Cucurbit[7]uril-based vesicles formed by self-assembly of supramolecular amphiphiles

Article abstract of DOI:10.1002/cjoc.201200603

Cucurbituril (CB), a well-known macrocyclic cavitand, has been used extensively to construct supramolecular aggregates. Based on host-guest intertactions, an adamantanyl derivative guest molecule was designed and synthesized to prepare a supramolecular am

Full text of DOI:10.1002/cjoc.201200603

FULL PAPER
DOI: 10.1002/cjoc.201200603
Cucurbit[7]uril-Based Vesicles Formed by Self-assembly of
Supramolecular Amphiphiles†
Li, Jiaxi(李佳锡)
Zhou, Lipeng(周黎鹏)
Lu, Wei(卢伟)
Luo, Quan*(罗全)
Xu, Jiayun(徐家云)
Wang, Yongguo(王永国)
Liu, Junqiu*(刘俊秋)
Zhang, Chunqiu(张春秋)
Key Laboratory of Supramolecular Structure and Materials College of Chemistry, Jilin University, Changchun,
Jilin 130012, China
Cucurbituril (CB), a well-known macrocyclic cavitand, has been used extensively to construct supramolecular
aggregates. Based on host-guest intertactions, an adamantanyl derivative guest molecule was designed and syn-
thesized to prepare a supramolecular amphiphile with cucurbit[7]uril. In aqueous solution, the cucurbit[7]uril based
supramolecular amphiphiles self-assemble into well-defined vesicles, and their disassembly can be achieved by
addition of excess competitive agent 1-adamantanamine hydrochloride. This vesicle functions as a new nanocapsule
to encapsulate molecules within its hollow cavity. Through competitive disassembly of supramolecular amphiphiles,
the vesicles behave as a novel drug delivery carrier.
Keywords self-assembly, cucurbituril, vesicles, supramolecular amphiphiles, drug delivery
Introduction
CB[8]. CB[7] has an internal cavity with a diameter of
7.3 Å and a portal diameter of 5.4 Å.[41] The size of the
cavity is similar to that of β-cyclodextrin (β-CD), which
is a typical host-guest pair with adamantane. The previ-
ous complex experiments demonstrated that the cavity
inside CB[7] is ideally suitable for the encapsulation of
adamantane.
Supramolecular chemistry of CB[n], including their
host-guest chemistry, has been studied extensively by
Mock,[39,42] Kim,[41,43-49] and others.[50-61] As known,
however, the supramolecular architectures through mul-
tilevel self-assembly based on CB[7] have rarely been
reported before.[62] Herein, we report a novel way to
construct vesicles triggered by direct self-assembly of
CB[7]-based host-guest superamphiphiles (Scheme 1).
The disassembly of vesicles can be controlled by adding
a guest analogue to compete with the guest molecule
within the CB[7] cavity. Using fluorescent rhodamine B
(RB) as a drug model, the controlled release experi-
ments have been performed. The results demonstrated
that this new kind of supramolecular assembly has great
potential to function as drug-loaded nanocapsules for
controlled drug release.
Self-assembled vesicles have been a subject of in-
tense research for the last three decades[1-4] because of
their potential applications in various areas such as the
development of biomimetic systems,[5] drug/gene deliv-
ery systems[6-9] and nanostructured materials.[10-12] A-
mong a number of designed vesicles, vesicles formed by
self-assembled supramolecular amphiphilic molecules
have attracted much attention.[13-17] In recent years, su-
pramolecular amphiphiles formed through noncovalent
driving forces have been developed as a new type of
building block for future fabrication of supramolecular
architectures through multilevel self-assembly.[18-27] A-
mong them, the host-guest interactions, such as cucur-
bit[n]uril and cyclodextrin systems, have been proven to
be important in the construction of supramolecular am-
phiphiles. The self-assembly of host-guest superamphi-
philes can provide opportunities not only for structural
versatility but also for functional modulation of nanoma-
terials.[24,28-38] For example, the easily controlled assem-
bly and disassembly can be used to realize drug delivery.
Cucurbit[n]uril (CB[n] n＝5—8), a family of macro-
cyclic compounds comprising n glycoluril units, has a
hydrophobic cavity that is accessible through two iden-
tical carbony-fringed portals.[39,40] These homologues
have much larger internal cavities and portals and thus
can be expected to have much improved host abilities as
compared to cucurbituril itself, especially CB[7] and
Experimental
Materials
Glycoluril was purchased from Yinghua Co. (Tai-
*
E-mail: junqiuliu@jlu.edu.cn; luoquan@jlu.edu.cn; Tel.: 0086-0431-85168452; Fax: 0086-0431-85168491
Received June 20, 2012; accepted July 10, 2012.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201200603 or from the author.
Dedicated to 80th Anniversary of Chinese Chemical Society.
†
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Scheme 1 Synthetic routes of the guest molecule 1
1
recrystallization. H NMR (D2O, 500 MHz) δ: 5.75 (d,
14H, CH2×14), 5.50 (s, 14H, CH×14), 4.21 (d, 14H,
CH2×14).
COOCH3
COOCH3
(1) OH-
(2) H+
n-C12H25Br/K2CO3
N2/70 oC
Synthesis of adamantanol-1-3,4,5-trihydroxy-ben-
zoicamide (guest)
HO
OH
OH
C12H25O
OC12H25
OC12H25
5
The guest molecule was synthesized according to the
method reported previously (Scheme 1).[64] Methyl
3,4,5-trihydroxybenzoate (4, 1 g, 5.43 mmol) and
roasted K2CO3 (5 g) were added to a deoxygenated
mixture of DMF (15 mL) and 1-bromododecane (6 mL,
25.03 mmol) under nitrogen for 1 h. Then the mixture
was heated at 70 ℃ for 12 h. Plentiful water was added
when the reaction mixture was cooled to room tem-
perature. The crude product was filtrated and recrystal-
lized for three times in acetone to get 5 as a yellow solid
4
COCl
COOH
SOCl2
C12H25
O
OC12H25
C12H25
O
OC12H25
OC12H25
NEt3
OC12H25
7
6
+
-
NH2
NH3Cl
1
9
(3.31 g, 89% yield). H NMR (CDCl3, 500 MHz) δ:
8
7.25 (s, 2H, ArH), 4.03—3.99 (m, 6H, OCH2×3), 3.89
(s, 3H, OCH3), 1.84—1.78 (m, 4H, CH2×2), 1.77—
1.72 (m, 2H, CH2), 1.50—1.44 (m, 6H, CH2×3), 1.34
—1.26 (m, 48H, CH2×24), 0.88 (t, J＝7.1 Hz, 9H,
CH3×3).
C12H25
O
O
C
H
C12H25
O
N
C12H25
O
A solution of 5 (1.0 g, 1.45 mmol) and excess potas-
sium hydroxide (5.6 g, 0.1 mol) in ethanol (100 mL,
95%) was kept refluxing for 2 h. Hydrochloric acid
(15%) was added to make the solution acidic. A large
amount of water was added and the resulting precipitate
was collected by filtration. After recrystallization from
acetone for 3 times, 3,4,5-trihydroxybenzoic acid (6)
1
yuan, China) and further purified by recrystallization for
three times in water. Methyl 3,4,5-trihydroxybenzoate
(methyl gallate) of 98% was obtained from Alladdin Co.
(Shanghai, China). 1-Bromododecane of 98% and
1-adamantanamine hydrochloride of 99% were pur-
chased from Alfa Aesar. Dialysis membrane (MWCO:
2000) was obtained from Sangon Biotech Co., Ltd
(Shanghai, China). In all the experiments, organic sol-
vents were reagent or high-performance liquid chro-
matography (HPLC) grade, and doubly distilled water
was used.
1
was obtained as a white solid (0.78 g, 76% yield). H
NMR (CDCl3, 500 MHz) δ: 7.32 (s, 2H, Ar), 4.06—
4.01 (m, 6H, OCH2×3), 1.85—1.79 (m, 4H, CH2×2),
1.78—1.72 (m, 2H, CH2), 1.51—1.45 (m, 6H, CH2×3),
1.35—1.27 (m, 48H, CH2×24), 0.88 (t, J＝7.1 Hz, 9H,
CH3×3).
An aqueous solution of sodium hydroxide was added
slowly to a stirred solution of amantadine hydrochloride
(310 mg, 1.65 mmol) in deionized water (20 mL). The
solution was extracted by dichloromethane for three
times and the solvent was removed by rotary evaporator.
The residue was dried under vacuum to yield a white
solid 9, which was used for the next step without further
purification.
3,4,5-Trihydroxybenzoic acid (6, 1.0 g, 1.48 mmol)
was dissolved in some thionyl chloride (about 1 mL)
and stirred for 3 h at room temperature to get 7. The
volatiles were evaporated and the residue was redis-
solved together with dry triethylamine (3 mL) in anhy-
drous dichloromethane. The solution of 9 in 20 mL an-
hydrous dichloromethane was added into the above
mixture solution and stirred overnight with ice bath. The
volatiles were removed and the residue was purified by
column chromatography (silica gel, dichloromethane as
eluent) to give an orange solid 1 (0.82 g, 69% yield). 1H
NMR (CDCl3, 500 MHz) δ: 6.90 (s, 2H, ArH), 5.68 (s,
1H, CONH), 4.01—3.95 (m, 6H, OCH2×3), 2.12 (s,
9H, CH2×3 & CH×3 in adamantane), 1.82—1.70 (m,
12H), 1.49—1.43 (m, 6H, CH2×3), 1.35—1.26 (m,
Synthesis of cucurbit[7]uril (host)
The host molecule (cucurbit[7]uril) was synthesized
according to the method reported by Kim.[63] Glycoluril
(5.68 g, 40 mmol) was reacted with formaldehyde (w＝
37%, 7.0 mL) in 9 mol•L－ sulfuric acid (20 mL) at 75
1
℃ for 24 h and then at 100 ℃ for 12 h. After the reac-
tion mixture was poured into water (200 mL), acetone
(1.0 L) was added to produce precipitate. The precipi-
tate was separated by decantation, washed with water/
acetone (V∶V＝1∶4), and filtered. 300 mL of water/
acetone (V∶V＝1∶2) was added to the resulting solid
and stirred for a few minutes. The precipitate is the ma-
jor product CB[6] that was separated by filtration. Ace-
tone (800 mL) was added to the filtrate to produce pre-
cipitate which was filtered, washed with acetone and
dried under vacuum to produce a mixture of most CB[5]
and CB[7]. The mixture of CB[5] and CB[7] was dis-
solved in water (75 mL). Methanol (75 mL) was added
to the solution to produce precipitate which was filtered,
washed with methanol (10 mL) and dried under vacuum
to yield a white power containing most CB[7] (600 mg).
The further purification of CB[7] can be achieved by
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Cucurbit[7]uril-Based Vesicles Formed by Self-assembly of Supramolecular Amphiphiles
48H, CH2×24), 0.88 (t, J＝7.1 Hz, 9H, CH3×3).
MALDI-MS calcd for C53H93NO4 [M＋H]＋ 807.7,
found 808.8.
stirrer was used to slowly stir the release medium. Sam-
ples (1 mL) were taken out from the deionized water
outside the dialysis bag and replaced with the same
amount of deionized water at given time intervals. The
release experiment was analyzed over a time period of
12 h and the amount of released RB was determined
using fluorometry.
Preparation of vesicles
In a general preparation procedure, the purified guest
molecules 1 (8.08 mg) were dispersed in DMF (1 mL)
by ultrasonic till a clear solution was obtained at 40 ℃.
Then DMF solution (100 μL) was slowly injected into a
cuvette with 9.9 mL CB[7] solution (100 μmol/L) in it
at 40 ℃ with ultrasonication. Opalescence appeared
immediately, which indicated the formation of aggre-
gates. The fine dispersion was cooled to room tempera-
ture at ambient conditions overnight for further study.
Results and Discussion
Formation and self-assembly of supramolecular
amphiphiles
β-CD, which has seven glucose units, can recognize
adamantane well, and this typical host-guest pair has
been widely utilized to build up supramolecular materi-
als and devices[28-32] because it can form stable 1∶1
complex in water.[28-31] CB[7], which has an similar
cavity with β-CD, also has this property. As reported,
adamantane and its derivant can strongly bind to
CB[7].[65,66] Thus we designed and synthesized an ada-
mantane derivative 1 as guest molecule to prepare su-
pramolecular amphiphile with CB[7] (Schemes 2a, 2b).
The formation of the stable host-guest complex 3 can be
driven by hydrophobic interactions between the ada-
mantanyl moiety of guest molecule 1 and the hydropho-
bic cavity of CB[7]. Because of the amphiphilic nature
of amphiphiles, supramolecular amphiphile 3 was found
to self-assemble into vesicles (Scheme 2c).
Disassembly of vesicles
Disassembly of the vesicles can be achieved easily
by adding a competitive molecule of guest. After the
vesicles were formed, excess 1-adamantanamine hydro-
chloride, an analogue of the guest molecule that can
compete with the guest molecule within the CB[7] cav-
ity, was added. The vesicles can be disassembled in 10
min with ultrasonication.
Scanning electron microscopy (SEM)
Samples for scanning electron microscopy were
prepared by transferring a drop of sample solution onto
a silicon wafer. The samples were freeze-dried before
measurement. All SEM measurements were carried out
on a JEOL JSM 6700F electron microscopy, and oper-
ated at an acceleration voltage of 3.0 kV.
Scheme 2 Structures of (a) guest (1) and host molecule CB[7]
(2), (b) supramolecular amphiphile 3, (c) supramolecular vesicles
and possible self-assembled multilayers, and (d) schematic of the
controlled disassembly of CB[7] based supramolecular vesicles
Transmission electron microscope (TEM)
Samples for transmission electron microscopy were
prepared by placing a drop of sample solution on a
300-mesh, carbon-coated copper grid on filter paper and
air-dried before measurement. TEM experiments were
performed with a JEOL1011 transmission electron mi-
croscope and operated at an acceleration voltage of 100
kV.
Dynamic light scattering (DLS)
The mean size distribution of the vesicles was meas-
ured by Malvern-ZetaSizer Nano ZS dynamic light
scattering instrument at 25 ℃. Each experiment was
repeated 3 times. The average diameter given by this
instrument is number weighed.
In vitro drug release studies
Fluorescent rhodamine B (RB) was used as the drug
model compound. The release of RB from vesicle solu-
tion and disassembled vesicle solution was examined
using the dialysis method at room temperature and was
compared with free RB dissolved in water. Before the in
vitro release experiments, the concentrations of RB
The morphology of the self-assembled supra-mo-
lecular amphiphiles was investigated by SEM. The SEM
analysis shows that a typical vesicle aggregate was ob-
tained with a diameter ranging from 150 to 200 nm
(Figure 1a). Indeed, the CB[7] and guest molecule 1 can
not form this morphology. When the guest molecule
dispersed in DMF was slowly injected into water, it can
self-assemble into a fiber-like aggregate with a diameter
of about 1 μm and a length of more than 20 μm (Figure
were adjusted to 10－ mol/L. An aliquot of each solu-
5
tion (10 mL) was placed in a dialysis membrane
(DWCO: 2000 Da) and tightly sealed. The dialysis bag
was immersed in 500 mL deionized water. A magnetic
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1c). Moreover, the vesicles can also be obtained by an
alternative approach that CB[7] was added under ultra-
sonic after the fiber like aggregate formed. Dynamic
light scattering (DLS) was then used to measure the
vesicle size. DLS analysis shows that the average di-
ameter of the vesicles is about 190 nm with a width of
53.5 nm (Figure 1d), which was identical with SEM
analysis.
The image of transmission electron microscopy
(TEM) provided more detailed structural information
for the vesicles (Figure 1b). The results demonstrated
that the hollow feature, as it shows a strong contrast
between the center and the periphery, was vital for vesi-
cle structure with a diameters of about 200 nm and a
wall thickness of about 20 nm.
Figure 2 SEM image of the disassembly of the vesicles when
1-adamantanamine hydrochloride was added at the amounts of
1∶1 (a) and 10∶1 (b) with the guest molecule. (c) DNS analysis
of the disassembly of the vesicles (1-adamantanamine hydrochlo-
ride was added at the amount of 10∶1 with the guest molecule).
pounds is important and promising for drug delivery
system. At present, the controlled-release system gener-
ally contains two patterns. One is that the delivery car-
riers can release the drug automatically and selectively
under the triggered physical and chemical conditions,
such as pH, ionic strength, and temperature, etc. The
other is that the drug release can be modulated by
alteration of the structure of drug carriers in vitro.[67] We
take an easy way to change the structure of the vesicles
so as to realize the drug release (Scheme 2d). RB-loaded
assembly solution was separated into two equal parts.
One part was directly dialyzed against the deionized
water. The other part was added 1-adamantanamine hy-
drochloride, at the amount of 10∶1 with the guest
molecule, to disassemble the vesicles and then dialyzed
against the deionized water. RB solution which has the
same concentration was used as control to dialyze
against the deionized water. From the data in Figure 3, it
was observed that the speed of RB release in the solu-
tion of RB-loaded vesicles was much slower than that in
the RB solution. After 1-adamantanamine hydrochloride
was added to disassemble the vesicles, the speed of RB
release increased significantly, that is in accordance
with the RB solution.
Through investigating the release time of the RB
(Figure 4), we found that the RB solution had a release
time of about 7 h. After the RB-loaded vesicles were
disassembled, the release time became to be about 8 h,
which is as same as that of the RB solution. This means
that the RB loaded in the vesicles was almost totally
released to the solution once the vesicles were disas-
sembled. These results demonstrated that a controlled
release system can be obtained through the control of
the disassembly of the CB[7] based vesicles. Therefore,
the controllable CB[7] based vesicles can serve as new
nanocapsules to release a variety of functional mole-
cules or drugs.
Figure 1 (a) SEM image of the aggregates formed by
self-assembly of supramolecule amphiphile 3. (b) TEM image of
the aggregates formed by self-assembly of supramolecule am-
phiphile 3. (c) SEM image of the aggregates formed by self-as-
sembly of 1 alone. (d) DLS analysis of the vesicles.
Disassembly of vesicles
As supramolecular amphiphiles 3 was formed by
host-guest chemistry, this supramolecular complex can
be affected by the competitive recognition of guest ana-
logue. By this means, the disassembly of the vesicles
can be realized. SEM was used to observe the disas-
sembly of the vesicles. When a small amount of 1-ada-
mantanamine hydrochloride (1∶1 with the guest mole-
cule) was added, the vesicles were partly disassembled,
and a few fibers formed by direct assembly of guest
molecules in water (Figure 2a) were obtained. With the
continuous addition of 1-adamantanamine hydrochlo-
ride (10∶1 with the guest molecule), the vesicles were
completely disassembled and a lot of fibers were ob-
tained (Figure 2b). Dynamic light scattering (DLS)
analysis provided effective evidence for the disassembly
of the vesicles. As demonstrated by the DLS analysis
(Figure 2c), the average size of the aggregate changed
from 200 to 2000 nm with the disassembly of vesicles,
revealing the formation of new aggregates.
In vitro release of Rhodamine B
The controlled release of vesicle-encapsulated com-
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Cucurbit[7]uril-Based Vesicles Formed by Self-assembly of Supramolecular Amphiphiles
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In this study, we have developed an effective strat-
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