Controllable self-assembled laminated nanoribbons from dipeptide-amphiphile bearing azobenzene moiety

Article abstract of DOI:10.1039/c0sm01050b

Artificial peptide self-assembly is an appealing research subject which has been demonstrated to be a reliable approach to create hierarchical nanostructures and biomaterials. In this paper, a dipeptide-amphiphile incorporated with an azobenzene moiety is synthesized, which are found to self-assemble into well-defined laminated nanoribbons as well as macroscopic hydrogel. The nanoribbons are formed by nanofibers aligning in nearly lamellar arrays. The driving force of dipeptide self-assembly is proposed to be a synergic effect of hydrophobic interaction, aromatic packing, and hydrogen bond. The addition of NaCl is found to promote hydrogelation and nanoribbon formation. Finally photoisomerization of the azobenzene group is utilized to rationally control dipeptide self-assembly and hydrogel formation by remote light input.

Full text of DOI:10.1039/c0sm01050b

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PAPER
Controllable self-assembled laminated nanoribbons from dipeptide-amphiphile
bearing azobenzene moiety†
a
Yiyang Lin, Yan Qiao, Peifeng Tang, Zhibo Li and Jianbin Huang
a
a
b
a
*
Received 23rd September 2010, Accepted 27th November 2010
DOI: 10.1039/c0sm01050b
Artificial peptide self-assembly is an appealing research subject which has been demonstrated to be
a reliable approach to create hierarchical nanostructures and biomaterials. In this paper, a dipeptide-
amphiphile incorporated with an azobenzene moiety is synthesized, which are found to self-assemble
into well-defined laminated nanoribbons as well as macroscopic hydrogel. The nanoribbons are formed
by nanofibers aligning in nearly lamellar arrays. The driving force of dipeptide self-assembly is
proposed to be a synergic effect of hydrophobic interaction, aromatic packing, and hydrogen bond.
The addition of NaCl is found to promote hydrogelation and nanoribbon formation. Finally
photoisomerization of the azobenzene group is utilized to rationally control dipeptide self-assembly
and hydrogel formation by remote light input.
a composite material in which the crystallographic c axes of
Introduction
hydroxyapatite are aligned with the long axes of the fibers.6 A
systematic study on single tail peptide-amphiphiles was
conducted by the same group.7 Hartgerink and co-workers have
focused on the research of polypeptides without a hydrocarbon
chain in aqueous solution.8 Shimizu has synthesized dipeptide
bolaamphiphiles which can self-assemble into vesicle-encapsu-
lated microtubes.9 It can be found that peptide derivatives pref-
erentially self-organize into one-dimensional nanostructures
such as fibers, tubes, ribbons, etc, which offer many potential
applications in electronically and biologically active materials.10
Inspiration by these works has led us to synthesize novel
dipeptide-amphiphiles bearing a glycylglycine group, and focus
on the study of controllable supramolecular self-assembly. For
the consideration of further bio-functional application, the
research is conducted in aqueous solutions rather than organic
solvents. Moreover, an azobenzene group was attached to the
dipeptide-amphiphile to give light-activity. Light is expected to
be an advantageous and important tool to change molecular
structure as well as to manipulate amphiphile self-assemblies at
different hierarchical levels.11 This is because light can be oper-
ated in a clean environment free of any additional reagent.
Another advantage of light over electric or ultrasound is that,
light can be directed at a precise spatial location. In our study,
hydrophobic hydrocarbon chains are also incorporated to the
amphiphile skeleton to promote molecular self-assembly in
water.
Many natural biomolecules (proteins, lipids, and DNA) can self-
assemble into functional materials displaying a hierarchy of
structure over several length scales.1 In particular, structural
protein-based hydrogels are used for many applications, ranging
from cosmetic thickeners to drug delivery and tissue engi-
neering.2 These materials are usually extracted from natural
sources, which may give rise to inconsistent properties unsuitable
for biomedical applications. Artificial peptides, which are
employed as the building blocks of proteins, begin to allow the
incorporation of the structures and functions of proteins into
synthetic materials.3 These peptide materials are particularly
desirable for control over nanoscale ordering through self-
assembly in the field of nanotechnology and biotechnology with
the increased capability to respond to living systems for tissue
engineering and therapeutic delivery.4
Recently low molecular peptide-derivatives have attracted
considerable attention because of their facile synthesis, vari-
ability of amino acid sequence, and ease of functionalization.5
For example, Stupp has reported pH-induced self-assembly of
a peptide-amphiphile to make nanostructured fibrous scaffold
reminiscent of extracellular matrix. The cross-linked fibers are
able to direct mineralization of hydroxyapatite to form
aBeijing National Laboratory for Molecular Sciences (BNLMS), State
Key Laboratory for Structural Chemistry of Unstable and Stable
Species, College of Chemistry, Peking University, Beijing, 100871,
People’s Republic of China. E-mail: JBHuang@pku.edu.cn; Fax: +86-
10-62751708; Tel: +86-10-62753557
bCAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese
Academy of Sciences, Beijing, 100080, People’s Republic of China
† Electronic supplementary information (ESI) available: Fluorescence
intensity curve, gelation temperature, dynamic rheology, and additional
TEM images. See DOI: 10.1039/c0sm01050b
Combined with different techniques, the self-assembly
behavior of dipeptide-amphiphiles in aqueous solution is inves-
tigated, which are demonstrated to spontaneously aggregate into
well-defined nanoribbons at low concentrations. The formation
and overlap of nanoribbons can further result into water gela-
tion. From the results of FT-IR and CD spectrum, the molecular
2762 | Soft Matter, 2011, 7, 2762–2769
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origin of 1D nanostructure formation is proposed. The effect of
salt addition and azobenzene photoisomerization on dipeptide-
amphiphile self-assembly is also investigated. It is hopeful that
this study can help to understand the mechanism of peptide-
amphiphile self-assembly and will thereby help to design func-
tional peptides materials.
at 0 ꢀC. The aqueous phase was acidified at room temperature to
pH 2 with aqueous HCl. The solid was isolated by filtration and
dissolved in hot alkaline solution. To this solution, aqueous HCl
was slowly added and the precipitates were collected and dried in
vacuum. 1H NMR (400 MHz, DMSO): 0.87 (t, 3H), 1.02
(m, 2H), 1.30 (m, 2H), 1.66 (t, 2H), 3.79 (s, 2H), 3.80 (s, 2H), 4.63
(s, 2H) 7.13 (d, 2H), 7.36 (d, 2H), 7.73 (d, 2H), 7.83 (d, 2H), 8.20
(m, 1H), 8.41 (m, 1H). Elemental analysis calcd (%) for
C22H26N4O5: C 61.96, H 6.15, N 13.14; found: C 61.23, H 6.14, N
12.70.
Experimental section
Materials
4-Butylaniline and 4-hexylaniline were purchased from Alfa
Aesar. Ethyl bromoacetate, glycylglycine, sodium nitrite,
potassium hydroxide, hydrochloric acid (37%), and sodium
hydroxide were bought from Beijing Chemical Reagents
Company. All chemicals were used as received without further
purification. The Millipore Milli-Q deionized water (18.2 MU/cm
resistivity) was used for producing aqueous solution.
Sample preparation
The C4AzoGlyGly solution mentioned in this paper is prepared
by adding the desired amount of C4AzoGlyGly solid into equal
amount of sodium hydroxide solution and heated under stirring
till completely soluble (ꢁ80 ꢀC). The sample is then slowly cooled
to room temperature. The gel–sol transition temperature (Tgel
)
was determined by the test-tube inversion method.
Amphiphile synthesis
Rheological measurement
The synthesis of dipeptide-amphiphile is according to the
following and outlined in Scheme 1.
The rheology was measured with a ThermoHaake RS300
rheometer. A cone-plate sensor was used with a diameter of 35
mm and a cone angle of 2ꢀ. A sample cover provided with the
instrument was used to minimize the change in sample compo-
sition by evaporation during the measurement. Frequency sweep
measurements were performed in the linear viscoelastic regime
determined previously by dynamic stress sweep measurements.
(1) Compound 1:12 14.9 g p-butylaniline was dissolved in 200
mL water and 25 mL concentrated hydrochloric acid. The
solution was cooled to 5 ꢀC in an ice bath and reacted with 15 mL
of aqueous sodium nitrite (6.67 M) at 5 ꢀC for 1 h. The resulting
diazonium solution was then coupled a solution containing 9.4 g
phenol, 4 g sodium hydroxide, 10 g sodium carbonate, and 100
mL water at 0 ꢀC for 2 h. The product was collected by filtration,
dried in vacuum, and purified by recrystallization in hexane.
(2) Compound 2:13 compound 1 (5.1 g, 20 mmol), ethyl bro-
moacetate (30 mmol), and potassium hydroxide (1.12 g, 20
mmol) was added to a round-bottomed flask containing 200 mL
ethanol. The mixture was refluxed for 4 h and cooled in an ice
bath. The solid product was collected by filtration and dissolved
in 30 mL of water–methanol (10/90) and to the solution were
added sodium hydroxide pellets. The mixture was heated to
reflux for 30 min and a yellow solid formed. The compound 2 was
obtained by collecting the solid and recrystallized from chloro-
form containing 2% glacial acetic acid.
Negative-staining transmission electron microscopy
TEM micrographs were obtained with a JEM-100CXII trans-
mission electron microscope (working voltage of 80–100 kV) by
the negative-staining method with uranyl acetate solution (1%)
as the staining agent. One drop of the solution or a piece of
hydrogel was placed onto a carbon Formvar-coated copper grid
(230 mesh) for 3ꢁ5 min. The excess liquid or hydrogel was
sucked away by filter papers. Then one drop of the staining agent
was placed onto the copper grid for 2ꢁ5 min. The excess staining
agent was also sucked away by filter papers.
(3) The compound 2 (1.63 g, 5 mmol) was added into a round-
bottomed flask containing excessive thionyl chloride. The
mixture was refluxed for 8 h and the resulting mixture was
distilled under reduced pressure to remove most of the thionyl
chloride. The product was added to a solution (20 mL)
containing 15 mmol glycylglycine and 5 mmol sodium hydroxide
Cryogenic transmission electron microscopy (cryo-TEM)
Cryo-TEM samples were prepared in a controlled environment
vitrification system (CEVS). A small drop of sample was placed
on a copper grid, and a thin film was produced by blotting off
the redundant liquid with filter paper. This thin film was then
quickly dipped into liquid ethane, which was cooled by liquid
nitrogen. The vitrified samples were then stored in liquid
nitrogen until they were transferred to a cryogenic sample holder
(Gatan 626) ꢀand examined by a JEM2200FS TEM (200 kV) at
about ꢂ174 C.
UV-vis absorbance
UV-vis absorbance measurements were carried out on the spec-
trophotometer (Cary 1E, Varian Australia PTY Ltd.) equipped
with a thermostated cell holder. The UV-vis measurement was all
ꢀ
Scheme 1 Organic synthesis of dipeptide-amphiphile C4AzoGlyGly.
This journal is ª The Royal Society of Chemistry 2011
carried out at 25 C.
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Fig. 1 (a) Molecular structure of dipeptide-amphiphile C4AzoGlyGly and C6AzoGlyGly; (b) macroscopic appearance of C4AzoGlyGly hydrogel (5
mM).
Fourier transform IR
(Fig. 1a). C4AzoGlyGly solution is prepared by adding the
desired amount of solid into an equal amount of sodium
hydroxide solution and heated until completely soluble. After
cooling, the sample becomes thicker and can even gelate water.
The fluorescence intensity curve of the Nile Red probe indicates
the critical aggregates concentration of C4AzoGlyGly solution is
about 2 mM (Fig. S1, ESI†). Fig. 1b shows the test-tube
inversion of 5 mM C4AzoGlyGly hydrogel in a test tube. The
gelation temperature, varied by gelator concentration, is also
measured by the test-tube inversion method (Fig. S2, ESI†). To
understand the effect of the hydrophobic tail, the dipeptide-
amphiphile C6AzoGlyGly (with C6 hydrocarbon chain) is also
synthesized. It is found that the lowest gelation concentration
(LGC) of C6AzoGlyGly can be lowered to 2 mM while that of
C4AzoGlyGly amphiphile is 5 mM.
The Fourier transform IR spectrum of C4AzoGlyGly solid or
hydrogel in D2O is collected with a Nicolet iN10 MX spec-
trometer (Thermo Scientific, America).
Fluorescence measurement
Steady-state fluorescence spectra were obtained with an Edin-
burgh FLS920 fluorescence spectrophotometer. In these studies,
a fixed amount (5.0 mM) of the fluorescent probe Nile Red was
added to the aphiphile solution and stirred to reach equilibrium.
The excitation wavelength of Nile Red is 575 nm and the fluo-
rescence emission was monitored.
Circular dichroism (CD)
Dynamic rheology is further performed to investigate the
mechanical properties of dipeptide-amphiphile hydrogels. Time
sweep rheology in Fig. 2a indicates that a stable hydrogel is
quickly formed with 20 mM C4AzoGlyGly in 100 s, in which the
storage modulus G0 and the loss modulus G00 can reach a plateau
soon after preparation. Nondestructive frequency sweep exhibits
typical solidlike rheological behavior with the storage modulus
G0 (ꢁ60 Pa) dominating the loss modulus G00 (ꢁ10 Pa) over the
investigated oscillating frequency (Fig. 2b). This is a typical
solidlike behavior of a hydrogel.14 Besides, the storage modulus
shows strong dependence on gelator concentration (Fig. S3,
ESI†). On the other hand, when the length of the hydrocarbon
tail is increased, the hydrogel mechanical intensity can be greatly
enhanced (Fig. S4, ESI†). For example, the storage modulus and
loss modulus of C6AzoGlyGly is nearly two orders larger than
that of C4AzoGlyGly.
CD spectra were recorded using a JASCO J-810 spectrometer.
The path length of the quartz cell was 0.1 mm and each
ꢀ
measurement was repeated three times at 25 C.
Photoisomeric experiment
For light-triggered trans–cis transition, the samples were irradi-
ated with 365 nm UV light from a Spectroline FC-100F fan-
cooled, long wave UV lamp. The power of the mercury arc lamp
is 100 W. Samples were placed in a quartz tube, and irradiation
was done for a specific duration. For cis–trans transition,
irradiation by visible light was performed using a 200 W incan-
descent light bulb (>440 nm). The quartz tube was placed in front
of the light source 15 cm away.
Results and discussion
Hydrogel formation by dipeptide-amphiphiles
Self-assembled 1D nanostructures from dipeptide amphiphiles
The dipeptide-amphiphile bearing azobenzene and glycylglycine
moieties are synthesized as described in the Experimental section
Transmission electronic microscopy (TEM) sampes were
prepared by negative-staining technique to reveal the
Fig. 2 Dynamic rheology of self-assembled C4AzoGlyGly hydrogel (C ¼ 20 mM): (a) time sweep; (b) frequency sweep.
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to the amide I band of C4AzoGlyGly solid.15 In the gel state (in
D2O), however, the amide I band of C4AzoGlyGly shifts from
1663 cmꢂ1 to 1635 cmꢂ1. The 28 cmꢂ1 shift of the C]O stretching
(1663–1635) may be attributed to further hydrogen bonding
involving C]O.14 The low-frequency amide I bands at 1635 cmꢂ1
are characteristic of a b-sheet configuration. On the other hand,
C4AzoGlyGly does not form a b-sheet structure in the solid state.
To obtain further insight into the submolecular information
for the dipeptide-amphiphile self-assembly, circular dichroism
(CD) studies are performed (Fig. 4b). When C4AzoGlyGly
concentration is 1 mM (below CAC), no notable CD signals
are observed. This is in agreement with the TEM result in
which self-assembled nanoribbons are not detected. When the
C4AzoGlyGly concentration is 2 mM, CD split signals can be
noted. The exciton splitting CD bands are observed in the
trans-azobenzene region (Fig. 4b). A dichroic bands of oppo-
site sign centered at 354 nm are related to the p–p* transition
of the azobenzene chromophore. It is suggested that the
dichroic couplet of C4AzoGlyGly in the p–p* region is the
result of exciton splitting owing to dipole–dipole interactions
between trans-azobenzene disposed in a mutual chiral geometry
along the b-sheet network. The CD signal at low wavelength
may be also indicative of b-sheet formation as suggested by
FT-IR.
Fig. 3 Negative-staining TEM images of self-assembled nanostructures
in the system of 5 mM C4AzoGlyGly hydrogel.
microstructure of the C4AzoGlyGly hydrogel. As shown in
Fig. 3a, the dipeptide-amphiphile C4AzoGlyGly (5 mM) can
self-organize into nanoribbons of several micrometres long and
tens of nanometres wide. These 1D nanostructures are highly
flexible and easy to overlap and entangle with each other, which
are responsible for the robustness of hydrogel. The enlarged
images (Fig. 3b, 3c and 3) indicate that the nanoribbons are
composed of highly ordered lamellar structures with obvious
light and shade boundaries. The images show that the ribbons
are formed by bundles of parallel nanofibers. The arrows in
Fig. 3b indicate a seven-layered nanoribbon with a width of 33
nm. A calculation from the TEM images gives the diameter of
a single nanofiber to be 5 nm, which is comparative to double the
molecular length of C4AzoGlyGly (ꢁ2.5 nm, by MM2 force field
simulation). Enlarged images indicate the ribbons are particu-
larly thin so that the overlap patterns of nanoribbons can even be
seen from TEM. It is estimated that the thickness of each ribbon
is several nanometres which corresponds to a bilayer structure.
Even when the amphiphile concentration decreases to 2 mM, the
nanoribbons with a laminated structure can be observed (Fig. S5,
ESI†).
Combined with these results, we propose
a molecular
arrangement as well as interpretation for laminated nano-
ribbons. As shown in Fig. 5a and 5b, the nanoribbons are con-
structed by the parallel alignment of fibers. The growth of these
nanostructures along each dimension is obviously regulated by
different forces. It is suggested that one dimensionality is linked
to fast growth along the direction of amide hydrogen bonding in
b-sheets (the arrows in Fig. 5c and 5d indicate the H-bond
direction). Also the p–p stacking between azobenzene groups
will contribute to 1D directional growth of nanofiber. These
directional forces (H-bond and p–p stacking) together with
nondirectional hydrophobic interaction ultimately promote
molecular self-assembly into one-dimensional nanofiber. In the
lateral direction, however, hydrogen bond and p–p stacking is
lacking. Another question is why the nanofibers will parallel
align into ribbons. It is supposed that the carboxyl on the
nanofiber surface will be partially hydrolyzed in solution. As
a consequence, surface hydrogen bonds will be formed between
adjacent fibers which facilitate the affinity and well-alignment of
neighboring fibers.
Mechanism illustration of molecular self-assembly
Fourier transform infrared (FT-IR) spectroscopy of C4Azo-
GlyGly in solid state and hydrogel state provides important
information about the intermolecular hydrogen bonds among
the component molecules (Fig. 4a). The spectrum of the
C4AzoGlyGly solid exhibited characteristic absorption bands
centered at around 1730 cmꢂ1 and 1663 cmꢂ1, which is attributed
Fig. 4 (a) FT-IR spectrum of C4AzoGlyGly solid (dark line) and C4AzoGlyGly hydrogel in D2O (light line). (b) CD spectrum of C4AzoGlyGly
solution at 25 ꢀC: 2 mM (dark line) and 1 mM (light line).
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Fig. 5 Dipeptide-amphiphile self-assembly of C4AzoGlyGly. (a) TEM image; (b) cartoon picture of nanoribbon; (c) an individual fiber; (d) molecular
packing in the plane along fiber axis.
Fig. 6 TEM images of 1 mM C4AzoGlyGly solution stained with uranyl acetate solution (1%): (a) with 10 mM NaCl; (b, c) with 20 mM NaCl; (d–g)
with 40 mM NaCl. The scale bar in these images is 50 nm.
Salt-promoted water gelation and nanofiber parallel alignment
1 mM. With addition of 10 mM NaCl, a large amount of
one-dimensional nanoribbons can be observed under TEM
(Fig. 6a). As NaCl concentration increases to 20 mM or 40 mM,
nanoribbons with well-aligned laminated structures become the
dominant morphology (Fig. 6b–6g). The CD spectrum shows
a notable signal can be detected in C4AzoGlyGly solution
(1 mM) with the addition of 20 mM (Fig. 7a). It is suggested that
sodium ions can weaken electrostatic repulsion between carboxyl
Considering the electrostatic repulsion between ionic headgroups
of C4AzoGlyGly is profound, the addition of inorganic salts are
expected to bring remarkable effect towards supramolecular self-
assembly. Herein, the self-assembly behavior of dipeptide
amphiphile is studied in the absence and presence of NaCl. As
demonstrated by TEM, C4AzoGlyGly cannot form aggregates at
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Fig. 7 (a) CD curve of C4AzoGlyGly (C ¼ 1 mM) solution without NaCl (dark line) or with 20 mM NaCl; (b) Fluorescence intensity of 5 mM Nile Red
in C4AzoGlyGly (C ¼ 1.0 mM) solution with the addition of NaCl.
Fig. 8 (a) UV-vis spectrum of C4AzoGlyGly solution before and after 365 nm light irradiation; (b) Shear viscosity of 5 mM C4AzoGlyGly solution after
repeated irradiated by 365 nm UV light and visible light. (The UV irradiation time is 30 min while the visible light irradiation time is 1 h.)
headgroups, which will promote molecular aggregates and
facilitate nanofiber alignment into ribbons. This is supported by
Nile Red fluorescence intensity as shown in Fig. 7b. Nile Red
fluorescence intensity is much greater in hydrophobic environ-
ments than in a hydrophilic environment.16 As shown in Fig. 7b,
the fluorescence emission of Nile Red in 1 mM C4AzoGlyGly
solution increases gradually with the addition of NaCl. This
implies the environment around Nile Red is becoming hydro-
phobic and the molecular packing inside the aggregates turns to
be more closely. In a control experiment, the fluorescence
intensity of Nile Red in water does not show an increase with the
addition of NaCl (Fig. S6, ESI†).
hydrogel (Fig. 8b). It is believed that the variation of flowing
property at the macroscale originates from the structural
evolution at the nanoscale. The TEM images show that the
nanoribbon networks in 5 mM C4AzoGlyGly hydrogels have
transformed into short fibers after 30 min UV light irradiation
(Fig. 9a). Cryo-TEM results also demonstrate the formation of
a small amount of short fibers in 5 mM C4AzoGlyGly solution
after UV irradiation (Fig. 9b). It is worthwhile that laminated
nanoribbons can be re-formed after irradiation by visible light
(Fig. S7, ESI†). Fig. 10a shows the light-triggered trans–cis
transition causes the disappearance of the CD signal after UV
light illumination. Meanwhile, the fluorescence intensity of Nile
Red in C4AzoGlyGly solution decreases sharply after UV irra-
diation (Fig. 10b), implying molecular packing inside the
aggregates becomes more loose.
Light-triggered supramolecular self-assembly
The incorporation of azobenzene group can impart light-active
behavior to the self-assembled C4AzoGlyGly solution. As shown
in Fig. 8a, before UV light irradiation, the spectrum is dominated
by the 349 nm absorption which is ascribed to the p–p*
absorption band of the trans-azobenzene moiety. As UV irradi-
ation proceeds, the 349 nm absorption band decreases with
concomitant increase of the p–p* and n–p* bands of the cis
isomer at around 310 nm and 440 nm, respectively. The UV-vis
result strongly demonstrates the azobenzene trans–cis transition
triggered by UV or visible light. Meanwhile, such a conforma-
tional change can be reversibly repeated by light.
Rheological measurements show a dramatic viscosity decrease
after exposing to UV light. When visible light is applied, the
viscosity can be reverted and the sample turns into highly viscous
Fig. 9 (a) Negative-staining TEM and (b) cryo-TEM images of a 5 mM
C4AzoGlyGly sample illuminated by UV light for 30 min.
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Fig. 10 (a) CD spectrum of C4AzoGlyGly solution before and after 365 nm light irradiation; (b) Fluorescence intensity of Nile Red in C4AzoGlyGly
solution (5 mM) before and after 365 nm light irradiation. (The UV irradiation time is 30 min while the visible light irradiation time is 1 h.)
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reflected in the dipole moment and change in geometry.17 On one
hand, the planar tran-azobenzene group transforms into
nonplanar cis-azobenzene and the aromatic packing between
azobenzene groups is suppressed. On the other hand, trans-
azobenzene has no dipole moment while the dipole moment of
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Conclusion
In conclusion, novel dipeptide derivatives bearing azobenzene
group are synthesized which can spontaneously self-assemble
into well-defined nanoribbons as well as sample-spanning
networks forming hydrogels in water. The nanoribbons are
found to be fabricated by the alignment of nanofibers parallel to
the fiber’s long axis. The driving force for self-organized nano-
structures is believed to come from directional forces (hydrogen
bonding and aromatic packing) and the nondirectional hydro-
phobic effect. Inorganic salt is demonstrated to suppress the
electrostatic repulsion and favor the nanoribbon formation.
Furthermore, laminated nanoribbons can transform into short
fibers under UV irradiation, which revert to nanoribbons by
visible light exposure. It is expected that this work can shed light
on stimuli-responsive molecular self-assembly and the rational
design of hierarchical nanostructures through a bottom-up
approach.
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Acknowledgements
This work is supported by National Natural Science Foundation
of China (20873001, 20633010, 50821061 and 21073006) and
National Basic Research Program of China (Grant No.
2007CB936201).
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