A Natural Glycyrrhizic Acid-Tailored Light-Responsive Gelator

Article abstract of DOI:10.1002/asia.201800180

The construction of stimuli-responsive materials by using naturally occurring molecules as building blocks has received increasing attention owing to their bioavailability, biocompatibility, and biodegradability. Herein, a symmetrical azobenzene-functionalized natural glycyrrhizic acid (trans-GAG) was synthesized and could form stable supramolecular gels in DMSO/H₂O and MeOH/H₂O. Owing to trans–cis isomerization, this gel exhibited typical light-responsive behavior that led to a reversible gel–sol transition accompanied by a variation in morphology and rheology. Additionally, this trans-GAG gel displayed a distinct injectable self-healing property and outstanding biocompatibility. This work provides a simple yet rational strategy to fabricate stimuli-responsive materials from naturally occurring, eco-friendly molecules.

Full text of DOI:10.1002/asia.201800180

DOI: 10.1002/asia.201800180
Full Paper
Sol–Gel Processes
A Natural Glycyrrhizic Acid-Tailored Light-Responsive Gelator
[a, c]
[c]
[c]
[a]
[b, c]
Heshu Fang,
Xia Zhao, Yuan Lin, Song Yang,* and Jun Hu*
Abstract: The construction of stimuli-responsive materials
by using naturally occurring molecules as building blocks
has received increasing attention owing to their bioavailabil-
ity, biocompatibility, and biodegradability. Herein, a symmet-
rical azobenzene-functionalized natural glycyrrhizic acid
sponsive behavior that led to a reversible gel–sol transition
accompanied by a variation in morphology and rheology.
Additionally, this trans-GAG gel displayed a distinct injecta-
ble self-healing property and outstanding biocompatibility.
This work provides a simple yet rational strategy to fabricate
stimuli-responsive materials from naturally occurring, eco-
friendly molecules.
(
trans-GAG) was synthesized and could form stable supra-
molecular gels in DMSO/H O and MeOH/H O. Owing to
2
2
trans–cis isomerization, this gel exhibited typical light-re-
[10]
Introduction
cal agents.
Liu et al. found that the curvature of protein
nanowires could be alternated by using a light-isomerizable
azobenzene-cored poly(amidoamine) (PAMAM) dendrimer
evoked SP1 (stable protein 1) protein assembly, which offers a
Low-molecular-weight gels (LMWGs), or supramolecular gels,
which arise from the self-assembly of small molecules into 3D
networks to immobilize solvents, are a fascinating class of ma-
potential strategy for the design of intelligent protein-based
[
1]
[9a]
terials. Compared with traditional polymer gels, the proper-
ties of LMWGs can be reversibly tuned by the input of chemi-
nanobiomaterials.
Obviously, although light-responsive
LMWGs have been around for many years, they are still receiv-
ing considerable current interest.
[
2]
[3]
[4]
cal or physical stimuli, such as pH, ultrasound, light, li-
[
5]
[6]
[7]
gands, enzymes, and mechanical force. Of these, light has
attracted immense attention because it is a fast and non-con-
taminating stimulus that can provide a broad range of tunable
parameters, for example, wavelength, intensity, and dura-
Glycyrrhizic acid (GA, Scheme 1), a natural triterpenoid sapo-
nin mainly found in licorice root, is widely used in candies and
[11]
sweets on account of its intense sweetness
and exhibits
many biological activities, such as antitumor, anti-inflammatory,
[
1d,8]
[12]
tion.
By incorporating light-active moieties (diarylethene,
antiviral, and antibacterial effects. From a structural point of
spiropyran, azobenzene) into the gelator, the microstructure of
view, GA is comprised of a hydrophobic triterpenoid aglycon
moiety (glycyrrhetinic acid) and a hydrophilic diglucuronic
unit, which endows it with a classic amphiphilic nature. In
2015, Mezzenga et al. first reported its fibrillar self-assembled
[9]
LMWGs can be manipulated easily upon exposure to light.
For example, Nilsson and co-workers have fabricated a light-
sensitive azobenzene–peptide hydrogel that showed a reversi-
ble sol–gel transition induced by UV/Vis light, which thus al-
lowed the light-triggered release of encapsulated pharmaceuti-
[13]
features in the preparation of Au nanoparticle–hybrid gels.
Next, Yang and co-workers used GA to fabricate plant-oil-struc-
[14]
tured emulsion gels. To date, only four works on GA gels
[13,14]
[a] H. Fang, Dr. S. Yang
have been reported,
and the combination of GA with
State Key Laboratory Breeding Base of Green Pesticide & Agricultural Bioen-
gineering, Centre for R&D of Fine Chemicals
Guizhou University
functional units to afford responsive LMWGs is still rare.
Guiyang 550025 (China)
E-mail: yangsdqj@126.com
[b] Dr. J. Hu
Beijing Advanced Innovation Center for Soft Matter Science and Engineer-
ing, Beijing University of Chemical Technology
Beijing 100029 (China)
E-mail: jhu@mail.buct.edu.cn
[
c] H. Fang, Dr. X. Zhao, Dr. Y. Lin, Dr. J. Hu
State Key Lab of Polymer Physics and Chemistry
Changchun Institute of Applied Chemistry
Chinese Academy of Sciences
Changchun 130022 (China)
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under https://doi.org/10.1002/
asia.201800180.
Scheme 1. Schematic representation of the gelation of trans-GAG and its re-
versible trans–cis isomerization.
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Herein, a symmetrical molecule, trans-GAG, in which two
natural GA moieties are linked by an azobenzene group
through the amide bond, was designed and synthesized as
shown in Scheme 1. In this molecule, the azobenzene chromo-
phore can participate in an efficient, reversible trans–cis iso-
merization, whereas GA supplies the amphiphilic backbone to
form LMWGs. Furthermore, naturally occurring GA has good
bioavailability, biocompatibility, and biodegradability, which
endow it with potential applications in the field of biomed-
icine. The results showed that trans-GAG formed supramolec-
ular gels in DMSO/H O and MeOH/H O. Upon irradiation with
2
2
alternating UV and visible light, it underwent a reversible cis-
GAG/trans-GAG isomerization, which thus resulted in gel–sol
transitions accompanied by a variation in morphology and
rheology. Additionally, the trans-GAG gel exhibited a distinct
injectable self-healing property and outstanding biocompatibil-
ity, which provides a way to fabricate natural-product-contain-
ing stimuli-responsive biomaterials for potential drug release
and 3D printing.
Results and Discussion
GA is a polyprotic weak acid and its dissociation constants are
pK =3.98, pK =4.62, and pK =5.17. By using the difference
a1
a2
a3
in pK values, the carboxyl groups on the diglucuronic unit
a
were selectively protected by acetyl groups, as shown in
Scheme 2, whereas the one on the triterpenoid backbone was
conjugated with trans-azodianiline (trans-Azo) by an amidation
reaction to give the intermediate trans-GAGP. After hydrolysis
under basic conditions, resin was used to remove the ions and
afford trans-GAG due to its high polarity. This selective protec-
tion of the carboxyl groups provides a convenient way to syn-
thesize other GA derivatives.
Scheme 2. Synthetic route of trans-GAG.
Table 1. Gelation results for trans-GAG in DMSO/H
different volume ratios.
2
O and MeOH/H
2
O at
[
a]
À1 [b]
Entry
Solvent
Ratio
State
MGC [mgmL ]
As a general procedure, trans-GAG was initially dispersed
and heated in different solvents, then subsequently cooled to
room temperature. After standing for 20 min, the bottle-inver-
1
2
3
4
5
6
7
8
9
DMSO/H
DMSO/H O
DMSO/H
DMSO/H
MeOH/H
MeOH/H
2
O
1:10
1:9
2:8
3:7
1:9
2:8
3:7
4:6
5:5
6:4
P
–
–
17.6
–
–
2
PG
G
S
2
O
2
O
2
O
2
O
[
15]
sion method was used to determine whether a supramolec-
ular gel had formed or not. As illustrated in Table S1 and Fig-
ure S1, only precipitates or clear solutions were formed in de-
ionized (DI) water (Table S1, entry 1), Tris-HCl (Table S1, en-
tries 2–6), phosphate buffer saline (PBS; Table S1, entries 7–8),
Kphos (Table S1, entries 9–10), and Naphos buffer (Table S1, en-
tries 11–13), regardless of the buffer concentration or pH (1–
P
PG
G
G
G
P
–
MeOH/H O
2
13.9
16.1
28.2
–
MeOH/H
MeOH/H
MeOH/H
2
O
2
O
2
O
1
0
[
a] G=gel, PG=partial gel; P=precipitate, S=solution. [b] MGC=mini-
mum gelation concentration at RT.
100 mm, pH 6–8). Additionally, different mixed solvents were
also tested. As can be seen, trans-GAG formed a transparent
gel and a partial gel in DMSO/H O and MeOH/H O, respectively
2
2
(Table S1, entries 14–15), but only clear solutions in other sys-
4–6, and 10). Therefore, DMSO/H O (2:8, v/v) was chosen as
2
tems (Table S1, entries 16–21).
the solvent in the following study.
To optimize the gelation conditions, the gelation ability of
trans-GAG in DMSO/H O and MeOH/H O with different volume
To clarify the driving forces during the gelation process, UV/
1
Vis and H NMR spectroscopy experiments were performed. As
2
2
ratios was further tested. As shown in Table 1 and Figure 1,
shown in Figure S2, trans-GAG in DMSO exhibited a broad
band that ranged from l=250 to 500 nm with maxima cen-
tered at l=262 and 378 nm, which were attributed to the p–
p* transition of GA and the trans-Azo moiety, respectively.
Upon addition of water, a blueshift of 10 nm in the absorption
maximum was observed, which was mainly due to the forma-
stable gels were obtained with DMSO/H O at a volume ratio of
2
2
:8 (Table 1, entry 3), and with MeOH/H O at ratios of 3:7, 4:6,
2
and 5:5 (Table 1, entries 7–9). An increase in either water or or-
ganic phase content resulted in collapse of the gel on account
of the hydrophilic/hydrophobic balance (Table 1, entries 1–2,
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Figure 1. Photographs of the gelation results of trans-GAG in DMSO/H
2
O
and MeOH/H O at different volume ratios (the numbers correspond to the
2
entries in Table 1).
[
15c,16]
tion of H-aggregates primarily driven by p–p stacking.
It
1
was further confirmed by the H NMR spectra (Figure S3), in
Figure 2. AFM and SEM images of a,b) trans-GAG gel and c,d) cis-GAG sol in
DMSO/H O (2:8 v/v) at a concentration of 14 mm.
2
which the signals of trans-GAG became broad compared with
those recorded in [D ]DMSO. Additionally, the resonance of ar-
6
omatic protons belonging to the trans-Azo unit shifted from
d=7.87 to 7.78 ppm when the water content reached 20%
(
Table S2), which clearly indicated the existence of p–p stack-
[
3b,4d,17]
ing.
Additionally, signals for 12-H and methyl protons on
the triterpenoid backbone were shifted upfield from d=5.46
and 0.71 to 1.39 ppm to d=5.38 and 0.64 to 1.26 ppm
(
Table S2), respectively, which revealed the involvement of hy-
[
18]
drophobic effects. Notably, the signal from the amide proton
shifted downfield from d=9.59 to 9.61 ppm when the water
content reached 10%, and eventually disappeared with a fur-
[
19]
ther increase in water because of hydrogen bonding. Appa-
rently, the synergetic combination of p–p stacking, hydropho-
bic effect, and hydrogen bonding promotes gel formation.
Upon irradiation with UV light at l=365 nm for 30 min, the
yellow gel of trans-GAG in DMSO/H O (2:8 v/v) gradually con-
2
verted into a sol, as shown in Scheme 1, which strongly im-
plied the collapse of aggregation caused by trans-to-cis light
isomerization of the Azo moiety. Moreover, the resulting sol
could become a gel again within 12 h under visible light due
to reversible isomerization that promoted the reaggregation of
GAG molecules in the trans form. This gel–sol transition was
evaluated by examining AFM and SEM images. Spheres with a
diameter of around 200 to 500 nm were observed in the trans-
GAG gel (Figure 2a,b), whereas the microscopic structures
within the cis-GAG sol were converted to irregular aggregates
after irradiation with UV light (Figure 2c,d).
2
Figure 3. Conformation conversion of GAG (0.04 mm) in DMSO/H O (2:8 v/v)
a) from trans to cis by irradiation with UV light (l=365 nm, 12 W) and
b) from cis to trans by irradiation with visible light (40 W). c) Conversion
cycle test of cis-GAG/trans-GAG under alternated UV and visible light for
3
min. d) Conformation conversion from cis-GAG to trans-GAG after heating
at 808C.
ing irradiation with UV and visible light, this cycle could be re-
peated multiple times with no macroscopic retrogression (Fig-
ure 3c). In addition, the transition between trans- and cis-GAG
The switchable conformation behavior of trans-GAG was fur-
ther investigated by examining the UV/Vis spectra. As shown
in Figure 3a, trans-GAG displayed an absorption peak at l=
[1d,21]
could also be realized by heating.
As shown in Figure 3d,
the continuous heating of cis-GAG at 808C for 60 min resulted
in conversion to trans-GAG with absorption values from 0.28
to 0.74. Notably, this conversion was much slower than the
one observed with visible-light irradiation. Similar to the results
in Figure 3c, the reversible change between cis-GAG and trans-
GAG induced by UV light or heat could be repeated numerous
times (Figure S4). Given the above, the symmetrical GAG mole-
cule could reversibly convert between cis and trans conforma-
tions under either visible/UV light, or heat/UV light conditions.
To obtain a better insight into the reversible conformation
3
71 nm, which was attributed to the p–p* transition of the
[
4b,16a,20]
trans-Azo unit.
After irradiation with UV light at l=
365 nm, the band at l=371 nm decreased from 0.84 to 0.45
within 140 s until it reached a plateau, accompanied by an
slight increase at l=470 nm, which was attributed to the n–p*
[
3c]
transition of the cis-Azo moiety. This strongly indicated that
trans-GAG underwent a change in conformation to cis-GAG,
and that dynamic equilibrium was attained within 140 s. Con-
versely, the isomerization from cis-GAG to trans-GAG proceed-
ed relatively faster under visible light, and needed only 80 s to
recover from 0.45 to 0.84 (Figure 3b). Moreover, under alternat-
1
of GAG, H NMR spectroscopy experiments as a function of il-
lumination time were performed. As illustrated in Figure 4 and
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Figure 5. a) Oscillatory frequency and b) amplitude sweep of trans-GAG gel
on exposure to UV light for 0 and 30 min. c) Digital photos of the sol–gel
transition between shaking and resting, and the injection ability of the gel.
d) Oscillatory time sweep for the trans-GAG gel under alternating step
À1
strains of 1 and 70% (268C). Concentration 20 mgmL (DMSO/H
2
O 2:8 v/v).
the value of G’ from 220 to 150 Pa was observed, which re-
vealed that a low viscosity and a fast relaxation process were
produced. Furthermore, the critical point (crossing point of G’
and G’’) shifted from 11% to a low strain value of 7% under an
1
Figure 4. H NMR spectra of GAG upon irradiation with UV light (12 W) and
visible light (40 W) for different time periods (6.40 mm, * indicates
[D ]DMSO).
6
À1
applied frequency of 10 rads as the irradiation time pro-
Table S3, initially a mixture of 82% trans-GAG and 18% cis-
GAG was observed. Upon irradiation at l=365 nm, the proton
integrations of the Azo unit at l=7.87 ppm (a-H, b-H), amide
at d=9.59 ppm (c-H), and triterpenoid aglycon at around d=
gressed (Figure 5b), which further confirmed that rheological
properties of the GAG gel relied heavily on the percentage of
trans-GAG and cis-GAG in the material.
It should be noted that after conversion into a sol by shak-
ing, the trans-GAG gel turned back into a robust gel after rest-
ing (Figure 5c). This self-healing behavior was verified by the
alternate step strain test. According to the results in Figure 5b,
alternating 1 and 70% strains were applied with the frequency
5.46 and 0.71 to 1.39 ppm (12-H, 23-H–29-H) in trans-GAG
gradually declined, whereas those of the corresponding pro-
tons in cis-GAG (a’-H, b’-H, c’-H, 12’-H, 23’-H–29’-H) increased.
Eventually the spectrum reached a photostationary state after
À1
180 min and revealed around 23% trans-GAG and 77% cis-
fixed at 10 rads . As shown in Figure 5d, initially a 1% strain
GAG, which clearly indicated the conversion from trans to cis
conformation. It should be noted that, due to the concentrat-
ed solution used in the NMR spectroscopy experiment, more
time was needed to reach the photostationary state compared
with the one in UV/Vis measurement. Similar to the results in
Figure 3, a shorter time (only 5 min) was required when cis-
GAG reverted to its original level under visible light. It was ap-
parent that GAG could reversibly transform between cis and
trans conformation under alternating UV and visible light.
Rheological studies provide direct information on the viscoe-
lastic behavior of soft materials. Normally, the storage modulus
was applied to trans-GAG gel, for which G’ showed a higher
value than G’’, which revealed the distinct gel characteristics.
Subsequently, a 70% strain was applied, which resulted in a
drastic decrease in the value of G’ to below G’’, indicating that
the gel behavior was destroyed and the sol behavior was
prominent. After removing the high strain and applying a con-
stant strain of 1% again, the original mechanical strength of
the trans-GAG gel was almost recovered. Clearly, the large
strain (70%) destroyed the microstructure of the trans-GAG gel
and thus resulted in gel collapse and a drastic decrease in the
G’ value, whereas a small strain (1%) exerted a force to help
the remixing of trans-GAG molecules and, therefore, refabricat-
ed the inner network of the gel. These results strongly indicat-
ed the self-healing property of the trans-GAG gel, and made it
suitable as an injectable material. As shown in Figure 5c, trans-
GAG gel could easily be transferred into a syringe and extrud-
ed to create desired patterns on the substrate, which may
enable it to create desired structures by 3D printing.
(
G’) describes the ability of deformed materials to store energy,
and the loss modulus (G’’) corresponds to the ability to dissi-
pate energy. The G’ and G’’ values as a function of angular fre-
quency for trans-GAG before and after the exposure to UV
light are shown in Figure 5a. Before UV irradiation, the G’ value
was greater than the respective G’’ value, and invariant to the
frequency under an applied strain of 1% in the frequency
À1
range of 0.5 to 100 rads , which revealed the viscoelastic
Furthermore, the cell viability of trans-GAG was investigated
by co-culturing with the breast cancer cell line MCF-7. On ac-
count of the natural origin of GA, trans-GAG displayed out-
nature of the trans-GAG gel over the entire frequency
[
15b,22]
range.
After irradiating the gel for 30 min, a decrease in
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Characterization
1
13
H and C NMR spectra were recorded in [D ]DMSO or D O at
6
2
2
58C by using a JEOL JNM-ECA 400 spectrometer. Electrospray ion-
ization mass spectroscopy (ESI-MS) was performed by using a
Bruker ESQUIRE-LC spectrometer in positive mode. Matrix-assisted
laser desorption/ionization- time of flight-mass spectrometry
(MALDI-TOF-MS) was performed by using an AXIMA-Performance
in positive mode. UV/Vis spectra were measured by using a
Thermo Scientific Nanodrop 2000C spectrophotometer. Scanning
electron microscopy (SEM) was recorded by using a Hitachi S-4800
microscope operated at 10–15 kV. Atomic force microscopy (AFM)
measurements were performed by using a NanoScope 3D AFM
(Veeco, USA) in the tapping mode with a SiN4 tip (radius 10–
Figure 6. a) Cell viability and cell images after treatment b) without and
c) with trans-GAG for 24 h. Cells were stained with fluorescein diacetate
2
0 nm). Rheological experiments were performed in a plate geom-
etry (diameter 25 mm) on the rheometer plate by using a TA In-
strument.
(FDA, green) and propidium iodide (PI, red).
Cell viability
standing biocompatibility even at concentrations of up to
TM
50 mm, as determined by using a CellTiter-Blue assay (Prome-
The breast cancer cell line MCF-7 was provided by Norman Be-
thune Health Science Center, Jilin University, and grown in Dulbec-
co’s modified Eagle’s medium (DMEM) supplemented with 10%
ga; Figure 6a). As shown in Figure 6b,c, live cells were stained
green and dead cells were stained with red, which indicated
that the majority of MCF-7 cells survived in the presence of
trans-GAG (50 mm) for 24 h. However, cell viability and mor-
phology were the only parameters examined in this study as
preliminary indicators of the suitability of trans-GAG gel for
tissue-engineering applications. Future studies will include the
construction of cell-contained self-healing multilayer structures
in hybrid tissue engineering.
fetal bovine serum (FBS) in an atmosphere of 5% CO
Cells were seeded in a 96-well culture plate with a density of 1ꢁ
at 378C.
2
5
1
0 . After overnight incubation, the medium was replaced with
fresh DMEM that contained trans-GAG at different concentrations
and the plates were incubated for another 24 h. Next, CellTiter-
Blue reagent (20 mL) was added to each well and the cells were in-
cubated for a further 3 h to perform the cell viability assay. The
fluorescence intensity was measured at l=560/590 nm (E /E ) by
x
m
using a BioTek Synergy H1 microplate reader.
Conclusions
Cell imaging
In summary, a symmetrical glycyrrhizic acid functionalized azo-
MCF-7 cells were seeded in 24-well culture plate with a seeding
density of 4ꢁ10 . After overnight incubation, cells were treated
benzene gelator, trans-GAG, was synthesized and could form
4
stable gels in MeOH/H O and DMSO/H O, mainly promoted by
2
2
with DMEM that contained 50 mm trans-GAG and then co-cultured
at 378C. After 24 h, the samples were stained with a solution of
the synergistic effects of hydrogen bonding, p–p stacking, and
hydrophobic forces. Moreover, this gel exhibited a typical
light-responsive behavior, that is, a reversible gel–sol transition
occurred upon irradiation with alternating UV and visible light,
due to a cis-GAG/trans-GAG isomerization that could be con-
firmed by using UV/Vis spectroscopy, NMR spectroscopy, mor-
phology, and rheology studies. Furthermore, the trans-GAG gel
displayed a distinct injectable self-healing property and out-
standing biocompatibility, which gives the trans-GAG gel po-
tential applications in 3D printing. This work provides a simple
yet rational strategy to fabricate stimuli-responsive materials
from naturally occurring eco-friendly molecules.
À1
À1
FDA (5 mgmL ) and PI (10 mgmL ). Cells were imaged by using
an LSM 700 confocal laser scanning microscope imaging system
(
Carl Zeiss). FDA was excited by using a l=488 nm laser to emit
l=500–600 nm fluorescence, and PI was excited by using a l=
55 nm laser to emit l=560–700 nm fluorescence.
5
[9a]
Synthesis of trans-4,4’-azodianiline (trans-Azo)
Potassium superoxide (11.84 g, 166.45 mmol) and p-phenylenedia-
mine (4.50 g, 41.61 mmol) were dispersed in anhydrous tetrahydro-
furan (180 mL) and degassed with high-purity nitrogen for 3 min.
After heating at reflux for 24 h, the mixture was filtered and the fil-
trate was evaporated to remove the solvent. The crude product
was purified by using silica column chromatography (dichlorome-
thane/ethyl acetate 80:1 v/v) to afford trans-Azo as a crimson
Experimental Section
1
powder (0.62 g, yield 15%). H NMR (400 MHz, [D ]DMSO): d=7.52
6
(
4
1
d, J=8.0 Hz, 4H; Ph-H), 6.62 (d, J=8.0 Hz, 4H; Ph-H), 5.74 ppm (s,
General materials
13
H; -NH2); C NMR (100 MHz, [D ]DMSO): d=151.0, 143.1, 123.8,
6
Glycyrrhizic acid (GA) was purchased from J&K Chemical. N-(3-di-
methylaminopropyl)-N’-ethylcarbodiimide hydrochloride (EDC), po-
tassium dioxide, p-phenylenediamine, and 4-dimethylaminopyri-
dine (DMAP) were purchased from Aladdin. Other common chemi-
cals were from local commercial suppliers.
+
+
13.4 ppm; ESI-MS (+): m/z: 213 [M+H] , 254 [M+CH CN+H] .
3
Synthesis of acetyl- and methoxyl-protected glycyrrhizic
acid (GAP)
[23]
A solution of GA (10.00 g, 12.15 mmol) in methanolic HCl (2 wt%,
3
60 mL) was stirred at RT for 10 h, and then an appropriate
amount of triethylamine was added to tune the pH to 7. After re-
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1
moval of the solvents, the intermediate was dissolved in a mixture
of pyridine and acetic anhydride (1:1 v/v, 300 mL), and the reaction
solution was stirred at RT for 36 h. Then the mixture was poured
slowly into ice water and the precipitate was filtered. The crude
product was further purified by using silica column chromatogra-
(2.20 g, yield 95%). H NMR (400 MHz, [D ]DMSO): d=0.71 (s, 2ꢁ
6
3H; 24-CH ), 0.75 (s, 2ꢁ3H; 28-CH ), 0.95 (s, 2ꢁ3H; 23-CH ), 1.03
(s, 2ꢁ2ꢁ3H; 25, 26-CH ), 1.21 (s, 2ꢁ3H; 29-CH ), 1.39 (s, 2ꢁ3H;
27-CH ), 2.34 (s, 2ꢁ1H; 9-H), 3.56 (d, J=8.0 Hz, 2ꢁ1H; 5’-H), 3.64
(d, J=8.0 Hz, 2ꢁ1H; 5’’-H), 4.41 (d, J=8.0 Hz, 2ꢁ1H; 1’’-H), 4.50
(d, J=4.0 Hz, 2ꢁ1H; 1’-H), 5.46 (s, 2ꢁ1H; 12-H), 7.87 (m, 8H; Azo-
H), 9.58 (s, 2ꢁ1H; CONH), 12.60 ppm (s, 2ꢁ2ꢁ 1H; COOH);
3
3
3
3
3
3
phy (dichloromethane/methanol 80:1 v/v) to give GAP as a white
1
solid (5.27 g, yield 41%). H NMR (400 MHz, [D ]DMSO): d=0.75 (s,
6
13
3
3
1
H; 28-CH ), 0.76 (s, 3H; 24-CH ), 0.98 (s, 3H; 23-CH ), 1.04 (s, 2ꢁ
C NMR (100 MHz, [D ]DMSO): d=198.9, 174.9, 170.1, 169.9, 169.5,
6
3
3
3
H; 25, 26-CH ), 1.10 (s, 3H; 29-CH ), 1.35 (s, 3H; 27-CH ), 1.91,
147.7, 142.0, 127.5, 123.1, 120.4, 104.7, 103.5, 88.2, 82.6, 76.2, 75.8,
75.6, 75.2, 74.8, 71.5, 71.2, 61.1, 54.3, 47.9, 44.8, 44.2, 42.9, 40.6,
38.5, 36.3, 32.1, 31.5, 30.4, 28.3, 27.9, 27.1, 26.1, 25.9, 25.7, 22.9,
3
3
3
.94, 1.95, 1.96, 2.11 (s, 5ꢁ3H; OCCH ), 2.33 (s, 1H; 9-H), 3.06 (dd,
3
J =12.0 Hz, J =4.0 Hz, 1H; 3-H), 3.62 (s, 2ꢁ3H; OCH ), 3.67 (t, J=
1
2
3
+
8
1
.0 Hz, 1H; 5’-H), 4.38 (t, J=8.0 Hz, 1H; 5’’-H), 4.54 (d, J=12.0 Hz,
H; 1’-H), 4.67 (t, J=10.0 Hz, 1H; 2’’-H), 4.76 (d, J=4.0 Hz, 1H; 2’-
18.6, 18.3, 16.2, 15.9 ppm; ESI-MS (+): m/z: 1845 [M+Na] .
H), 4.86 (t, J=10.0 Hz, 1H; 4’-H), 4.93 (t, J=10.0 Hz, 1H; 4’’-H), 4.99
d, J=8.0 Hz, 1H; 3’-H), 5.24 (t, J=8.0 Hz, 1H; 3’’-H), 5.32 (t, J=
Acknowledgements
(
1
0.0 Hz, 1H; 1’’-H), 5.40 (s, 1H; 12-H), 12.18 ppm (s, 1H; COOH);
1
3
C NMR (100 MHz, [D ]DMSO): d=199.0, 169.80, 169.76, 169.5,
This work is supported by National Key R&D program of China
6
1
7
4
2
69.3, 169.1, 167.7, 166.9, 127.3, 102.0, 99.3, 89.4, 76.5, 73.0, 71.7,
1.1, 70.9, 70.7, 69.4, 69.2, 61.0, 54.2, 52.4, 48.0, 44.9, 43.1, 42.9,
0.7, 37.5, 36.3, 32.1, 31.5, 28.4, 27.9, 27.0, 26.1, 25.8, 25.4, 23.0,
0.6, 20.2, 20.1, 18.3, 16.9, 16.2, 15.6 ppm; ESI-MS (+): m/z: 1062
(
no. 2017YFD0200302), the NSFC (no. 21604085), the Jilin Sci-
ence Foundation for Youths (no. 20160520135JH), and the Fun-
damental Research Funds for the Central Universities (ZY1831).
J.H. thanks the State Key Laboratory of Polymer Physics and
Chemistry, CIAC, for support.
+
+
[M+H] , 1079 [M+NH ] .
4
Synthesis of N,N’-(azobenzene-4,4’-diyl)-diprotected glycyr-
rhizic amide (trans-GAGP)
Conflict of interest
trans-Azo (0.33 g, 1.57 mmol) was added to a solution of GAP
The authors declare no conflict of interest.
(
5.00 g, 4.71 mmol), EDC (0.96 g, 5.03 mmol), and DMAP (0.61 g,
.03 mmol) in chloroform (55 mL), and the mixture was heated at
5
reflux for 62 h. After removal of the solvent, the crude product was
dispersed by water with ultrasonication for 10 min, then the pre-
cipitate was collected and redissolved in dichloromethane. The or-
ganic phase was washed with water and brine, and then dried
over Na SO . After evaporation of the solvent, the crude product
Keywords: azo compounds · gels · glycyrrhizic acid · self-
assembly · terpenoids
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.12 (s, 2ꢁ5ꢁ3H; OCCH ), 2.35 (s, 2ꢁ1H; 9-H), 3.05 (dd, J =4.0 Hz,
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Version of record online: && &&, 0000
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FULL PAPER
Sol–Gel Processes
Make light work of gelation: A sym-
metrical azobenzene-functionalized nat-
ural glycyrrhizic acid (trans-GAG; see
figure) was synthesized, and could form
a stable light-responsive supramolecular
gel due to cis–trans isomerization. This
work provides a simple yet rational way
to fabricate functional materials from
naturally occurring eco-friendly triter-
penes in light of their biocompatibility
and biodegradability.
Heshu Fang, Xia Zhao, Yuan Lin,
Song Yang,* Jun Hu*
&& – &&
A Natural Glycyrrhizic Acid-Tailored
Light-Responsive Gelator
&
&
Chem. Asian J. 2018, 00, 0 – 0
www.chemasianj.org
8
ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!