Water-induced gel formation of an oleanlic acid-adenine conjugate and the effects of uracil derivative on the gel stability

Article abstract of DOI:10.1039/c2sm26085a

The conjugate of oleanlic acid with adenine was synthesized and it could be gelled in mixed solvents of THF and water, but no gel was formed in the single organic solvent. The self-aggregation behaviour and physical properties of the organogel were characterized by ¹H NMR, FT-IR, and scanning electron microscopy. The morphology and the stability of the gel were remarkably affected by the uracil derivative through destroying hydrogen-bonding.

Full text of DOI:10.1039/c2sm26085a

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PAPER
Water-induced gel formation of an oleanlic acid–adenine conjugate and the
effects of uracil derivative on the gel stability†
a
a
ac
Jinrong Lu,a Jun Hu,ab Chulong Liu, Hongxin Gao and Yong Ju
*
Received 9th May 2012, Accepted 11th July 2012
DOI: 10.1039/c2sm26085a
The conjugate of oleanlic acid with adenine was synthesized and it could be gelled in mixed solvents of
THF and water, but no gel was formed in the single organic solvent. The self-aggregation behaviour
and physical properties of the organogel were characterized by 1H NMR, FT-IR, and scanning electron
microscopy. The morphology and the stability of the gel were remarkably affected by the uracil
derivative through destroying hydrogen-bonding.
scaffolds in organogel molecules requiring multiple non-cova-
Introduction
lent interactions between the building blocks.8 Among LMOGs,
aromatic–linker–steroid (ALS) is one classic type,9 and there are
many gelators belonging to this type, such as cholesterol-based
gelators10 and bile acid-based gelators.11 Besides, steroids used
as building blocks in gelator molecules, and another natural
product—triterpenoids, which are biogenic homologues of
steroids also began to emerge in architectures of organogel.12
Oleanlic acid (I), a natural pentacyclic triterpenoid, has been
reported for its therapeutic effect on human liver disorders and
antitumor-promotion effects due to its relatively low-toxicity,
biocompatibility and bioactivity.13 In recent years, there are
more and more reports about triterpenoids as scaffolds applied
in molecular recognition14 and suprachemistry,15 since imitating
and modifying these natural products can minimize the tedious
synthetic work in designing new self-assembling architectures. In
our previous work, the organogelation ability of a modified
triterpenoid, 2,3-dihydroxyiminooleanlic acid was reported.16
The supramolecular gel of pentacyclic triterpenoid derivatives
were focused on and then we further developed a fan-shaped C3
molecule containing three glycyrrhetinic acid units with gelation
properties in mixed organic solvents and a tweezer based on
glycyrrhetinic acid appended with uracil moiety as a dual-
responsive organogel, respectively.17 We have taken a journey
with the organogel chemistry based on triterpenoid molecules
starting from the matter of serendipity toward expected designs.
In this paper, an oleanlic acid–adenine conjugate IV (Scheme 1)
was designed and synthesized, which not only keeps the rigid
skeleton structure of oleanlic acid and capacity for self-assembly,
but also imparted non-covalent bond association through the
hydrogen-bonding interactions and p–p stacking provided by
adenine units, which was often reported as the building block in
constructing functional molecules in biology and supramolecular
chemistry.18 The results showed that conjugate IV could self-
assemble into fibrillar networks (SAFINs) in THF and water
affording the formation of gels, which can respond to the
complementary uracil derivative V.
The recent focus of interest in supramolecular chemistry has been
directed towards the creation of one-dimensional supramolec-
ular nanoarchitectures.1 To control the molecular arrangement
in one-dimensional structure, low molecular-mass organic gela-
tors (LMOGs) are fascinating candidates due to their variety of
self-assembling capabilities and functionalities.2 These molecules
could form one-dimensional fibrous aggregates to trap the
solvent molecules through self-assembly, and the driving forces
used for gel formation are always hydrogen-bonding interac-
tions, van der Waals forces, p–p stacking and donor–acceptor
interactions, and so on.3 With the development of gel chemistry,
further initiatives towards functionalization of the conventional
LMOGs to develop ‘‘smart’’ materials are put on the agenda
gradually.4 Advantageously, when additional additives, which
can interact with gel molecules through exact molecular recog-
nition are added, the morphology and the functionality of the
created one-dimensional architecture is easily affected.5 So the
response of the designed molecular gels to various stimuli has
considerable potential in the development of tunable and
responsive materials for controlled drug delivery.6
Up to now, much effort has been made to explore the
structure–property relationship of gelators, but it is still difficult
to judge the gelation ability of a compound.7 To our knowledge,
functional groups, such as amide, urea, carbohydrate, steroid,
nucleobase, and long chain alkanes, are often found as potent
aKey Laboratory of Bioorganic Phosphorus Chemistry
& Chemical
Biology, Ministry of Education, Department of Chemistry, Tsinghua
University, Beijing, 100084, China. E-mail: juyong@tsinghua.edu.cn
bDepartment of Chemistry and Biochemistry, University of South Carolina,
Columbia, SC, 29208, USA
cState Key Laboratory of Applied Organic Chemistry, Lanzhou University,
Lanzhou, 730000, China
† Electronic supplementary information (ESI) available: synthesis and
structure data of all compounds. Binding study of conjugate IV with
uracil derivative V by 1H NMR. See DOI: 10.1039/c2sm26085a
9576 | Soft Matter, 2012, 8, 9576–9580
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Fig. 3 IR spectra of IV in solid, xerogel (1.0 g 100 cm3) made from
THF : H2O (1 : 1).
Scheme 1 (a) CH3I, DMF, rt; (b) BrCOCH2Br, K2CO3, dry CHCl3, rt;
(c) adenine, K2CO3, DMF, rt; (d) K2CO3, DMF, rt.
Table 1 Gelation test results of IV in a THF–water mixture (G ¼ gel)
Fig. 4 The process of gel formation in THF as is water added. (a) Turbid
solution of conjugate IV in THF-d8 (10 mg/0.5 mL), (b) clear solution of
conjugate IV in THF-d8 with 10 mL D2O, (c) opaque gel in THF-d8 with
300 mL D2O.
GMC (g 100
cm3)
Entry
Solvent
State
1
2
3
2 : 1 THF : H2O
2 : 2 THF : H2O
2 : 3 THF : H2O
1.3
0.7
0.2
G
G
G
Fig. 1 The gels of conjugate IV in (a) THF : H2O ¼ 2 : 1 (b) THF :
H2O ¼ 1 : 1 (c) THF : H2O ¼ 2 : 3.
Fig. 5 Partial 1H NMR (600 MHz) of a solution of IV in THF-d8 with
(a) 10 mL D2O (sol); (b) 120 mL D2O (sol); (c) 300 mL D2O (gel).
Fig. 2 Scanning electron micrographs of gels from IV in (a) THF :
H2O ¼ 2 : 1 (b) THF : H2O ¼ 1 : 1 (c) THF : H2O ¼ 2 : 3.
Fig. 6 (a) Evolution of G0 as a function of the applied shear stress at
different contents of water in the mixture solvents. (b) Evolution of G0
and G00 of the gel as a function of the applied shear stress.
Synthesis
The synthesis of the oleanlic acid–adenine conjugate IV was out-
lined in Scheme 1. Oleanlic acid I reacted with iodomethane in
DMFto give II, and wasthentreated with bromoacetylbromide to
afford III in high yield. Finally, conjugate IV was obtained by
reaction of III with adenine in DMF with the yield of 60%.
Meanwhile, ann-butylderivativeofuracilwassynthesizedthrough
the treatment of uracil with n-butyl bromide in DMF in the pres-
ence of K2CO3. Compound VI without an oleanlicacid moietywas
synthesized by the same procedure as that described for IV.
Results and discussion
Water-induced physical gelation of THF
To investigate the optimized gelation conditions, conjugate IV
was tested in 20 different organic solvents including protic/aprotic
and polar/apolar solvents. It was found that conjugate IV had
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revealed that the volume of solvents gelled by a given amount of
gelator IV depend upon the volume of added water and the MGC
decreased with the increased water volume. Moreover, the opacity
of the gels was more and more apparent by increasing the volume
ratios of water (Fig. 1). Like other gel systems,19 this gel was also
thermoreversible in this mixed solvent.
In general, the gelation system must be the result of a gelator
network formed during the shaking process, and scanning elec-
tron microscopy (SEM) was used to measure these kinds of
networks or gelator aggregates.20 As shown in Fig. 2, conjugate
IV could self-assemble into nanoscale fibrous structures with
regular fiber diameters of ca. 25–80 nm in THF and water. By
entanglement of such nano-fibers, a closely packed 3D network
structure was created to trap the solvent molecules into its
interstices. On the other hand, the proportion of water had a
considerable effect on the morphology of the assembly. Obvi-
ously, comparing Fig. 2a, b and c, the more transparent the gel
was, the more extensive thin entanglement and flexible fibrous
structure it had, which was consistent with the volume ratio of
water. A reasonable explanation was that more water in the
system leads to a much looser arrangement of fibres in the same
direction and in parallel, which could not capture the solvents
effectively,21 resulting in a more apparent gel.
Fig. 7 Tgel of gels formed from IV (16 mmol dmꢁ3) in THF and H2O
(1 : 1) plotted against additive V concentrations.
Fig. 8 Binding isotherms for imide protons of the uracil units of n-butyl
In order to confirm the driving forces of gel formation, we
compared the infrared spectra of the solid and xerogel (made from
THF and water) (Fig. 3).22 The stretching vibration bands of the
N–H moiety for xerogel (3333 cmꢁ1) appeared at a lower wave-
number compared with that of the solid (3347 cmꢁ1), and this
phenomenon revealed that hydrogen bonding was involved in the
organogel formation. Compound VI without an oleanlic acid
moiety was designed and synthesized as a control to further
exploretheroleoftheoleanlicacidskeletonintheformationofgels
(Scheme 1). It displayed no gelation abilities in the same condi-
tions, suggesting packing interaction between triterpenoid skele-
tons also had a synergistic effect in the formation of gel besides
intermolecular hydrogen bonding between adenines. In the recent
decade, Bag et al.23 and our group16,17 had already reported several
organogels based on pentacyclic triterpenoid derivatives to inter-
pret the special functions provided by these rigid skeletons.
Obviously, our new oleanlic acid–adenine gel system offered
another approach to understand the structure–property relation-
ships of this kind of triterpenoid natural products.
uracil.
Fig. 9 1 : 1 complex of conjugate IV and V.
The role of water
The subtle self-assembly process of conjugate IV as described
above was influenced by different factors including concentra-
tion, solvent, and especially additive water. Gelator IV is crys-
talline and has a high melting point 575–578 K, which must be
due to stronger hydrogen-bonding interactions between adenine
groups and van der Waals forces. Though conjugate IV was
unable to dissolve and form gel in any single solvent including
water or THF due to the strong intermolecular hydrogen-
bonding and van der Waals forces it could gel THF in the
presence of water. It was conceivable that the addition of water
as an hydrogen-bonding solvent broke or weakened the
hydrogen bond among gelators and thus enhanced the solubility
in solvents, which clearly suggested that water played a major
role in the gelation process. In fact, it is now well accepted that
Fig. 10 SEM picture of the xerogel prepared from conjugate IV and V
mixture (a) [V] : [IV] ¼ 2; (b) [V] : [IV] ¼ 6.
poor solubility in most of the solvents except DMF and DMSO
even at 60 ꢀC. However, to our surprise, when the suspensions of
conjugate IV in THF were added to a small amount of water, a
clear solution formed, and it could transform to an opaque gel
upon further continuously increasing water under sonication. In
this process, some flocculants appeared slowly. It was noted that
the gelation volume ratio of the two solvents are within the range
of 2 : 1 to 2 : 3 (THF : H2O). In order to determine the gel ability
of IV, the minimum gelator concentration (MGC) in different
volumes of water in THF was summarized in Table 1. The results
9578 | Soft Matter, 2012, 8, 9576–9580
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gelation occurs as a result of a subtle balance between a tendency
of the gelator molecules to dissolve and a tendency to aggregate
leading to precipitation or crystallization.24
always readily affected by the additive.28 It thus occurred to us
that when some complementary nucleobase is added to the gel
system, it should interact with the gelator molecule to change the
gelation properties. To clarify the additive effect, we measured
the Tgel values of IV as a function of added uracil derivative V
concentration. As shown in Fig. 7, the addition of V induced a
significant decrease in the Tgel values, suggesting this derivative
efficiently cleaves the intergelator hydrogen bonds, which
contributes to gel stabilization. Meanwhile, the binding study of
conjugate IV with uracil derivative V was also carried out by
using 1H NMR titration method to confirm that the Tgel decrease
was mainly due to the hydrogen-bond cleavage (ESI†). To a 0.01
M solution of n-butyl uracil V in CDCl3, various aliquots of
In order to investigate how the intermolecular interactions of
water with the gelator regulate such supramolecular arrange-
ments of individual gelators in the 3-D framework, 1H NMR
analysis of conjugate IV in THF-d8 with different volumes of
D2O which contains HDO as an impurity was performed. When
10 mL of water was added to the turbid solution of 10 mg of
conjugate IV in 0.5 mL of THF-d8, the solution transformed to
transparent and it would change to an opaque gel until the added
D2O was up to 300 mL (Fig. 4). The change of d values of HDO
with its concentration is shown in Fig. 5. It exhibited a significant
downfield shift from d 3.4 to d 4.4 ppm with the increase of D2O
due to the formation of hydrogen bonds between the first added
D2O molecule and the NH group. The hydrophobic and
hydrophilic interactions as well as the hydrogen bonding
promoted molecules to become arrange more orderly when more
water was added, resulting in the gel formation. Moreover, the
variable-temperature 1H NMR of gel IV in THF and water
(1 : 1) were also performed. The results showed that the HDO
resonance signals were gradually shifted upfield with a temper-
ature increase (Fig. S1 in ESI†), clearly indicating formation of
hydrogen bonds between the D2O molecule and the adenine
group. It illustrated that the gelation can be regulated by tuning
the hydrogen-bonding interaction of the molecule, and water
could induce conjugate IV to self-assemble into fibrillar
networks, which could trap THF molecules.
1
0.002 M solution of IV were added and H NMR spectra were
recorded after each addition. It was observed that the shifts of
amide N–H protons of uracil moieties moved upfield as the
concentration of IV increased (Fig. 8). The 1 : 1 binding mode
was confirmed by plotting changes in the chemical shift of N–H
as well as the binding constant of 47.1 Mꢁ1 being calculated
(see ESI†).29 Analysis of the 1H NMR spectra revealed that each
addition of adenine derivative to the uracil derivative molecule
resulted in the upfield shift of only H-2 of adenine moiety, and it
suggested that the Watson–Crick site of adenine was involved in
complexation (Fig. 9).30
SEM images further revealed that the complementary
hydrogen-bonding interaction between gelator IV and V caused
drastic changes of the molecular packing among gelator IV
molecules. As shown in Fig. 2a, one could recognize that a gel of
IV gave a well-developed fibrous network structure. However,
when gelator IV was mixed with the complementary nucleobase
derivative IV, morphological changes can be observed accom-
panied with a destroyed closely packed 3D network structure
and stuck together fibers, appearing as a sheet like crystalline
structure (Fig. 10).31
Rheological studies
The mechanical performance of a material is extremely impor-
tant for its practical uses.25 To explore this property of the gel in
detail, rheological measurements were conducted on a series of
gels, which had been prepared in different volume ratios of the
mixture solvents. The storage modulus G0 and the loss modulus
G00 which characterize the visco-elastic behaviour of the gels were
measured as functions of shear stress at a constant frequency of
1.0 Hz at 25 ꢀC (Fig. 6a). It showed that the value of G0 increased
from 9180 to 37 000 Pa with increasing water content from 33%
to 60% and the yield stress increased from 99 to 370 Pa, sug-
gesting that both the stability and the elastic property of the gels
are dependent upon the water amount. The results may be
understood by the fact that increase in the volume ratio of water
must enhance the strength of the gel networks.26
Conclusions
We have demonstrated that incorporation of an adenine moiety
affording hydrogen-bonding interactions is an effective
approach to guide the self-assembly of an oleanlic acid skeleton.
The oleanlic acid–adenine conjugate self-assembled fibrillar
networks to form a gel in THF induced by water, which is due to
the hydrophobic interaction of the oleanane skeleton and the
balance between a tendency to dissolve and a tendency to
aggregate. The supramolecular gel stability and morphology
were sensitively affected by addition of complementary uracil
derivatives through cleaving the intergelator hydrogen bonds.
Due to the biocompatibility and bioactivity of oleanlic acid, it
may be used in biomedical engineering and the drug industry.
All these gels exhibit a clear thixotropic property and the gel of
a mixed solvent of THF and water (2 : 3 in volume) with the
compound (1.0 g 100 cm3) was chosen as an example and its
thixotropic property was studied. As shown in Fig. 6b, the
beginning G0 was about fourteen times greater than G00, indi-
cating the dominant elastic character of the gel.27 A sudden
decrease in the two values is observed above 350 Pa indicating the
breakup of the gel networks under high shearing force, in other
words an indication of a dominant fluidity character.
Acknowledgements
The project is supported by NSFC (nos. 20972086, 21172130)
and NBRPC (973 Program, no. 2012CB821600).
Notes and references
The effects of uracil derivative on the gel stability
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