Design and evaluation of selective recognition on supramolecular gel using soft molecular template effect

Article abstract of DOI:10.1246/cl.130902

A supramolecular cyclodextrin (CyD) gel having selective monosaccharide recognition function was prepared by polymerization of γ-CyD containing a lipophilic phenylboronic acid azoprobe. The non-template gel exhibited galactose selectivity in adsorption, whereas the glucose-template gel exhibited glucose selectivity.

Full text of DOI:10.1246/cl.130902

CL-130902
Received: September 28, 2013 | Accepted: October 25, 2013 | Web Released: October 31, 2013
Design and Evaluation of Selective Recognition on Supramolecular Gel
Using Soft Molecular Template Effect
Takeshi Hashimoto,* Masafumi Yamazaki, Hiroyuki Ishii, Taiji Yamada, and Takashi Hayashita*
Department of Materials and Life Sciences, Faculty of Science and Technology, Sophia University,
7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554
(E-mail: ta-hayas@sophia.ac.jp)
A supramolecular cyclodextrin (CyD) gel having selective
monosaccharide recognition function was prepared by polymer-
ization of γ-CyD containing a lipophilic phenylboronic acid
azoprobe. The non-template gel exhibited galactose selectivity
in adsorption, whereas the glucose-template gel exhibited
glucose selectivity.
N
a)
b)
O
C8H17
C8H17
N
O
B
N
O
N
N
O
HO
B
N
c)
d)
O
C8H17
OH
HO
Sugars are essential biological molecules for metabolism,
nutrition, and cell structure. They play vital roles in regulating
birth, differentiation, and immunity.1 The design of artificial
sugar receptors is very difficult due to the diversity of sugar
structures by forming anomers in water.2 Though natural
enzymes such as GOx or GDH are known to be an efficient
receptor for sugars,3 their labilities in pH and temperature also
make the development of continuous separation systems
difficult.
O
O
OH
H2n+1Cn
2n+1Cn
O
N
N
B
OH
H
O
O
N
OH
N
B
O
OH
n
HO
On the other hand, phenylboronic acid can readily form
stable cyclic esters with the diol moiety of sugars in aqueous
solution.4 Most monophenylboronic acid sensors exhibit fruc-
tose selectivity,5 but this sugar recognition selectivity can be
tuned by controlling the microenvironment of the binding sites.
Pioneering studies by Shinkai and Norrild indicated that some
diboronic acids exhibit D-glucose selectivity.6,7 Two boronic
acids having the proper spatial positioning can selectively form
a 2:1 complex with D-glucose.8 Also, we previously studied
selective sugar recognition by using a supramolecular phenyl-
boronic acid probe/CyD complex in water.9 Our studies
revealed that conformational changes of the probes inside the
CyD molecule contributed to the large difference in selectivity
for saccharides. This supramolecular recognition function is
expected to act as a novel sugar separation system in which the
hydrophobic phenylboronic acid ligands are dynamically fixed
inside the polymeric CyD gel.
Herein, we report efficient sugar separation based on the
template effect by using the recognition system of phenyl-
boronic acid azoprobe/γ-CyD gel complexes. We synthesized 4-
(4¤-alkoxyphenylazo)phenylboronic acid (B-Azo-Cn (n = 6, 8,
and 10), Figure 1) and introduced it into a CyD molecule to
form a gel for the selective adsorption of monosaccharides based
on the template effect. This system has several merits: i) the
selective concentration of sugars is feasible via specific
adsorption; ii) the template effect can be efficiently activated
by using an adsorption system; and iii) the repositioning of the
binding sites for target sugar recognition by repeating sugar
adsorption (self-learning ability10 of CyD gel) is expected.
The azoprobes (Bp-Azo-Cn, B-Azo-Cn, and Azo-C8) were
prepared by azocoupling of 4-(4,4,5,5-tetramethyl-1,3-dioxaboro-
lan-2-yl)aniline with phenol, followed by Williamson ether
coupling with alkyl bromides (see ESI11). The template γ-CyD
Figure 1. Structures of azoprobes and supramolecular gel.
a) Azo-C8, b) Bp-Azo-C8, c) B-Azo-C8, d) B-Azo-Cn/γ-CyD
gel.
gel was prepared by polymerization of azoprobe-containing
γ-CyD with ethylene glycol digrycyl ether (EGDE)12 in the
presence or absence of monosaccharide (see ESI11). The
template sugars were completely removed by washing with
HCl solution at pH 3, to avoid the influence of sugar residues.
The adsorption properties of the template gel were evaluated
by measuring changes in sugar concentrations before and after
immersion of the gel in sugar solution. The concentrations were
estimated by the phenol-sulfuric acid colorimetric method.13
The amount adsorbed (Q) was determined with the following
equation;14
ðcbefore ꢀ cafterÞ ꢁ V
Q ¼
ð1Þ
M
where c represents sugar concentration (mol dm¹3), V is the
volume of sugar solution (cm3) before gel adduction, and M
is the weight of dry gel (g). Application to the Langmuir
adsorption isotherm gave the saturated amount of adsorption
(Ws) and the adsorbed equilibrium constant (a), as shown in
eq 2.14
1
1
1
1
¼
¢
þ
ð2Þ
Q
aWs
c
Ws
The sugar adsorption ability was influenced by the presence
of target sugar during the gel formation. The absorption behavior
as a function of equilibrium sugar concentration (cafter) is shown
in Figure 2. It is evident that the amount adsorbed was increased
as the sugar concentration was increased.
228 | Chem. Lett. 2014, 43, 228–230 | doi:10.1246/cl.130902
© 2014 The Chemical Society of Japan

a)
Figure 3. Adsorption isotherm for D-glucose in water.
[NaCl] = 0.1 M, [Na2CO3] = 10 mM, [sugar] = 0-6 mM by B-
Azo-C6, -C8, and -C10 gel (glucose template).
b)
¹1
Table 1. Saturated amount of adsorption/¯mol g (adsorbed
equilibrium constant/mol¹1 m3) of gel
Gel
Glucose
n.d.a
n.d.a
Galactose Fructose
n.d.a n.d.a
Blank
B-Azo-C8 (Non-template)
0.151 (303) n.d.a
B-Azo-C6 glucose template ¹88.5 (n.d.)
B-Azo-C8 glucose template 1.38 (125)
B-Azo-C10 glucose template 1.35 (208) 0.41 (49.0) n.d.a
aNot determined.
n.d.a
n.d.a
n.d.a
n.d.a
c)
in Figure 3. In the case of n = 6 (B-Azo-C6/γ-CyD gel), the
adsorption ability at 490 nm showed negative values, indicating
that the leaking of azoprobe took place which affected the
phenol-sulfuric acid method (see ESI11). The ability of
adsorption increased with increasing the length of alkyl chain
(for n = 10). These adsorption parameters are summarized in
Table 1.
To examine the mechanism of recognition of azoprobes in
the gel, the UV-vis spectra and the induced circular dichroism
(ICD) spectra of B-Azo-C8 in γ-CyD monomer were measured
in water.
Figure 2. Adsorption isotherm of D-glucose, D-fructose, and
D-galactose by azoprobes/γ-CyD gel in water. a) Azo-C8/γ-
CyD gel without template sugar, b) B-Azo-C8/γ-CyD gel
without template sugar, and c) B-Azo-C8/γ-CyD gel D-glucose
template. [NaCl] = 0.1 M, [Na2CO3] = 10 mM, [sugar] = 0-6
mM at 30 °C.
The effects of sugar addition on UV-vis and ICD spectra
are shown in Figure 4. In the UV-vis spectra, little difference
was noted among the spectra and only a slight increase in
absorption around 450 nm was found in the presence of fructose
(Figure 4a). The ICD spectra were markedly affected by the
sugar species (Figure 4b). In the case without sugar, a large split-
type Cotton effect was noted, where the twisting of two probes
took place inside the γ-CyD molecule.16 This Cotton effect was
also noted in the presence of galactose. This phenomenon is
consistent with the adsorption selectivity of the template-free gel
for galactose. On the other hand, a single positive Cotton effect
was observed by the addition of glucose, and a negative Cotton
effect was noted in the case of fructose addition. These variations
in ICD patterns denote differences in the probe positioning
inside γ-CyD upon the addition of sugar species. This behavior
suggests that the adsorption selectivity was controlled by the
position of the azoprobe dimer inside the γ-CyD gel.
The γ-CyD gel containing azoprobe Azo-C8, which has no
phenylboronic acid group, exhibited no adsorption of any of the
sugars in the solutions (Figure 2a). These results indicate that
the phenylboronic acid group acts as a recognition site for the
absorption of sugars. On the other hand, the gel without template
sugar (using Bp-Azo-C815) exhibited selective adsorption for
D-galactose, and there was no adsorption for D-glucose and
D-fructose (Figure 2b). It is evident that the B-Azo-C8 posi-
tioned at proper recognition sites for D-galactose inside γ-CyD
of template-free gel. In contrast, glucose-template gel showed
selective and high adsorption ability for D-glucose, as expected
(Figure 2c).
In conclusion, we have developed a novel supramolecular
γ-CyD gel containing phenylboronic acid azoprobes as dynamic
binding sites for sugar recognition. The most hydrophobic
The effect of alkyl chain length of the azoprobes (n = 6, 8,
and 10) upon the adsorption ability of the template gel is shown
Chem. Lett. 2014, 43, 228–230 | doi:10.1246/cl.130902
© 2014 The Chemical Society of Japan | 229

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mM, [KOH] = 0.2 M, [γ-CyD] = 0.4 mM, [B-Azo-C8] = 0.05
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molecule and exhibited the highest selectivity. In addition, the
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adsorption. This difference was caused by changes in the
position of the two azoprobes inside γ-CyD, as confirmed by the
ICD spectra of sugar-B-Azo-C8/γ-CyD complexes in water.
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This study was supported by a Grant-in-Aid for Scientific
Research (Nos. 22350039 and 23550104) from the Ministry of
Education, Culture, Sports, Science and Technology of Japan.
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230 | Chem. Lett. 2014, 43, 228–230 | doi:10.1246/cl.130902
© 2014 The Chemical Society of Japan