Modular fluorescence sensors for saccharides

Article abstract of DOI:10.1039/b105994g

Modular and modular polymer supported fluorescence photoinduced electron transfer (PET) sensors 2 and 3 with two boronic acid receptor units, a pyren-1-yl fluorophore, and hexamethylene linker show selective saccharide binding in aqueous methanolic soluti

Full text of DOI:10.1039/b105994g

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Modular fluorescence sensors for saccharides
Susumu Arimori,a Michael L. Bell,b Chan S. Oh,b Karine A. Frimata and Tony D. James*a
a Department of Chemistry, University of Bath, Bath, UK BA2 7AY. E-mail: T.D.James@Bath.ac.uk
b Beckman Coulter, Inc., Advanced Technology Center, 200 S. Kraemer Blvd., P.O. Box 8000, Brea, CA
92822-8000, USA
Received (in Cambridge, UK) 5th July 2001, Accepted 6th August 2001
First published as an Advance Article on the web 4th September 2001
Modular and modular polymer supported fluorescence
photoinduced electron transfer (PET) sensors 2 and 3 with
two boronic acid receptor units, a pyren-1-yl fluorophore,
and hexamethylene linker show selective saccharide binding
in aqueous methanolic solution at pH 8.21.
If a practically useful sensor is to be developed then the
modular approach should also allow efficient solution based
systems to be coupled to a polymer support. Polymeric boronic
acid based sensors have been previously prepared14–20 but these
Fluorescent receptors for saccharides have recently received
considerable attention. Boronic acids are known to bind
saccharides via covalent interactions in aqueous basic media.
The most common interaction is with cis-1,2- or 1,3-diols of
saccharides to form five- or six-membered rings respectively.
The interaction between boronic acids and amines has been
used to create photoinduced electron transfer (PET) sensory
system for saccharides.1–10 The interaction of a boronic acid
(Lewis acid) and neighbouring tertiary amine (Lewis base) is
strengthened on saccharide binding. The strength of this boronic
acid–tertiary amine interaction modulates the PET from the
amine to the fluorophore. These compounds show increased
fluorescence at neutral pH through suppression of the PET from
nitrogen to the fluorophore on saccharide binding, a direct result
of the stronger boron–nitrogen interaction.
Over the last few years we have been interested in developing
new fluorescence sensors selective for saccharides employing a
modular approach. The basic idea was to break a sensor into
three components; receptor units, linker units, and fluorophore
units. The approach requires the selection and synthesis of a set
of molecular building blocks from which the selective fluores-
cent sensors can be easily constructed. The quick assembly of a
diverse selection of fluorescent sensors will require that the
receptor and fluorophore units are linked to a core unit using the
minimum of synthetic linkage reactions. The use of common
reactions means the synthetic routes towards the new sensors
will be convergent. The anthracene core unit has been used for
sugars such as
D
-glucose3,7 and
D
-glucosamine hydrochlor-
ide.11,12 However, using anthracene as both the core and
fluorophore units will limit the further development of the
system. Therefore, it is important to use different core and
fluorophore units. This will result in systems where the
selectivity (core unit) and emission wavelength (fluorophore)
can be varied independently, which hitherto has not been
possible. Sensors with different emission wavelengths will
allow the simultaneous detection of several analytes.
Shinkai has demonstrated with compound 1 the value of a
hexamethylene linker (core unit) in obtaining
D
-glucose se-
lectivity.13 However, two fluorophores as with sensor 1 are not
required and may in fact be detrimental to an operational sensor
since the fluorescence spectra of sensor 1 are complicated by
excimer emission due to stacking of the two-pyrene units.
Scheme 1 Synthesis of PET sensors 2 and 3. Reagent (yield): Sensor 2. i)
Benzaldehyde, toluene-p-sulfonic acid, THF–EtOH; ii) NaBH4, THF (78%)
(2 steps); iii) pyrene-1-carbaldehyde, THF–MeOH (93%); iv) NaBH4,
MeOH (92%); v) 2-(2-bromobenzyl)-1,3,2-dioxaborinane, K2CO3, MeCN
(35%); Sensor 3.23 i) Bis(6-aminohexamethyl)amine, HOBt, DIPC,
DMAP, DMF (88%); ii) pyrene-1-carbaldehyde, THF–MeOH; iii) NaBH4
(33%); iv) 2-(2-bromobenzyl)-1,3,2-dioxaborinane, K2CO3, MeCN
(65%).
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Chem. Commun., 2001, 1836–1837
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b105994g

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have not involved the linkage of selective solution based
systems onto a polymer support.
phenomena by the preparation of systems with longer linkers to
the polymer.
Following the criteria laid out above we designed PET
sensors 2 and 3 with two boronic acid units (saccharide
selectivity), one pyrene fluorophore unit and a hexamethylene
In conclusion we have shown that it is possible to prepare
saccharide selective sensors using simple building blocks and
then attach the sensor to a polymer support. Although, the
selectivity of the system has been modulated by attachment to
the polymer support the system retains significantly enhanced
linker unit (
-glucose selectivity).13
D
Synthesis of compounds 2 and 3 was achieved according to
Scheme 1 from readily available starting materials. Reference
compound 4 was prepared as previously reported.13
selectivity for both
D
-glucose and
D
-galactose over mono-
boronic acid sensor 4. Our ongoing research is directed towards
new modular PET sensors with different linkers and fluor-
ophore units.
Fluorescence titrations of 2 (1.0 3 1027 mol dm23), 3
(pyrene concentration 1.0 3 1027 mol dm23) and 4 (1.0 3 1027
mol dm23) with different saccharides was carried out in
aqueous methanolic buffer solution [52.1 wt% methanol at pH
T. D. J. wishes to acknowledge the Royal Society, the
EPSRC, and Beckman-Coulter for support. S. A. wishes to
acknowledge Beckman-Coulter for support through the award
of a Postdoctoral Fellowship. K. A. F. wishes to acknowledge
the EPSRC for support through the award of a studentship. We
would also like to acknowledge the support of the University of
Bath.
8.21 (KCl, 0.01000 mol dm23, KH2PO4, 0.002752 mol dm23
;
Na2HPO4, 0.002757 mol dm23)].21 The fluorescence intensity
increases with increasing saccharide concentration. The stabil-
ity constants (K) of PET sensors 2, 3 and 4 were calculated by
fitting the emission wavelength at 397 nm vs. concentration
curves12,22 and are given in Table 1.
The relative stability constants of the diboronic acids 2 and 3
relative to the monoboronic acid 4 are given in Table 1. The
ratio of stability constants shows how effective the molecular
design is at enhancing the binding towards a specific sacchar-
ide.
Notes and references
1 J. H. Hartley, T. D. James and C. J. Ward, J. Chem. Soc., Perkin Trans.
1, 2000, 3155.
2 T. D. James, K. R. A. S. Sandanayake and S. Shinkai, J. Chem. Soc.,
Chem. Commun., 1994, 477.
3 T. D. James, K. R. A. S. Sandanayake, R. Iguchi and S. Shinkai, J. Am.
Chem. Soc., 1995, 117, 8982.
4 T. D. James, K. R. A. S. Sandanayake and S. Shinkai, Nature (London),
1995, 374, 345.
5 T. D. James, K. R. A. S. Sandanayake and S. Shinkai, Angew. Chem.,
Int. Ed. Engl., 1996, 35, 1911.
6 T. D. James, P. Linnane and S. Shinkai, Chem. Commun., 1996, 281.
7 T. D. James, K. R. A. S. Sandanayake and S. Shinkai, Angew. Chem.,
Int. Ed. Engl., 1994, 33, 2207.
8 R. A. Bissel, A. P. de Silva, H. Q. N. Gunaratne, P. L. M. Lynch,
G. E. M. Maguire, C. P. McCoy and K. R. A. S. Sandanayake, Top.
Curr. Chem., 1993, 168, 223.
From Table 1, in all cases, the stability constants with
diboronic acid sensors 2 and 3 are higher than for monoboronic
acid sensor 4. Cooperative binding of the two boronic acid
groups is clearly observed as illustrated by the stability constant
differences between the mono- and di-boronic acid compounds.
The stability constant K of diboronic acid sensor 2 with
glucose and -galactose are 22 and 13 times greater than with
monoboronic sensor 4. Whereas, the stability constant K of
diboronic acid sensor 2 with -fructose is only 2 times stronger
than monoboronic acid sensor 4. This result can be explained
since it is well known that -glucose readily forms 1+1 cyclic
D
-
D
D
D
9 A. P. de Silva, T. Gunnlaugsson and T. E. Rice, Analyst (London), 1996,
121, 1759.
10 A. W. Czarnik, Fluorescent Chemosensor for Ion and Molecular
Recognition, American Chemical Society Books, Washington, 1993.
11 C. R. Cooper and T. D. James, Chem. Commun., 1997, 1419.
12 C. R. Cooper and T. D. James, J. Chem. Soc., Perkin Trans. 1, 2000,
963.
complexes with di-boronic acids, whereas -fructose tends to
D
form 2+1 acyclic complexes with di-boronic acids.1–7,13
Attachment of the solution-based receptor to a polymer
support has altered the selectivity of the system towards -
D
glucose. The stability constant K of diboronic acid sensor 3 with
-glucose and -galactose are 9 and 15 times greater than with
monoboronic sensor 4. Polymer bound system 3 shows the
strongest binding with -fructose however this value is only 3
D
D
13 K. R. A. S. Sandanayake, T. D. James and S. Shinkai, Chem. Lett., 1995,
D
503.
14 T. Nagasaki, T. Kimura, S. Arimori and S. Shinkai, Chem. Lett., 1994,
1495.
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Reinhoudt, Chem. Commun., 2001, 1038.
times stronger than the monoboronic acid sensor 4. The major
difference between the polymer bound system 3 and solution
based system 2 is the
D
-glucose selectivity. The
D
-glucose
selectivity drops for compound 3 whereas the selectivity with
other saccharides is similar to those observed for compound 2.
We believe the differences are due to the proximity of receptor
to the polymer backbone. We are currently investigating these
18 S. Patterson, B. D. Smith and R. E. Taylor, Tetrahedron Lett., 1997, 38,
6323.
Table 1 Stability constant K (coefficient of determination; r2) for the
saccharide complexes of molecular sensors 2, 3 and 4
19 S. Patterson, B. D. Smith and R. E. Taylor, Tetrahedron Lett., 1998, 39,
3111.
20 W. Wang, S. Gao and B. Wang, Org. Lett., 1999, 1, 1209.
21 D. D. Perrin and B. Dempsey, Buffers for pH and Metal Ion Control,
Chapman and Hall, London, 1974.
2 K
3 K
4 K
/dm3 mol21
2/4 /dm3 mol21
3/4 /dm3 mol21
22 The K values were analysed in KaleidaGraph using nonlinear (Leven-
berg–Marquardt algorithm) curve fitting. The errors reported are the
standard errors obtained from the best fit.
D
D
D
D
-glucose 962 70 (0.99) 22
-galactose 657 39 (1.00) 13
385 26 (0.99)
778 58 (0.99) 15 51 2 (1.00)
1124 109 (0.99) 3 395 11 (1.00)
9
44 3 (1.00)
-fructose 784 44 (1.00)
-mannose 74 3 (1.00)
2
2
23 Poly(styrene-co-maleic acid) partial isopropyl ester (Mw ca. 65000) was
79 7 (0.99)
2
36 1 (1.00)
purchased from Aldrich.
Chem. Commun., 2001, 1836–1837
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