was only 4 times stronger than monoboronic sensor 3 + 4. These
results are not surprising since it is well known that -glucose
easily forms 1+1 cyclic complexes with di-boronic acids,
D
whereas -fructose tends to form 2+1 acyclic complexes with
D
di-boronic acids.2,3
In conclusion, we have shown that it is possible to prepare an
electrochemical sensor 2 with enhanced
-galactose (17 fold) binding employing simple building blocks
D
-glucose (40 fold) and
D
and using a modular approach. We believe that these results
could be applied in the development of new saccharide selective
electrochemical sensors. Our ongoing research in directed
towards new modular electrochemical sensors with different
linkers and electroactive units.
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. S. U. wishes to acknowledge the
EPSRC for support through the award of an EPSRC student-
ship. A. T. A. J. wishes to acknowledge the Nuffield foundation
for an equipment grant. We wish to thank Dr Noel W. Duffy for
helpful advice and discussions. We would also like to
acknowledge the support of the University of Bath.
Fig. 1 Differential pulse voltammograms of 2 (5.0 3 1025 mol dm23) with
different concentration of
D
-glucose (0, 1.11 3 1024, 3.89 3 1024 6.66 3
1024 1.04 3 1023, 1.03 3 1022, 1.02 3 1021 mol dm23) at pH 8.21 in 52.1
wt% methanol. The area of the glassy carbon electrode is 0.28 cm2.
Voltammetric parameters are as follows: scan rate, 20 mV s21; modulation
time, 50 ms; interval time, 500 ms; step potential, 5.1 mV; modulation
amplitude 25.05 mV. The lack of an isobestic point is attributed to viscosity
effects.‡
Notes and references
‡ Selected data for 2: mp 155–158 ° C (decomp.); m/z (FAB) 1212
([M + H + 4(3-HOCH2C6H4NO2) 2 4H2O]+, 95%); Found: C, 69.25; H,
7.07; N, 4.12. C38H46B2FeN2O4-H2O+0.05 CHCl3 requires C, 69.21; H,
6.74; N, 4.24%. dH(300 MHz, CDCl3 + CD3OD (a few drops), Me4Si): 1.28
(4H, br s, NCC(CH2)2), 1.42 (4H, br s, NC(CH2) 2), 2.28 (2H, t, FcCNCH2),
2.36 (2H, t, NCH2), 3.56 (4H, s, NCH2Ph), 3.58 (2H, s, FcCH2N), 3.78 (2H,
s, NCH2Ph), 4.12 and 4.18 (5H, 4H, s each, Fc-H), 7.15–7.38, 7.86 (11H,
2H, m each, Ar-H). dC (75 MHz, CDCl3 + CD3OD (a few drops), Me4Si):
22.7, 24.7, 25.6, 27.0, 31.6, 51.2, 52.0, 57.1, 60.4, 61.3, 68.4, 68.5, 68.6,
68.9, 70.6, 127.2, 127.3, 127.5, 128.2, 128.5, 129.6, 129.7, 129.9, 130.6,
130.9, 136.2, 136.5, 141.7, 141.8.
Scheme 2
§ The observed DPV for 3 + 4 and 5 are available as ESI.† A decrease in
current intensity with increasing
Since reference compound 5 can not bind with
decrease in current intensity is due to a change in viscosity of the solution
D
-glucose concentration was observed.
D-glucose, the observed
The stability constants (K) of electrochemical sensors 2 and
the 1+1 mixture of 3 + 4 were calculated by fitting the current
intensity at 0.357 V vs. saccharide concentration and are given
in Table 1.13,14
The relative stability constants of the diboronic acid 2 relative
to the 1+1 mixture of monoboronic acids 3 + 4 are given in
Table 1. A 1+1 mixture of monoboronic acids 3 + 4 was used as
the reference to alow direct comparison with the diboronic acid
system 2. Using a 1+1 mixture ensures that the same
concentration of both ferrocene and boronic acid groups are
present in both cases.
with added D-glucose. With compound 5 for concentrations of D-glucose
greater than 0.1 mol dm23 the current dramatically decreases due to high
solution viscosity. Therefore, the ferrocene boronic acid sensors were
titrated until a concentration of 0.1 mol dm23 saccharide.
1 A. P. Davis and R. S. Wareham, Angew. Chem., Int. Ed., 1999, 38,
2979.
2 J. H. Hartley, T. D. James and C. J. Ward, J. Chem. Soc., Perkin Trans.
1, 2000, 3155.
3 T. D. James and S. Shinkai, Top. Curr. Chem., 2002, 218, 159.
4 S. Arimori, M. L. Bell, C. S. Oh, K. A. Frimat and T. D. James, Chem.
Commun., 2001, 1836.
5 S. Arimori, M. L. Bell, C. S. Oh, K. A. Frimat and T. D. James, J. Chem.
Soc., Perkin Trans. 1, 2002, 802.
6 P. D. Beer, P. A. Gale and G. Z. Chen, J. Chem. Soc., Dalton Trans.,
1999, 1897.
7 W. Schuhmann and H.-L. Schmidt, Adv. Biosensors, 1992, 2, 79.
8 A. Ori and S. Shinkai, J. Chem. Soc., Chem. Commun., 1995, 1771.
9 A. N. J. Moore and D. D. M. Wayner, Can. J. Chem.-Rev. Can. Chim.,
1999, 77, 681.
Although the highest binding constant for compound 2 is
with -fructose, cooperative binding of the two boronic acid
D
groups is clearly observed as illustrated by the stability constant
differences between the mono- and di-boronic acid compounds
(sensors 2 and 3 + 4, respectively). In particular the stability
constant K of diboronic acid sensor 2 with
times greater than with monoboronic sensor 3 + 4, whereas the
stability constant K of diboronic acid sensor 2 with -fructose
D-glucose was 40
D
10 J. P. Lorand and J. D. Edward, J. Org. Chem., 1959, 24, 769.
11 The reference compound 3 has recently been prepared and proposed as
a potentially active electrochemical unit by J. C. Norrild and I. Sotofte,
J. Chem. Soc., Perkin Trans 2, 2002, 303.
12 A. Bard and L. Faulkner, Electrochemical Methods, 2nd edition, Wiley,
New York, 2001.
13 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.
14 C. R. Cooper and T. D. James, J. Chem. Soc., Perkin Trans. 1, 2000,
963.
Table 1 Stability constant K (determination of coefficient; r2) for the
saccharide complexes of molecular sensors 2 and 3 + 4
K/mol21 dm3 at 25 °C
Saccharide
2
3+4
17 ± 2 (0.99)
362 ± 5 (1.00)
47 ± 2 (1.00)
54 ± 8 (0.99)
2/3 + 4
D-Glucose
D-Fructose
D-Galactose
D-Mannose
684 ± 54 (0.99)
40
4
17
3
1478 ± 72 (1.00)
782 ± 72 (0.99)
149 ± 9 (1.00)
CHEM. COMMUN., 2002, 2368–2369
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