Tuning saccharide selectivity in modular fluorescent sensors

Article abstract of DOI:10.1016/S0040-4039(03)00942-0

Five modular photoinduced electron-transfer (PET) sensors bearing two phenylboronic acid receptors with different fluorophores have been prepared. The sensors' interaction with saccharides was assessed via fluorescence spectroscopy. It was shown that monosaccharide selectivity is influenced by the choice of fluorescent moiety.

Full text of DOI:10.1016/S0040-4039(03)00942-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 4789–4792
Tuning saccharide selectivity in modular fluorescent sensors
Susumu Arimori, Giuseppe A. Consiglio, Marcus D. Phillips and Tony D. James*
Department of Chemistry, University of Bath, Bath BA2 7AY, UK
Received 5 March 2003; revised 31 March 2003; accepted 10 April 2003
Abstract—Five modular photoinduced electron-transfer (PET) sensors bearing two phenylboronic acid receptors with different
fluorophores have been prepared. The sensors’ interaction with saccharides was assessed via fluorescence spectroscopy. It was
shown that monosaccharide selectivity is influenced by the choice of fluorescent moiety. © 2003 Elsevier Science Ltd. All rights
reserved.
play enhanced selectivity for
sor also contains a fluorescent pyrene ‘read-out’ unit to
report saccharide binding events.
D
-galactose.19,20 This sen-
Receptors with the capacity to selectively bind saccha-
rides and signal this event by altering their optical
signature have attracted considerable interest in recent
years.1–13 Boronic acids are known to bind saccharides
via covalent interactions in aqueous media. The most
common interaction is with cis-1,2- or 1,3-diols of
saccharides to form five- or six-membered rings respec-
tively.14 The interaction between boronic acids and
amines has been used to create photoinduced electron-
transfer (PET) sensory systems for saccharides.15–17 The
interaction of a boronic acid (Lewis acid) and neigh-
bouring tertiary amine (Lewis base) is strengthened on
saccharide binding. The strength of this boronic acid–
tertiary amine interaction modulates the PET mecha-
nism between nitrogen and the fluorophore. These
compounds show increased fluorescence through sup-
pression of PET on saccharide binding, a direct result
of the stronger boronꢀnitrogen interaction.
Having determined the effect of the linker on saccha-
ride selectivity, we set out to probe the other factors
affecting saccharide selectivity. The next logical compo-
nent to vary is the fluorophore or ‘read-out’ unit.
Although not directly involved in saccharide binding,
the nature of this unit will directly influence both the
solvation and steric crowding of the binding site. We
now report our investigations into the effect of the
fluorophore units on saccharide binding in our modular
fluorescent sensors (Table 1).
Diboronic acid PET sensors 3a–e and monoboronic
acid PET sensors 5a–e were synthesised according to
Scheme 1 from readily available starting materials. The
synthesis allows the facile preparation of diboronic acid
PET sensors appended with a range of different
fluorophores.
Over the last few years we have been involved in the
development of sensors with increased selectivity for
saccharides.18–22 We have constructed our sensors using
a modular approach. The sensors consist of three com-
ponents; receptor units, linker units, and ‘read-out’
units. The approach can be illustrated by describing the
The fluorescence titrations of 3a–e and 5a–e with differ-
ent saccharides were carried out in aqueous methanolic
buffer solution [52.1 wt% methanol at pH 8.21 (KCl,
D
-glucose selective fluorescent sensor 3a which contains
two boronic acid units (receptors), a hexamethylene
unit (linker), and a pyrene unit (fluorophore-‘read-
out’).19,20 Sensor 3a contains two boronic acid units
because only through two point binding can saccharide
selectivity be controlled. Sensor 3a with a hexa-
Table 1. Fluorescence spectra measurement conditions
Fluorophore
Concentration
uex (nm)
uem (nm)
(mol dm−3
)
1.0×10−6
5.0×10−6
1.0×10−6
2.5×10−6
2.5×10−6
342
299
370
275
274
397
369
420
335
335
methylene linker unit displays enhanced
D-glucose
a
b
c
d
e
selectivity, whilst systems with longer linker units dis-
* Corresponding author. Tel.: +44-1225-383810; fax: +44-1225-
386231; e-mail: t.d.james@bath.ac.uk
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)00942-0

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S. Arimori et al. / Tetrahedron Letters 44 (2003) 4789–4792
Scheme 1. Reagents and conditions: (i) p-Toluenesulfonic acid, THF/EtOH; (ii) NaBH4 79% (1); (iii) fluorophore-aldehyde,
THF/MeOH; (iv) NaBH4 86% (2a), 73% (2b), 37% (2c), 77% (2d), 54% (2e); (v) 2-(2-bromobenzyl)-1,3,2-dioxaborinane, K2CO3,
MeCN, 35% (3a), 53% (3b), 56% (3c), 49% (3d), 55% (3e); (vi) methylamine, THF/MeOH; (vii) NaBH4, 91% (4a), 86% (4b), 86%
(4c), 85% (4d), 90% (4e); (viii) 2-(2-bromobenzyl)-1,3,2-dioxaborinane, K2CO3, MeCN, 36% (5a), 44% (5b), 86% (5c), 39% (5d),
8% (5e).
0.01000 mol dm−3; KH2PO4, 0.002752 mol dm−3;
Na2HPO4, 0.002757 mol dm−3)].23 The fluorescence
intensity of 3a–e increased with increasing saccharide
concentration. The observed stability constants (K) of
PET sensors 3a–e and 5a–e were calculated by the
fitting of emission intensity versus saccharide concen-
tration curves and are reported in Table 2.24
It is well known that
form 1:1 cyclic complexes with diboronic acids whereas
-fructose and -mannose prefer to form 2:1 acyclic
complexes.1
D-glucose and D-galactose tend to
D
D
To help visualise the trends in the observed stability
constants from Table 2, the stability constants of the
diboronic acid sensors 3n are reported in Figure 1
relative to the stability constants of the equivalent
monoboronic acid analogues 5n. The relative stability
clearly illustrates that an increase in selectivity is
obtained by cooperative binding through the formation
of 1:1 cyclic systems. The large enhancement of the
relative stability observed for the 1:1 cyclic systems
tose. By reducing the size of the hydrophobic p-surface,
as with 3d and 3e, (appended with 1- and 2-naphtha-
lene), then the selectivity for the monosaccharides
switches from
D-glucose to D-galactose. Since, the size
of the p-surface affects the solvation of the receptor, the
larger the p-surface the more hydrophobic the receptor.
These results indicate that the best match between
receptor and guest is for 3a, 3b and 3c with
and 3d and 3e with -galactose.
D-glucose
D
(
D
-glucose,
small two-fold enhancement observed for the 2:1
acyclic systems ( -fructose, -mannose).
D-galactose) are clearly contrasted with the
Steric crowding at the receptor site must also be consid-
ered. There is little difference between the size of 1-
D
D

S. Arimori et al. / Tetrahedron Letters 44 (2003) 4789–4792
4791
Table 2. Stability constant K (coefficient of determination; r2) and fluorescence enhancement for the saccharide complexes of
molecular sensors 3 and 5
Sensor
D
-Glucose
D
-Galactose
D
-Fructose
D-Mannose
K (dm3 mol−1
)
Fluorescence K (dm3 mol−1
enhancement
)
Fluorescence K (dm3 mol−1
enhancement
)
Fluorescence K (dm3 mol−1
enhancement
)
Fluorescence
enhancement
3a
5a
3b
5b
3c
5c
3d
5d
3e
5e
962 70 (0.99)
44 3 (1.00)
325 58 (0.97)
30 7 (0.98)
441 76 (0.98)
61 3 (1.00)
417 60 (0.98)
52 1 (1.00)
2.8
4.5
1.5
1.5
3.2
3.4
6.1
5.7
4.2
4.5
657 39 (1.00)
3.1
4.2
1.4
1.4
3.1
3.0
5.4
5.0
4.1
4.3
784 44 (1.00)
3.2
3.6
1.4
1.4
3.0
3.0
5.5
5.4
3.8
4.6
74 3 (1.00)
36 1 (1.00)
134 18 (0.98)
58 8 (0.98)
111 6 (1.00)
61 3 (1.00)
101 3 (1.00)
46 1 (1.00)
98 4 (1.00)
40 2 (1.00)
2.8
3.7
1.4
1.4
2.8
3.0
5.0
5.2
3.5
3.8
51 2 (1.00)
611 101 (0.97)
77 12 (0.98)
536 31 (1.00)
93 6 (1.00)
1072 68 (0.99)
78 5 (1.00)
894 66 (0.99)
49 4 (1.00)
395 11 (1.00)
1013 126 (0.98)
548 55 (0.99)
1000 69 (0.99)
713 35 (1.00)
964 41 (1.00)
529 45 (0.99)
1068 63 (1.00)
399 34 (0.99)
532 57 (0.99)
35 2 (1.00)
In conclusion, we have shown that it is possible to
adopt a modular approach in the preparation of PET
saccharide sensors 3a–e and do so using simple building
blocks. We have also shown that the sensitivity and
selectivity displayed by fluorescent sensors is
fluorophore dependent.
We believe that these observations will need to be
considered in the construction of future fluorescent
saccharide sensors. Our ongoing research is directed
towards exploring these systems in detail and develop-
ing a theoretical model to explain these phenomena.
Only by understanding these results can we rationally
design new saccharide selective sensors.
Acknowledgements
Figure 1. Relative stability with saccharides for 3n/5n.
Financial support from the Royal Society, Beckman-
Coulter and the EPSRC are gratefully acknowledged.
We would also like to thank the University of Bath for
support.
naphthalene’s p-surface and that of 2-naphthalene. The
same is true for phenanthrene and anthracene. In these
cases the observed trends are probably due to increased
steric crowding at the binding site. In both cases a
decrease in the relative stability is coupled to an
increase in the number of peri-hydrogens. Anthracene
has 2 peri-hydrogens, more than any other fluorophore
in the series and clearly displays the lowest relative
stability. It is not surprising that increasing the steric
crowding of the receptor site reduces binding efficiency.
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sensor with two phenylboronic acid groups, a hexa-
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