Highly stable giant supramolecular vesicles composed of 2D hydrogen-bonded sheet structures of guanosine derivatives

Article abstract of DOI:10.1002/anie.200704535

(Figure Presented) Stabilized net: A molecular design approach to protect a 2D hydrogen-bonding network with nonpolar shielding layers has allowed the fabrication of micrometer-scale supramolecular vesicles in water (see picture). These hydrogen-bond-dire

Full text of DOI:10.1002/anie.200704535

Communications
DOI: 10.1002/anie.200704535
Supramolecular Chemistry
Highly Stable Giant Supramolecular Vesicles Composed
of 2D Hydrogen-Bonded Sheet Structures of Guanosine
Derivatives**
Isao Yoshikawa, Jun Sawayama, and Koji Araki*
Angewandte
Chemie
1038
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Angewandte
Chemie
The rapid progress in nanoscale technology has resulted in the
design and fabrication of nano- to micrometer-scale materials
composed of highly organized structures attracting much
attention.[1] Although covalently bonded polymers have been
used predominantly as macroscale materials, attempts to
control their chain arrangement and/or alignment has
encountered considerable difficulty because of the robustness
of the covalent bonds. In contrast, supramolecular materials
which are assembled by relatively soft intermolecular inter-
actions are suitable for this purpose. In aqueous media, lipids
and amphiphiles self-assemble into nano- to mesoscopic-scale
membranes and vesicles mainly through hydrophobic inter-
actions.[2] Although these structures have dynamic properties,
as a result of the reversible nature of the intermolecular
interactions, the relatively poor stability of the structures,
especially giant micrometer-sized vesicles, is a major draw-
back for practical applications.[3] Polymerized vesicles,[4]
diblock copolymer systems,[5] and other types of vesicles[3a,6]
have been studied extensively to circumvent this problem.
Supramolecular vesicles prepared from specially designed
nonlipidic molecules are being increasingly reported.[7]
Although the use of relatively strong and directional hydro-
gen bonds seems to be a promising approach to this end,
hydrogen-bonding interactions only operate effectively in
nonpolar environments[8] and not in highly polar aqueous
media.[9] Herein, we show that a rational molecular design to
sandwich two-dimensional (2D) hydrogen-bonding networks
between nonpolar protective layers and proper control of the
polar surfaces of the resultant sheet structure allowfor the
fabrication of nano- and microcapsules with high stability,
even in aqueous media.
Figure 1. a) Structures of the alkylsilylated derivatives of guanosine,
b) their 2D hydrogen-bonding pattern, and c) the film structure.
gave only a highly viscous liquid (1b) or a gumlike solid (1c);
destruction of the 2D hydrogen-bonding networks was
suggested from X-ray diffraction (XRD) analysis (see
Figure S1 in the Supporting Information). Since a simple
increase in the hydrophobic alkyl chain from propyl (1b) to
hexyl (1c) was not effective, the size and rigidity of the
alkylsilyl unit had to be re-designed to protect the hydrogen-
bonding networks from the hydrophilic oxyethylene moieties.
The newly designed guanosine derivatives, 2a and 2b,
have a rigid phenyl unit within the alkyl chain of the extended
oxyethylene units. Macroscale flexible supramolecular films
of 2a and 2b were successfully obtained by casting their THF
solutions (5 wt%) on to a teflon plate. The absence of a free
NH stretching band of 2-NH2 at n˜ = 3495 cmÀ1 in the IR
spectrum of the film of 2a (Figure 2a) suggested the
formation of 2D hydrogen-bonded sheet structures (see
Figure S2 in the Supporting Information).[11] The presence
of a sharp diffraction peak at 3.808 (2.32 nm), which
corresponds to the thickness of the sheet structure in the
X-ray diffraction pattern (Figure 2b), confirmed the layered
structure. Spreading the solution of 2a or 2b in THF gently on
to water gave the same flexible films at the surface after
diffusion of the THF, which suggests that the hydrogen-
We previously showed that alkylsilylated deoxyguanosine
derivative 1a can be fabricated into a flexible macroscale
supramolecular film by
a simple solvent-cast method
(Figure 1).[10] In this macroscale film, 1D tapes of guanine
1
7
6
À
=
molecules with N H···N and 2-NH2···O C hydrogen bonds
are further connected by two 2-NH2···N3 hydrogen bonds to
form a 2D hydrogen-bonded layer. This hydrogen-bonded
guanine layer is sandwiched between nonpolar and flexible
alkylsilyl side-chain layers, thereby forming a 2D sheetlike
structure. The oxyethylene groups located at the surface of
the sheet structures enhance the intersheet interactions and
results in the formation of the lamellar-like film structure.
However, 1a did not dissolve or disperse in water at all. To
increase the hydrophilicity, 1b and 1c, which have extended
oxyethylene units, were prepared. An attempt to prepare a
solvent-cast film from their tetrahydrofuran (THF) solutions
[*] I. Yoshikawa, J. Sawayama, Prof. K. Araki
Institute of Industrial Science
University of Tokyo
4-6-1 Komaba, Meguro-ku, Tokyo 153-8505 (Japan)
Fax: (+81)3-5452-6364
E-mail: araki@iis.u-tokyo.ac.jp
[**] The study is partly supported by a Grant-in-Aid for Scientific
Research (14350482) from the Ministry of Education, Culture,
Sports, Science, and Technology (Japan). We thank Dr. T. Kuzumaki
and Prof. Y. Mitsuda for the TEM observations.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Figure 2. a) IR spectrum and b) X-ray diffraction pattern of a flexible
supramolecular film of 2a.
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Communications
bonding network sandwiched by the alkylsilyl-protecting
layers was retained even after contact with water.
mation was further confirmed by fluorescence microscopy
after the incorporation of a water-soluble fluorescent marker
(eosin Y) inside the vesicle. The vesicle, after replacing the
outer aqueous phase with fresh water, clearly showed an
orange color and green fluorescence as a result of the trapped
eosin Y inside the vesicle (Figure 3 f).
The vesicle solution retained the same turbidity and gave
the same DICM image when it was heated at 808C, after
addition of methanol (16.7 v/v%), or after being kept under
ambient conditions for more than a month. Centrifugation of
the vesicle solution (10000 G) for 10 minutes caused sed-
imentation of the vesicles at the bottom of the tube, which
could be re-dispersed in fresh water by gentle shaking. The re-
dispersed vesicles were confirmed to be intact (see Figure S3
in the Supporting Information). These results demonstrated
that the vesicles have sufficiently high stability in water to be
processed or handled in a variety of ways.
To test the stability of the vesicles further, a drop of the
vesicle solution on a silicone substrate was air-dried and
observed by atomic force microscopy (AFM) in the AC-mode
under ambient conditions. The AFM image showed ellipsoi-
dal micrometer-sized vesicles (Figure 4a). The vesicles before
and after being kept in vacuo (2 ” 10À3 Pa) for 12 h on the
substrate retained essentially the same shape. The results
indicated that the internal water was preserved even under
vacuum, thus demonstrating that the vesicle membrane has
high stability and lowwater permeability. When the AC-mode
unidirectional scan was carried out with an increased
maximum force,[12] typically from 2.3 to 4.7 nN, deformation,
elongation, or lateral relocation of the vesicle along the
scanning direction was observed, but without rupture
For the preparation of the vesicles, a small amount of the
solution of 2a or 2b in THF was injected into deionized water
and the solution was subjected to sonication for 5 minutes at
808C (see the Supporting Information). The resultant trans-
lucent aqueous solution was stable for more than a day
without the formation of precipitates or oily droplets.
Although slowsedimentation was noticed for the solution
of the less hydrophilic 2a after a prolonged period, the
translucent aqueous solution of 2b remained unchanged for
more than a month. The differential interference contrast
microscopy (DICM) image showed the formation of small
particles, and the size distribution measured by dynamic light
scattering (DLS) indicated the diameter of the particles to be
mostly within 190 to 335 nm (Figure 3a and b). A trans-
mission electron microscope (TEM) image (Figure 3c) of the
particles showed vesicular structures with diameters of
150 nm.
Figure 3. The vesicles of 2b prepared by the injection method (a–c)
and the thin-film method (d–f). a) DIC image, b) size distribution
determined by DLS, and c) TEM image (stained with sodium phospho-
tungstate) of the vesicle prepared by the injection method. d) DIC
image, e) size distribution determined by DLS, and f) microscopy
(upper half) and fluorescence (lower half) images of the vesicles
containing eosin Y (labs =524 nm, lfl =544 nm) and prepared by the
thin-film method.
Figure 4. The AC-mode AFM images of the vesicles of 2b prepared by
the thin-film method. Height profiles along the lines within the images
are shown under each image. a) Vesicles under ambient conditions
observed with the default setting (maximum applied force: 2.3 nN).
b) and c) Vesicles observed by unidirectional scanning with a max-
imum applied force of 4.7 nN. An arrow indicates the scanning
direction of the tip. The image showed deformation of the vesicle in
the scanning direction. d) The vesicle before rupture and e) 30 min
after rupture during the repeated scan with an increased maximum
applied force. The thickness of the membrane was estimated from the
height of the inner flat part that was composed of the upper and lower
membranes.
Stable micrometer-sized giant vesicles were obtained by
the thin-film method. After preparing a thin film from the
solution of 2b in benzene inside a glass tube and subsequent
drying at 508C in vacuo for 3 h, water was added and the
aqueous solution was subjected to sonication at 808C for
10 minutes. The DICM image of the resultant translucent
solution clearly showed the presence of the micrometer-sized
giant vesicles as the major fraction, and the DLS profile
confirmed this observation (Figure 3d and e). Vesicle for-
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(Figure 4b), which suggests the vesicles have a high stability
and flexibility.
Keywords: hydrogen bonds · materials science · nucleosides ·
supramolecular chemistry · vesicles
.
Although it rarely happened, we encountered rupture of
the vesicle membranes twice during the repeated AC-mode
scan with a maximum force of approximately 6–17 nN
(Figure 4b). From the heights of the inner flat parts of the
residual solids after evaporation of the trapped water in these
two cases, the thickness of the vesicular membranes was
estimated to be approximately 11 and 15 nm, which corre-
spond to 5 and 7 layers of the sheet assembly, respectively. A
flat CaF2 plate was pressed hard on to a second flat CaF2 plate
on which the vesicles were densely placed, and kept in vacuo
overnight. The residual 2b film on the CaF2 plate showed
essentially the same IR spectrum as that prepared from the
THF solution (see Figure S2 in the Supporting Information),
further confirming that the vesicle membrane was composed
of the 2D hydrogen-bonded sheet structure of 2b.
Thus, it has been shown that hydrogen-bond-directed
micrometer-sized giant vesicles can be conveniently prepared,
and these giant vesicles showsufficiently high stability in
water. Furthermore, a cationic surfactant, hexadecyltri-
methylammonium chloride (HTAC), induced an effective
destruction of the vesicles. Upon adding a small amount of
HTAC (5 ” 10À4 moldmÀ3), the translucent vesicle dispersion
immediately turned to a clear transparent solution, and no
vesicular particles were observed in the DIC image. This
dynamic response is the direct manifestation of the reversible
nature of the intermolecular interactions and, therefore, is the
major advantage of the self-assembled supramolecular mate-
rials.
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Received: October 2, 2007
Published online: November 21, 2007
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