nm, right.

4920
W.-Z. Xu et al. / Tetrahedron Letters 49 (2008) 4918–4921
Figure 3. Fluorescence emission spectra of 3a and 3b in the presence of different substrates (a: free, b: D-glucose, c: D-galactose, d: galacturonic acid, e: lactic acid, f: malic
acid, g: tartaric acid) with same concentration. Left: [3a] = 1.4 Â 10À2 mM, [Substrate] = 5.0 mM, kex = 297 nm; right: [3b] = 1.6 Â 10À2 mM, [Substrate] = 5.0 mM,
kex = 257 nm.
Table 1
Acknowledgements
Stability constant K (MÀ1) for
a
-hydroxy acids of fluorescent sensor 3a and 3b, in pH
8.71 buffer at kex 297 nm (3a) and 257 nm (3b)
We thank the National Natural Science Foundation of China
(No. 20773134), the Major State Basic Research Development
Program of China (No. 2007CB8008005) and the Chinese Academy
of Sciences (KJCX2-YW-H13) for financial support.
a
-Hydroxy acids
3a
3b
Tartaric acid
Malic acid
Lactic acid
440
290
100
100
2300
700
270
270
Galacturonic acid
References and notes
1. (a) James, T. D.; Sandanayake, K. R. A. S.; Shinkai, S. Angew. Chem., Int. Ed. 1996,
35, 1910–1922; (b) James, T. D.; Shinkai, S. Top. Curr. Chem. 2002, 218, 159–200;
(c) James, T. D. In Boronic Acids: Preparation and Applications in Organic Synthesis
and Medicine; Hall, D. G., Ed.; Wiley-VCH: Weinheim, 2004; pp 441–480; (d)
James, T. D.; Philips, M. D.; Shinkai, S. Boronic Acids in Saccharide Recognition;
RSC Publishing: Cambridge, 2006.
2. (a) Friedman, S.; Pizer, R. J. Am. Chem. Soc. 1975, 97, 6059–6062; (b) Zhao, J.;
Fyles, T. M.; James, T. D. Angew. Chem., Int. Ed. 2004, 43, 3461–3464; (c) Zhao, J.
Z.; Davidson, M. G.; Mahon, M. F.; Kociok-Kohn, G.; James, T. D. J. Am. Chem. Soc.
2004, 126, 16179–16186; (d) Wiskur, S. L.; Lavigne, J. J.; Metzger, A.; Tobey, S.
L.; Lynch, V.; Anslyn, E. V. Chem. Eur. J. 2004, 10, 3792–3804; (e) Zhao, J.; James,
T. D. Chem. Commun. 2005, 1889–1891.
In this sensing system, fluorescence decrease may involve inter-
nal charge-transfer (ICT) mechanism because the amino groups are
integrated into the fluorophore in the receptors.11 The
p- electron
system of the receptors has very different dipole moments in their
ground and lowest energy singlet excited states due to the internal
charge-transfer. Upon complexing, the heteroatom will change the
interaction state with the substrate and therefore affect the dipole
moment of the fluorophore. It is known that the B–N bond can be
broken to form boronate species B on addition of saccharide. But
the stability constant for this reaction balance is low.6 In our sys-
3. Frirdich, E.; Bouwman, C.; Vinogradov, E.; Whitfield, C. J. Biol. Chem. 2005, 280,
27604–27612.
4. (a) James, T. D.; Sandanayake, K. R. A. S.; Iguchi, R.; Shinkai, S. J. Am. Chem. Soc.
1995, 117, 8982–8987; (b) Gao, X.; Zhang, Y.; Wang, B. Org. Lett. 2003, 5, 4615–
4618; (c) Wang, Z.; Zhang, D.; Zhu, D. J. Org. Chem. 2005, 70, 5729–5732; (d)
Zhang, Y.; Gao, X.; Hardcastle, K.; Wang, B. Chem. Eur. J. 2006, 12, 1377–1384.
5. Gamsey, S.; Miller, A.; Olmstead, M. M.; Beavers, C. M.; Hirayama, L. C.;
Pradhan, S.; Wessling, R. A.; Singaram, B. J. Am. Chem. Soc. 2007, 129, 1278–
1286.
6. (a) Arimori, S.; Bosch, L. I.; Ward, C. J.; James, T. D. Tetrahedron Lett. 2001, 42,
4553–4555; (b) Arimori, S.; Bosch, L. I.; Ward, C. J.; James, T. D. Tetrahedron Lett.
2002, 43, 911–913; (c) Bosch, L. I.; Mahonb, M. F.; James, T. D. Tetrahedron Lett.
2004, 45, 2859–2862.
tem, the two hydroxyl groups of
the less stable five-membered cyclic esters with boronic acid.
However, the additional carbonyl group of the -hydroxyl acid
a-hydroxyl acid could also form
a
may form a new hydrogen bond with the NH group of aniline
branch, which could play an important role for the high selectivity
of the recognition (Scheme 2).
In summary, the two new fluorescent ICT sensors 3a and 3b dis-
played large decrease of fluorescence intensity upon binding with
7. He, J.; Machida, S.; Kishi, H.; Horie, K.; Furukawa, H.; Yokota, R. J. Polym. Sci. A:
Polym. Chem. 2002, 40, 2501–2512.
a
-hydroxy acids. The different position of the amino-group at the
phenyl has a significant effect on the fluorescence properties of
these sensors. We believe that the high selectivity on recognition
for
8. Compound 3a: Yield: 19%. 1H NMR (300 MHz, DMSO-d6): d 9.48 (s, 3H), 7.89 (d,
3H), 7.80–7.84 (m, 9H), 7.69–7.74 (m, 6H), 7.45–7.48 (m, 6H), 7.34 (t, 3H), 4.62
(s, 6H); 13C NMR (75.5 MHz, DMSO-d6): 148.5, 145.2, 141.4, 132.2, 130.1, 129.7,
a-hydroxy acids over most saccharides is due to the additional
127.4, 126.3, 122.4, 117.8, 52.5; IR (KBr): m 3384, 2923, 2854, 1610, 1515, 1460;
hydrogen bonds between the carbonyl groups and NH groups.
ESI/MS (negative): m/z 754.23 ([M+1]À). Compound 3b: Yield: 23%. 1H NMR: d
OH
HO
OH
OH
OH
B
OH
OH
O
R
S
O
B
B
R
OH
O
O
S
O
N
N
NH
O
H
Ar
Ar
Ar
C
A
B
less stable
more stable
Scheme 2. Proposed reaction balance in the buffer.

W.-Z. Xu et al. / Tetrahedron Letters 49 (2008) 4918–4921
4921
9.48 (s, 3H), 8.02 (s, 3H), 7.95 (s, 3H), 7.89 (d, 3H), 7.70 (d, 3H), 7.38–
7.48 (m, 12H), 7.31 (t, 3H), 4.67 (s, 6H); 13C NMR: 149.0, 146.7, 142.9, 141.5,
135.7, 130.6, 130.1, 129.9, 126.7, 125.0, 122.9, 119.6, 117.5, 116.8, 53.0;
9. Perrin, D. D.; Dempsey, B. Buffers for pH and Metal Ion Control; Chapman and
Hall: London, 1974.
10. Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 3rd ed.; Springer: New
York, 2006.
11. De Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C.
P.; Rademacher, J. T.; Rice, T. E. Chem. Rev. 1997, 97, 1515–1566.
IR (KBr):
m 3406, 2923, 2852, 1602, 1486; ESI/MS (negative): m/z 752.41
([MÀH]À).