A benzthiazole-based simple receptor in fluorescence sensing of biotin ester and urea

Article abstract of DOI:10.1016/j.tetlet.2009.04.117

A benzthiazole-based receptor 1 has been designed and synthesized for recognition of biotin ester and urea in CHCl₃ containing 1% CH₃CN. The receptor binds biotin methyl ester and urea with moderate binding constant values and shows

Full text of DOI:10.1016/j.tetlet.2009.04.117

Tetrahedron Letters 50 (2009) 4096–4100
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A benzthiazole-based simple receptor in fluorescence sensing
of biotin ester and urea
*
Kumaresh Ghosh , Tanushree Sen
Department of Chemistry, University of Kalyani, Kalyani, Nadia 741 235, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A benzthiazole-based receptor 1 has been designed and synthesized for recognition of biotin ester and
Received 19 March 2009
Revised 22 April 2009
Accepted 25 April 2009
Available online 3 May 2009
3 3
urea in CHCl containing 1% CH CN. The receptor binds biotin methyl ester and urea with moderate bind-
ing constant values and shows significant increase in emission of benzthiazole motif. The emission char-
acteristics of 1 upon complexation clearly distinguishes biotin methyl ester and urea from thiourea and
0
1
N,N -dimethylurea. Characterization and sensing properties of the receptor 1 were evaluated by H NMR,
UV–vis, and fluorescence spectroscopic methods.
Keywords:
Biotin ester binding
Urea binding
Ó 2009 Elsevier Ltd. All rights reserved.
Thiourea binding
Benzthiazole
The design and synthesis of artificial receptors to study the
hydrogen bonding interaction with biologically relevant molecules
are important in the area of molecular recognition.1 It is worth
mentioning that the recognition and detection of bioactive sub-
ami et al. have reported the fluorometric detection of urea by a
1
6
macrocyclic receptor. A 2,6-bis(2-benzimidazole)pyridine recep-
tor as developed by Iyer and co-workers shows urea binding with
,2
1
7
concomitant change in fluorescence. Non-fluorescent urea recep-
tors with considerable binding ability are also known. Crown
strates such as urea, biotin,4 carbohydrates,5 carboxylic acids,
3
6
7
3
and amino acids have gained considerable success. Among the
substrates biotin (referred to as vitamin H in human) is an essential
cofactor for a number of enzymes that have diverse metabolic
ether-based receptors as developed by Pedersen and carboxylic
1
8
acid containing crown ether receptor of Reinhoudt are known
to bind urea. Reinhoudt and co-workers have also explored the
concept of using an electrophilic center to bind urea in the cavity
8
functions. Structurally it consists of a pentanoic acid side chain
1
9
and a cis-fused bicyclic moiety with sulfur atom in the ring. Crys-
tallographic studies have confirmed the relative stereochemistry at
of crown ether. Bell et al. have reported the synthesis of naph-
thyridine-fused polyaza heterocycles that were potentially effec-
9
,10
20
the asymmetric carbon.
The crystal structure of biotin shows
tive for urea recognition. An investigation of hydrogen bonding
that the carboxyl group of one biotin molecule is intermolecularly
hydrogen bonded to the urea linkage of the other biotin mole-
interaction of benzimidazole-based receptors with urea and barbi-
2
1
turate in polar solvent is reported. Goswami and Mukherjee re-
ported the solubilization and recognition of urea using a simple
1
1
cule. The valeryl chain is severely twisted from the maximally
extended all trans-conformation. To bind and sense this interesting
biological substrate of defined stereochemistry, considerable ef-
forts were directed at a limited number of designed receptors. In
this aspect, the use of Troger’s base receptor as reported by Wilcox
and co-workers, was noteworthy.12 Claramunt and co-workers
have reported the recognition of biotin methyl ester.13 Pyridine
amide-based simple receptor is also known to bind biotin itself
involving both the carboxylic acid and the cyclic urea part of the
2
2
dinaphthyridine receptor. Recently we have shown that a pyri-
dine amide-based macrocyclic receptor can make a strong inclu-
2
3
sion complex with urea. In contrast to the binding of small
molecule urea, simple bis amide receptor is sometimes very effi-
cient in sequence-specific binding of high molar mass
24
copolyimides.
Our ongoing interest in the selective binding of biologically rel-
evant species by our synthetic receptor inspired us to work on sim-
ple system which is easy to make for fluorometric detection of both
biotin methyl ester and urea in common organic solvent. In rela-
tion to this, we report here the design, synthesis, and host-guest
interaction of benzthiazole-based receptor 1 which shows signifi-
cant increase in emission upon complexation of biotin methyl ester
4
bicyclic unit of biotin. Similarly urea is also of particular impor-
tance as it is toxic, pollutant, and causes serious biological disor-
1
4,15
ders.
To the best of our knowledge, very few fluorescent
receptors for urea recognition are known in the literature. Gosw-
3 3
and urea in CHCl containing 1% CH CN.
*
Corresponding author. Tel.: +91 33 25828282/306; fax: +91 33 25828282.
E-mail address: ghosh_k2003@yahoo.co.in (K. Ghosh).
0
040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.04.117

K. Ghosh, T. Sen / Tetrahedron Letters 50 (2009) 4096–4100
4097
OCH (CH ) CH
2
2 6
3
OCH (CH ) CH
2
2 6
3
O
O
N
S
ClOC
COCl
NH2
Hb
NHa
HN
S
S
Et N, dry THF, reflux
3
N
N
67%
1
Scheme 1. Synthesis of receptor 1.
Receptor 125 was synthesized according to Scheme 1. The reac-
tion of 2-aminobenzthiazole with 5-octyloxyisophthaloyl dichlo-
ride (prepared by etherification of diethyl 5-hydroxyisophthalate
N,N'-Dimethylurea
Thiourea
2 3
with octyl bromide in dry acetone using K CO and hydrolysis of
the esters followed by reaction with oxalyl chloride) in the pres-
ence of triethyl amine in dry THF afforded the desired compound
in 67% yield.
Receptor 1 can adopt different conformations in solution. The
syn–syn conformation with nitrogen atoms into the cavity was
optimized with the guests containing urea moiety. Figure 1 shows
the MM2 optimized geometries26 of 1 with biotin methyl ester,
urea, and thiourea. In all the structures the urea motif is complexed
into the cleft with all possible hydrogen bonds. Thiourea is weakly
complexed with less number of longer hydrogen bonds due to its
more steric nature than urea.
Urea
Biotin Ester
0
2
4
6
8
10
In order to understand the recognition properties of 1 with bio-
ꢁ
5
0
1
Figure 2. Fluorescence ratio (I
upon addition of 3 equiv of a particular guest in CHCl
0
ꢁI/I
0
) of receptor 1 (c = 6.17 ꢀ 10 M) at 426 nm
tin methyl ester, urea, thiourea, and N,N -dimethylurea, H NMR,
fluorescence, and UV titrations were performed in CHCl contain-
CN. Receptor 1 (c = 6.17 ꢀ 10 M) showed the broad
emission at 426 nm when excited at 308 nm in CHCl containing
% CH CN. Upon gradual addition of biotin methyl ester, urea, thio-
urea, and N,N -dimethylurea to the solution of receptor 1 in CHCl
containing 1% CH CN, the emission of benzthiazole was increased
3
containing 1% CH CN.
3
3
ꢁ5
ing 1% CH
3
3
of biotin methyl ester and urea, respectively, in CHCl
1% CH CN. Upon complexation of both biotin ester and urea the
3
containing
1
3
0
3
3
emission peak at 426 nm of 1 underwent a red shift of 15 nm, sug-
gesting a strong hydrogen bonding interaction. During titration no
other spectral changes were observed in the emission spectra, that
is, there was no evidence of either exciplex or excimer emission.
Figure 5 represents the fluorescence titration curves for biotin
methyl ester, urea, thiourea, and N,N -dimethylurea. The linear nat-
ure of the curves for thiourea and N,N -dimethylurea indicated
their weak interactions with receptor 1. Simultaneous UV–vis titra-
tions of 1 with the same guests were carried out in CHCl
3
to different extents. Among the guests examined, receptor 1 dis-
played strong fluorescence enhancement effect with biotin methyl
ester. Figure 2 shows the comparison of change in emission of 1 in
the presence of 3 equiv amounts of a particular guest in CHCl
3
con-
0
taining 1% CH CN. It is evident from Figure 2 that receptor 1 shows
3
0
a clear-cut selectivity with biotin methyl ester. To investigate the
binding abilities of receptor 1 toward the guests examined, titra-
tion studies were performed. Figures 3 and 4, for example, show
the change in emission of 1 in the presence of increasing amounts
3
contain-
ing 1% CH
3
CN. The changes in absorbance of 1 upon complexation
0
0
0
0
0
0
Figure 1. MM2 optimized geometries of (a) 1 with biotin methyl ester [a = 2.25 ÅA , b = 2.04 ÅA , c = 2.42 ÅA , d = 2.04 ÅA , e = 2.17 ÅA , E = 13.79 kcal/mol]. (b) 1 with urea [a = 2.21 ÅA ,
0
0
0
0
0
0
0
0
b = 2.07 ÅA , c = 2.52 ÅA , d = 2.06 ÅA , e = 2.08 ÅA , E = ꢁ11.35 kcal/mol] and (c) 1 with thiourea [a = 2.25 ÅA , b = 2.81 ÅA , c = 2.85 ÅA , d = 2.34 ÅA , E = 3.65 kcal/mol].

4
098
K. Ghosh, T. Sen / Tetrahedron Letters 50 (2009) 4096–4100
2
1
1
0
0
.0
.6
.2
.8
.4
The sharp break of the fluorescence titration curves at [G]/[H] = 1
for both biotin methyl ester and urea confirmed the 1:1 stoichiom-
etries of the complexes (see Fig. 5).
5
4
3
2
1
00
00
00
00
00
0
Fluorescence titration data were used to determine the binding
2
7
constant values. As can be seen from Table 1, the simple receptor
, shows selectivity for biotin methyl ester. Urea being smaller in
1
size is preferentially complexed than thiourea (based on the mode
as shown in Fig. 1) in the excited state. Weak binding of thiourea is
attributed to the bigger size of the sulfur atom that poorly fits into
the cavity than urea. In this context, some characterization data of
the C@Sꢂ ꢂ ꢂHO and C@Oꢂ ꢂ ꢂHO hydrogen bonds as reported by
0.0
2
80
320
360
400
Wavelength (nm)
2
8
Kryachko et al. suggested that polarizability of the sulfur atom
and charge transfer phenomena are also responsible for the weaker
hydrogen bond interaction of sulfur. This cannot be ruled out in the
3
50
400
450
500
550
600
650
0
Wavelength (nm)
present study also. N,N -Dimethylurea is weakly complexed into
the open cleft due to its bulky nature. It is mentionable that CH
being polar solvent, reduces the binding constant values to some
extent. To realize the influence of polarity of CH CN in the binding
process the titration experiments were also conducted in dry
CH CN and the binding constant values are tabulated in Table 1.
The binding constant values in CH CN are significantly less com-
pared to the values determined in CHCl containing 1% CH CN. It
is important to note that the change in fluorescence ratio (I
ꢁI)/
upon addition of particular guest is also significantly reduced
Fig. 6). The 1:1 stoichiometry of the complexes in CH CN was also
3
CN
ꢁ
5
Figure 3. Change in emission of 1 (c = 6.17 ꢀ 10 M) upon gradual addition of
3 3
biotin methyl ester in CHCl containing 1% CH CN; inset: change in absorbance of 1
c = 6.17 ꢀ 10 M) upon gradual addition of biotin methyl ester in CHCl
ꢁ
5
(
3
contain-
3
3
ing 1% CH CN.
3
3
3
3
2.0
1.6
1.2
0.8
0.4
0.0
0
2
1
1
40
80
20
I
(
0
3
confirmed from UV job plot. In this context, Figure 7 shows the job
plot for biotin methyl ester and urea with 1.
However, to substantiate the mode of binding as shown in Fig-
1
ure 1, H NMR of 1 in the presence of the equivalent amount of
3 3
guests was recorded in dry CDCl containing 1% CD CN. Figure 8,
280
320
360
400
1
Wavelength (nm)
for example, shows the change in H NMR of 1 in the presence of
6
0
0
biotin methyl ester. During complexation the amide protons
Table 1
Binding constant values for 1 from fluorescence titration
3
50
400
450
500
550
600
650
Wavelength (nm)
(Mꢁ1)a in CHCl
(Mꢁ1)b in CH
3
CN
Guests
K
a
3
containing 1% CH
3
CN
K
a
3
2
ꢁ
5
Biotin methyl
ester
Urea
9.79 ꢀ 10
6.42 ꢀ 10
4.94 ꢀ 10
Figure 4. Change in emission of 1 (c = 6.17 ꢀ 10 M) upon gradual addition of urea
ꢁ
5
in CHCl
3
containing 1% CH
3
CN; inset: change in absorbance of 1 (c = 6.17 ꢀ 10 M)
3
2
2
2
3.63 ꢀ 10
upon gradual addition of urea in CHCl
3
containing 1% CH CN.
3
c
Thiourea
7.26 ꢀ 10
0
c
N,N -Dimethylurea
2.11 ꢀ 10
a
Binding constant values were determined at wavelength 426 nm.
Binding constant values were determined at wavelength 455 nm.
Binding constant values were not determined due to minor change.
b
c
5
4
3
2
1
00
00
00
00
00
0
Biotin Ester
Urea
N,N'-Dimethylurea
Thiourea
Thiourea
N,N'-Dimethylurea
Urea
0
.0
0.5
1.0
1.5
G]/[H]
2.0
2.5
3.0
3.5
[
Biotin Methyl Ester
Figure 5. Plot of change in emission of 1 at 426 nm versus the ratio of guest to host
concentration in CHCl containing 1% CH CN.
3
3
0.00
0.15
0.30
ΔI/I0
0.45
0.60
of biotin methyl ester and urea are represented in the inset of Fig-
ures 3 and 4, respectively. During complexation in the ground state
there was no shifting of the absorption band in either direction.
ꢁ5
Figure 6. Fluorescence ratio (I
upon addition of 30 equiv of a particular guest in CH
0
ꢁI/I
0
) of receptor 1 (c = 8.24 ꢀ 10 M) at 455 nm
CN (kex = 305 nm).
3

K. Ghosh, T. Sen / Tetrahedron Letters 50 (2009) 4096–4100
4099
1
1
0
0
0
.2
.0
.8
.6
.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(
a)
(b)
0.2
0.0
0
.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
XG
XG
3
Figure 7. Job plots of 1 with biotin methyl ester (a) and urea (b) from UV method in CH CN.
Acknowledgments
We thank CSIR, New Delhi, for financial support and DST, New
Delhi for providing facilities in the department under DST FIST pro-
gram. T.S. thanks CSIR, New Delhi, for a fellowship.
Supplementary data
Supplementary data (binding constant curves for 1 with biotin
3 3
methyl ester and urea in CHCl containing 1% CH CN, change in
emission of 1 in the presence of biotin methyl ester and urea in
Figure 8. Partial 1H NMR spectra of (a) receptor 1 (c = 4.8 ꢀ 10ꢁ3 M), (b) biotin
1
CH
3
CN, and change in H NMR of 1 in the presence of equivalent
containing 1% CD CN are available) asso-
3
methyl ester and (c) 1:1 complex of 1 with biotin methyl ester in CDCl containing
amount of urea in CDCl
3
3
1
3
% CD CN.
ciated with this article can be found, in the online version, at
doi:10.1016/j.tetlet.2009.04.117.
appearing at d 12.02 ppm moved downfield (
lesser extent and became broad. Furthermore, the isophthaloyl peri
proton H underwent significant downfield shift ( d = 0.6 ppm)
Dd = 0.18 ppm) to the
References and notes
b
D
1
2
.
.
Dugas, H. Bio-organic Chemistry; Springer: New York, 1996.
Lehn, J. M. Supramolecular Chemistry; VCH: Weinhein, New York, Basel,
Cambridge, Tokyo, 1995.
upon complexation. In presence of equivalent amount of urea,
0
thiourea, and N,N -dimethylurea this peri proton of 1 exhibited
3
4
.
.
Pedersen, C. J. J. Org. Chem. 1971, 36, 1690–1693.
Goswami, S.; Dey, S. J. Org. Chem. 2006, 71, 7280–7287. and references cited
therein.
the downfield chemical shifts of 0.50, 0.29, 0.16 ppm, respectively.
This clearly indicated the participation of the peri proton in the for-
mation of hydrogen bond with the urea carbonyl oxygen of the
5. Mazic, M.; Kuschel, M.; Sicking, W. Org. Lett. 2006, 8, 855–858. and references
cited therein.
guests. Negligible changes in chemical shift of H
b
of 1 in the pres-
6
7
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.
Goswami, S.; Ghosh, K.; Dasgupta, S. J. Org. Chem. 2000, 65, 1907–1914.
Wang, H.; Chan, W.-H.; Lee, A. W. M. Org. Biomol. Chem. 2006, 6, 929–934.
0
ence of thiourea and N,N -dimethylurea corroborated their weak
interaction compared to urea (for urea see supplementary data).
It is also of note that in the 1:1 complex of 1 with biotin methyl es-
ter, the cyclic urea protons of biotin ester exhibited an upfield
chemical shift with respect to the guest (biotin ester) itself. More-
over, the signals for aromatic protons of benzthiazole moiety
shifted drastically, demonstrating probably a subtle conforma-
tional change of benzthiazole moiety in 1 during the course of
binding of biotin methyl ester in solution phase. This experiment
thus confirmed that biotin ester forms a hydrogen-bonded com-
plex in the mode as shown in Figure 1a, where the cyclic urea part
instead of more bulkier ester group of biotin is involved in the
complexation into the isophthaloyl diamide core.
8. Nelson, D. L.; Cox, M. M. Lehninger Principles of Biochemistry, 3rd ed.; Worth:
New York, 2000.
9.
Traub, W. Nature 1956, 178, 649.
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98, 1920–1926. and references cited therein.
12. (a) Adrian, J. C., Jr.; Wilcox, C. S. J. Am. Chem. Soc. 1989, 111, 8055–8057; (b) Rao,
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1
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1
4. Cooke, I. J. Nature 1962, 194, 1262–1263.
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Eerden, J.; Harkema, S.; Reinhoudt, D. N. J. Am. Chem. Soc. 1988, 110, 8134–
In conclusion, we have shown that receptor 1 which is simple
and easy-to-make, can bind biotin ester and urea with moderate
binding constant values. The detection of these biologically rele-
vant guests is possible by fluorescence. The significant change in
emission of 1 in the presence of biotin methyl ester and urea
clearly distinguishes them from thiourea and substituted N,N’-
dimethylurea. The distinction is best possible in less polar solvent
8
144. and references cited therein.
19. van Staveren, C. J.; van Eerden, J.; Veggel, F. C. J. M.; Herkema, S.; Reinhoudt, D.
N. J. Am. Chem. Soc. 1988, 110, 4994–5008.
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2
2
2
2
007, 129, 16163–16174.
1
and in the present case CHCl
3
containing 1% CH
3
CN is the excellent
25. Mp 198 °C; H NMR (400 MHz, CDCl ) d 12.18 (br s, 2H, amide NH), 8.20 (s, 1H),
3
choice as solvent in monitoring the host–guest interaction effec-
tively. Further work in this direction is underway in our laboratory.
7.82 (d, 2H, J = 7.6 Hz), 7.73 (s, 2H), 7.67 (d, 2H, J = 7.6 Hz), 7.43 (t, 2H,
J = 7.6 Hz), 7.33 (t, 2H, J = 7.6 Hz), 3.99 (t, 2H, J = 6 Hz), 1.80–1.77 (m, 2H), 1.46–

4
100
K. Ghosh, T. Sen / Tetrahedron Letters 50 (2009) 4096–4100
1
.43 (m, 2H), 1.32–1.34 (m, 8H), 0.8 (t, 3H, J = 6.8 Hz); 13C (CDCl
79.6, 164.8, 160.1, 146.7, 133.6, 131.5, 126.3, 124.1, 121.5, 119.9, 118.9, 117.9,
3
, 125 MHz) d
26. Energy optimization was done using CS Chem 3D version 7.0.
27. Chou, P. T.; Wu, G. R.; Wei, C. Y.; Cheng, C. C.; Chang, C. P.; Hung, F. T. J. Phys.
Chem. B 2000, 104, 7818.
28. Kryachko, E.; Nguyen, M. T.; Zeegers-Huyskens, T. J. Phys. Chem. A 2001, 105,
3379–3387.
1
6
2
ꢁ
1
8.7, 31.8, 29.3, 29.2, 29.0, 25.9, 22.6, 21.7; FTIR (KBr,
m in cm ) 3334, 2924,
855, 1672, 1597, 1545, 1445; Mass (EI positive ionization mode, m/z) 559.0
+
+
(
M+H) , 581.1 (M+Na) .