Enzymatic Hydrolysis-Responsive Supramolecular Hydrogels Composed of Maltose-Coupled Amphiphilic Ureas

Article abstract of DOI:10.1002/asia.202100376

Maltose is a ubiquitous disaccharide produced by the hydrolysis of starch. Amphiphilic ureas bearing hydrophilic maltose moiety were synthesized via the following three steps: I) construction of urea derivatives by the condensation of 4-nitrophenyl isocya

Full text of DOI:10.1002/asia.202100376

CHEMISTRY
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Accepted Article
Title: Enzymatic Hydrolysis-Responsive Supramolecular Hydrogels
Composed of Maltose-Coupled Amphiphilic Ureas
Authors: Masamichi Yamanaka, Ryohei Yoshisaki, Shinya Kimura, and
Masashi Yokoya
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Enzymatic Hydrolysis-Responsive Supramolecular Hydrogels
Composed of Maltose-Coupled Amphiphilic Ureas
Ryohei Yoshisaki,[a,b] Shinya Kimura, Masashi Yokoya, and Masamichi Yamanaka*[a]
[a]
[b]
R. Yoshisaki, Dr. S. Kimura, Dr. M. Yokoya, Prof. Dr. M. Yamanaka
Meiji Pharmaceutical University
2-522-1 Noshio, Kiyose, Tokyo 204-8588 (Japan)
E-mail: yamanaka@my-pharm.ac.jp
R. Yoshisaki,
Department of Chemistry, Shizuoka University
836 Ohya, Suruga-ku, Shizuoka 422-8529 (Japan)
Supporting information for this article is given via a link at the end of the document.
Abstract: Maltose is a ubiquitous disaccharide produced by the
hydrolysis of starch. Amphiphilic ureas bearing hydrophilic maltose
moiety were synthesized via the following three steps: I) construction
of urea derivatives by the condensation of 4-nitrophenyl isocyanate
and alkylamines, II) reduction of the nitro group by hydrogenation, and
III) an aminoglycosylation reaction of the amino group and the
unprotected maltose. These amphiphilic ureas functioned as low
molecular weight hydrogelators, and the mixtures of the amphipathic
ureas and water formed supramolecular hydrogels. The gelation
ability largely depended on the chain length of the alkyl group of the
amphiphilic urea; amphipathic urea having a decyl group had the
highest gelation ability (minimum gelation concentration = 0.4 mM).
The physical properties of the supramolecular hydrogels were
evaluated by measuring their thermal stability and dynamic
viscoelasticity. These supramolecular hydrogels underwent gel-to-sol
phase transition upon the addition of -glucosidase as a result of the
-glucosidase-catalyzed hydrolysis of the maltose moiety of the
amphipathic urea.
increases the number of synthetic steps. An aminoglycosylation
reaction, which produces a N-linked glycoside by the reaction of
a sugar and an amine, can be applied even to unprotected sugars
making it an effective route for the efficient synthesis of the target
molecule.[11–13] We have previously reported the synthesis of low
molecular weight hydrogelators (LMWHGs) using an
aminoglycosylation reaction with unprotected lactose.[14,15] The
self-assembly of LMWHGs can form supramolecular hydrogels,
which have received significant attentions in recent decades
because of their broad applications, such as in drug delivery
systems (DDS), regenerative medicine, and cell cultures.[1,16–19]
Supramolecular hydrogels that respond to stimuli present in the
body, such as temperature, pH, redox, and enzymes, are an
important feature of these applications.[20,21] Herein, we report the
synthesis of amphiphilic urea derivatives having maltose as a
hydrophilic group that functioned as LMWHGs. The resulting
supramolecular hydrogels showed gel-to-sol phase transition in
response to enzymatic hydrolysis of the maltose moiety.
Results and Discussion
Introduction
Synthesis of Amphiphilic Ureas
Most LMWHGs are amphiphilic molecules that achieve regulated
self-assembly in aqueous media. Amphiphilic ureas (Mal-Cn) with
Sugars, which have excellent structural diversity and high
hydrophilicity, have been widely applied as a component of
supramolecular architectures involving gelatinous materials.[1,2]
Monosaccharides are often used in such studies because they
are easy to obtain and handle. However, it is desirable to apply
disaccharides, trisaccharides, and oligosaccharides for the
development of materials that exhibit higher-order functions.
Maltose is a disaccharide in which two molecules of glucose are
joined by an α-1,4-glycosidic bond. Starch, whose linear portion
is composed of repeating maltose groups, affords maltose by
enzymatic hydrolysis, making maltose one of the most readily
available disaccharides.[3] The application of maltose as a
substrate for functional materials would be an effective use of this
ubiquitous disaccharide.[4–10]
a
hydrophilic maltose moiety were designed as LMWHG
candidates. Mal-Cn can be synthesized in three steps from
commercially available compounds via an aminoglycosylation
reaction with unprotected maltose (Scheme 1). The hydrophobic
urea structure (NU-Cn) was obtained via the condensation of 4-
nitrophenyl isocyanate and alkylamine.[15] Reactions were carried
out using amines with alkyl chain lengths from heptyl (C7) to
dodecyl (C12). The nitro group of NU-Cn was reduced to an amino
group by hydrogenation in the presence of a palladium/carbon
catalyst to obtain the desired amine (AU-Cn).[15] The
aminoglycosylation reaction of the amino group of AU-Cn with
maltose proceeded in the presence of a catalytic amount of
ammonium sulfate to give the desired amphiphilic urea (Mal-Cn).
The reactions occurred in a stereoselective manner, and only β-
isomers were obtained as products.[12] The structures of Mal-Cn
were confirmed via 1H NMR, 13C NMR, and mass spectrometry.
An efficient synthesis that has only a few steps is an important
factor in the development of materials for commercial use.
However, when synthesizing a molecule bearing a sugar moiety,
it is generally necessary to protect the hydroxyl groups, which
1
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10.1002/asia.202100376
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at a rate of 0.2 °C/min. The Tgel of a 2.0 mM supramolecular
hydrogel of Mal-C8 had a wide temperature range of 28–44 °C.
Under constant temperature conditions of 35 and 40 °C, a 2.0 mM
supramolecular hydrogel of Mal-C8 required 80 and 60 min,
respectively, to complete the gel-to-sol phase transition. When
the concentration of Mal-C8 was increased to 5.0 and 10 mM, the
Tgel increased to 44–56 and 52–58 °C, respectively, and the
temperature ranges became narrower. Similar trends were
observed for the supramolecular hydrogels of Mal-C9 and Mal-C10.
The Tgel values of the 2.0, 5.0, and 10 mM supramolecular
hydrogels of Mal-C9 were 28–42, 38–56, and 54–58 °C,
respectively. The Tgel values of the 2.0, 5.0, and 10 mM
supramolecular hydrogels of Mal-C10 were 26–40, 34–52, and 62–
70 °C, respectively. Supramolecular hydrogels of Mal-C11 and
Mal-C12, which have long alkyl chains, tended to have lower Tgel
values. The 5 and 10 mM supramolecular hydrogels of Mal-C11
showed Tgel values of 26–40, and 46–56 °C, respectively. The Tgel
of 10 mM supramolecular hydrogels of Mal-C12 was 34–52 °C. A
good correlation was found between gelation ability and Tgel when
10 mM supramolecular hydrogels of Mal-Cn were compared
(Table 1).
The physical properties of the supramolecular hydrogels of
Mal-Cn were investigated by measuring their viscoelastic features
using a rheometer.[23,24] To obtain receiving reproducible results,
10 mM supramolecular hydrogels were used for Mal-C8, Mal-C9,
and Mal-C10, and 20 mM supramolecular hydrogels were used for
Mal-C11 and Mal-C12. Both the storage moduli (G′) and loss moduli
(G′′) were almost independent of the frequency from 0.01 to 10.0
Hz in supramolecular hydrogels of Mal-C8, Mal-C9, and Mal-C10,
which is typical of a supramolecular gel (Figure 1a).[25] The G′ and
G′′ values of the 10 mM Mal-C8 gel are 2490 and 257 Pa,
respectively, at 1.0 Hz. The G′ and G′′ values of the 10 mM Mal-
C9 gel are 2950 and 380 Pa, respectively, at 1.0 Hz. The G′ and
G′′ values of the 10 mM Mal-C10 gel are 2180 and 444 Pa,
respectively, at 1.0 Hz. Comparing these three supramolecular
hydrogels, there was a tendency for the tan (G′′/G′) to increase
as the alkyl chain length increased. For the supramolecular
hydrogels of Mal-C11 and Mal-C12, the G' values were frequency-
dependent and increased as the frequency increased (Figure 1b).
This is a typical trend observed for weak supramolecular gels.[26]
Even in a 20 mM supramolecular hydrogel of Mal-C11, the G'
values were small, and the tan values were large (tan: 0.32 at
0.1 Hz, 0.22 at 1.0 Hz). Mal-C12 had properties similar to those of
Mal-C11. The strain sweep of the supramolecular hydrogels of
Mal-Cn demonstrated an elastic response that is typical of
supramolecular hydrogels (Figures S1 and S2).
Scheme 1. Synthesis of Amphiphilic Ureas Mal-Cn.
Gelation Experiments
The gelation abilities of the amphiphilic ureas Mal-Cn are shown
in Table 1. A mixture of Mal-Cn and water in a glass vial was
heated until the Mal-Cn dissolved, whereupon the mixture was
cooled slowly at room temperature. When the glass vial was
inverted, the mixture remaining in the vial was defined as the gel.
The amphiphilic urea with a heptyl group (Mal-C7) did not form a
supramolecular hydrogel, even at a high concentration of 40 mM.
The amphipathic urea having an octyl group (Mal-C8) functioned
as a LMWHG and formed a supramolecular hydrogel at a
minimum gelation concentration (MGC) of 1.5 mM. Amphipathic
ureas having nonyl and decyl groups (Mal-C9 and Mal-C10) had
better gelation ability than Mal-C8, and amphipathic ureas with
long alkyl groups (Mal-C11 and Mal-C12) had worse gelation ability
than Mal-C8 (Table 1).
The gelation ability of the amphipathic ureas was greatly
influenced by the chain length of the alkyl group, Mal-C10 showed
the highest gelation ability (MGC = 0.4 mM); the MGCs of Mal-Cn
ureas having both shorter (n = 8 and 9) and longer (n = 11 and
12) alkyl groups were higher than that of Mal-C10. The balance
between hydrophilicity and hydrophobicity of a LMWHG is
extremely important in the formation of supramolecular hydrogels,
and a difference of one carbon chain had a dramatic effect on
gelation ability.
Table 1. Minimum Gelation Concentrations (MGCs) and Phase Transition
Temperatures (Tgel) of Mal-Cn.
MGC (mM)
Tgel (ºC) [a]
-
Mal-C7
Mal-C8
Mal-C9
Mal-C10
Mal-C11
Mal-C12
-
1.5
1.0
0.4
2.3
5.3
52–58
54–58
62–70
46–56
34–52
[a] The phase transition temperatures of 10 mM supramolecular hydrogels of
Mal-Cn were measured.
Properties of Supramolecular Hydrogels
The thermal stability of the supramolecular hydrogels was
evaluated by measuring gel-to-sol phase transition temperature
(Tgel) (Tables 1 and S1). Tgel was determined by the inverse flow
method and defined as the temperature at which the gel fell down
the vial.[22] The temperature of the oil bath was increased slowly
2
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10.1002/asia.202100376
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dynamic viscoelasticity measurements using a rheometer (Figure
2c). After the addition of α-glucosidase to the supramolecular
hydrogel, G′ decreased over time, and G′′ increased over time.
After approximately 3 h, the value of G′′ exceeded that of G′. The
difference in the time required for the phase transition between
that determined by examination of the glass vial and that
determined by rheometer measurements is probably due to the
thickness of the sample. Structural changes in Mal-C8 as a result
of α-glucosidase were monitored by 1H NMR (Figure 2d). The ¹H
NMR spectra showed that the amphiphilic urea Glu-C8, which
formed from hydrolysis of the Mal-C8 maltose moiety, was a major
component of the suspension obtained after mixing
a
supramolecular hydrogel of Mal-C8 and α-glucosidase. The
suspension also contained ~35% AU-C8, which resulted from
hydrolysis of the N-glycosidic bond of Mal-C8.
Figure 1. Storage moduli (G') and loss moduli (G'') of supramolecular hydrogels
via frequency sweep at a strain of 0.1%: (a) 10 mM of Mal-C8, Mal-C9, and Mal-
C10; (b) 20 mM of Mal-C11 and Mal-C12
.
Enzymatic Responsive Phase Transition
A gel-to-sol phase transition in response to external stimuli is a
characteristic property of supramolecular gels formed by highly
reversible intermolecular interactions.[20,21,27–30] The responsive
stimulus can be designed by choosing the functional group to be
introduced to the molecule. The development of supramolecular
hydrogels that respond to stimuli existing in the living body is an
important factor in various applications such as DDS. We
expected that Mal-Cn would show a gel-to-sol phase transition in
response to enzymatic hydrolysis of the maltose moiety.
-Glucosidase is an enzyme that catalyzes the hydrolysis of
the -1,4-glucoside bond of sugars.[31] -Glucosidase is present
in many organisms; in humans, it is primarily expressed in the
small intestine. When α-glucosidase acts on Mal-Cn, the
glucosidic bond at the maltose moiety is hydrolyzed to produce
amphipathic urea (Glu-Cn) and glucose (Figure 2a). Previous
studies have shown that Glu-C8 does not function as a LMWHG,
[15] so we used the supramolecular hydrogel of Mal-C8 to evaluate
the enzymatic responsiveness. When 10 units of α-glucosidase
were added to a 5 mM supramolecular hydrogel of Mal-C8 at room
temperature, the supramolecular hydrogel at the interface with α-
glucosidase collapsed. After 8 h, the mixture completely changed
from a hydrogel to a suspension containing a precipitate (Figure
2b). The amount of enzyme greatly affected the time required for
the phase transition; using 1 unit of α-glucosidase took 5 days for
the complete phase transition of a 5 mM supramolecular hydrogel
of Mal-C8. The process of supramolecular hydrogel
transformation into a suspension could also be followed by
Figure 2. (a) Reaction scheme of the enzymatic hydrolysis of Mal-Cn, (b)
Photographs showing the time course of a mixture of supramolecular hydrogel
(5.0 mM Mal-C8) and α-glucosidase (10 units) after 0, 4, and 8 h, (c) Storage
moduli (G') and loss moduli (G'') of a mixture of Mal-C8 and α-glucosidase (6
units) via time sweep at a strain of 0.1% and a frequency of 1.0 Hz, (d) 1H NMR
spectra (DMSO-d6, 298 K) of Mal-C8 (top), a mixture of a supramolecular
hydrogel of Mal-C8 and α-glucosidase after 8 h (middle), and Glu-C8 (bottom).
Supramolecular hydrogels of Mal-C9 and Mal-C10 showed
similar gel-to-sol phase transitions in response to enzymatic
hydrolysis. A 1 mM supramolecular hydrogel of Mal-C9 became a
suspension after 3 h at room temperature in the presence of 10
units of α-glucosidase (Figure 3a). A 0.5 mM supramolecular
hydrogel of Mal-C10 also underwent phase transition to a
3
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10.1002/asia.202100376
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added. The obtained solid was collected by filtration and washed with
MeOH that had been cooled to 0 ºC. The desired Mal-Cn was obtained as
a white solid.
suspension after 3 h at room temperature in the presence of 10
units of α-glucosidase (Figure S3). The rate of the phase
transition could be controlled by not only the amount of enzyme
but also an external additive. Acarbose is an inhibitor of α-
glucosidase and therefore inhibits the hydrolysis of
saccharides.[32,33] It was believed that if acarbose was allowed to
coexist in a supramolecular hydrogel, the hydrolysis of Mal-Cn
catalyzed by α-glucosidase would be delayed, and the phase
transition would require more time. Indeed, the addition of 10 units
of α-glucosidase to a 1 mM supramolecular hydrogel of Mal-C9
containing 1 mM acarbose resulted in the phase transition taking
18 h to complete (Figure 3b). When 10 units of α-glucosidase
were added to a 1 mM supramolecular hydrogel of Mal-C10
containing 1 mM acarbose, the phase transition also took 18 h to
complete (Figure S4).
Gelation experiment
A mixture of Mal-Cn and H2O in a glass vial was heated on a hot plate
(150 °C) until the Mal-Cn dissolved. The resulting solution was gradually
cooled to ambient temperature. Gel formation was evaluated by the
inverted tube test. When the vial was inverted, any mixture remaining in
the vial was defined as gel.
Acknowledgements
This work was supported by Grant-in-aid for the Scientific
Research (No. 20K06977 for M.Yo.; 17H06374 and 21K06485 for
M.Ya.) the Japan Society for the Promotion of Science (JSPS) or
the Ministry of Education, Culture, Sports, Science and
Technology (MEXT) and the NOVARTIS Foundation (Japan) for
the Promotion of Science (for S.K.).
Keywords: gels • hydrogels • phase transition • self-assembly •
urea
Figure 3. Photographs showing the time course of
a mixture of (a) a
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4
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5
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Entry for the Table of Contents
Amphiphilic ureas bearing hydrophilic maltose moiety functioned as low molecular weight hydrogelators. The gelation ability largely
depended on the chain length of the alkyl group of the amphiphilic urea; amphipathic urea having a decyl group had the highest
gelation ability (minimum gelation concentration = 0.4 mM). These supramolecular hydrogels underwent gel-to-sol phase transition
upon the addition of -glucosidase as a result of the hydrolysis of the maltose moiety.
6
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