Photo-induced reversible structural transition of cationic diphenylalanine peptide self-assembly

Article abstract of DOI:10.1002/smll.201402140

The photo-induced self-assembly of a cationic diphenylalanine peptide (CDP) is investigated using a photoswitchable sulfonic azobenzene as the manipulating unit. A reversible structural transition between a branched structure and a vesicle-like structure is observed by alternating between UV and visible light irradiation.

Full text of DOI:10.1002/smll.201402140

Self-Assembly
Photo-induced Reversible Structural Transition of
Cationic Diphenylalanine Peptide Self-Assembly
Hongchao Ma, Jinbo Fei, Qi Li, and Junbai Li*
Stimuli-responsive supramolecular assemblies have gener- condensation/hydrolysis, and gelation.[8] However, it is still
ated much research interest because of their applications in a challenge to control the reversible self-assembly of the
[
1]
[9]
drug carriers, sensors, protein probes, and memory storage.
A variety of external stimuli, including temperature, pH,
enzymes, oxidizing/reducing agents, and light, have been azobenzene 4-[(4-ethoxy)phenylazo]benzenesulfonic acid
dipeptide for biological applications.
In this report, we designed a photoswitchable sulfonic-
[
2]
used in these systems. Recently, light-triggered assembling (EPABS), aiming to optically manipulate the self-assembly
has attracted considerable attention because it works revers- of cationic diphenylalanine peptide (CDP, H-Phe-
ibly, rapidly, and remotely without generating any unde- Phe-NH ·HCl). The photo-induced trans–cis conformational
sired substances. Thus various photo-responsive groups change of EPABS significantly influenced the CDP assembly
have been employed as photoswitching units to manipulate and a reversible structural transition between a branched
the assembly of structures and their relevant functions in microstructure and a vesicle-like nanostructure was observed.
numerous supramolecular assembly systems. Among them, Sulfonic-azobenzene is the analog of Congo red, which is an
azobenzene and its derivatives have been proven to be par- important medical molecule for the detection and therapy
ticularly advantageous as photoswitches because of their of Alzheimer’s disease.[10] Thus we hope that this approach
trans–cis conformational change, which can be triggered by could contribute to shed light on the light-based diagnosis
UV-vis light irradiation. They have, therefore, been widely and therapy of Alzheimer’s disease and fabrication of novel
used in various supramolecular assemblies for optical con- smart biomaterials.
a
2
[
3]
[
4]
trol of these systems, including controlling dramatic structure
changes and molecular property variations.
Figure 1(i) illustrates the molecular structures of EPABS
(trans- and cis-form) and CDP. Before utilization EPABS was
Diphenylalanine (FF) peptide, the core recognition motif dissolved in water to investigate its photoswitching property.
of Alzheimer’s β-amyloid polypeptide, is an excellent biomo- As expected, the EPABS molecules could be photoisomer-
lecular building block to fabricate bio-functional nanoma- ized under UV-vis irradiation. As shown in Figure 1b, upon
terials. A variety of defined supramolecular structures, such irradiation by UV light (λ = 365 nm) the absorption band at
as, discrete nanotubes, nanowires, macroporous honeycomb around 352 nm decreased significantly, whereas the band at
scaffolds, and peony-flower-like hierarchical nanostructures, around 432 nm increased slightly. These absorption bands
can be formed through self-assembly of diphenylalanine and located around 352 and 432 nm are related to the π–π* and
[
5]
[11]
its derivatives. Recently, researchers are not just interested n–π* transitions, respectively. The variation in the absorp-
in assembling such peptides into diverse structures, the con- tion bands under UV irradiation indicates the photoisomeri-
trolled assembly of these peptides has attracted even more zation of EPABS from the trans-state to the cis-state. After
attention. Li et al. reported a structural transition from UV irradiation the above solution was irradiated by visible
organogels to flower-like microcrystals by introducing eth- light and, interestingly, the π–π* absorption band increased
[
6]
anol as a co-solvent to FF organogels formed in toluene.
again whereas the n–π* absorption band decreased slightly
Reches and co-workers fabricated biomolecular necklaces by after irradiation, revealing that EPABS underwent a cis–trans
controlling the molecular mole ratio of the two units in the photoisomerization changing back to its trans state.
co-assembly of FF and its tert-butyl dicarbonate protected
The supramolecular assemblies formed based on CDP
[
7]
analogue. Very recently, Ulijn and co-workers prepared a and EPABS were prepared by adding an aqueous solution of
supramolecular self-assembly system using FF-based deriva- EPABS to the 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) solu-
tives, which displays coupled light switching, biocatalytic tion of CDP with a mole ratio of 1:1 at room temperature.
Upon mixing, a clear yellow solution was initially obtained;
then some suspended solids appeared gradually and the clear
solution turned completely turbid in several minutes. The
formation of the suspended solids indicates the generation
Dr. H. Ma, J. Fei, Q. Li, Prof. J. Li
Beijing National Laboratory for Molecular Sciences
CAS Key Lab of Colloid
of the supramolecular assemblies. After aging for two hours,
the suspended solids were separated from the suspension
and characterized by scanning electron microscopy (SEM)
and transmission electron microscopy (TEM) (Figure 2).
Both the SEM and TEM results indicate that branched struc-
tures, about 60 µm in size, were generated via co-assembly
Interface and Chemical Thermodynamics
Institute of Chemistry
Chinese Academy of Sciences
Beijing 100190, PR China
E-mail: jbli@iccas.ac.cn
DOI: 10.1002/smll.201402140
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DOI: 10.1002/smll.201402140
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H. Ma et al.
branched structure at molecular level.
Figure 3 shows the FTIR spectra of CDP,
EPABS, and the branched structures. In
−
1
Figure 3A the bands located at 1201 cm
−
1
and 1039 cm can be ascribed to the asym-
metric and symmetric O=S=O stretching
vibrations of the sulfonic group.[ After
co-assembly with CDP to form branched
structures, these two bands were shifted
to 1183 cm and 1033 cm , respectively,
suggesting strong electrostatic interactions
between CDP and EPABS. Moreover, a
14]
−1
−1
−1
new band located at 1547 cm appeared
in the branched structure. This new band
Figure 1. i) Molecular structure of EPABS and CDP; ii) UV–vis spectrum of an aqueous solution may be ascribed to stacking of the aro-
of EPABS after UV or visible light irradiation.
matic rings, which was also the case in
[
15]
our previous work.
Furthermore, after
of CDP and EPABS. The detailed SEM images shown in
Figure 2B–E reveal that the branched nanostructures are at 1664 cm , which belongs to the amido group of the CDP
built by elongated nanoplates and helical nanobelts that molecules, also shifted compared to that of pure CDP to
are interconnected through a center core so as to construct 1644 cm , indicating a strong molecular interaction between
the final co-assembled structure. The elongated nanoplates the two kinds of building blocks.
co-assembly with CDP, the band located
−1
−
1
(Figure 2D) with a size of about 30 µm in length, 2 µm in
Because of the special trans–cis isomerization of EPABS
width, and 1 µm in thickness, majorly serve as the building under irradiation with UV light, it is reasonable to expect
blocks to form the branched structure. The helical nano- that the co-assembly system formed by CDP and EPABS
belts, which are also found in the TEM images (Figure 2F), may also possess this photoswitching property. To verify
are about 30 µm in length. However, they are thinner than this speculation, the suspension was transferred to a quartz
the nanoplates, only hundreds of nanometers in thickness tube and irradiated under UV light at 365 nm. Interestingly,
(Figure 2E). Various reports have demonstrated the forma- upon irradiation, the suspended solids disappeared gradu-
tion of nanoplates and hierarchical structures composed ally, and the suspension became completely clear after being
of nanoplates during the self-assembly of diphenylalanine irradiated for an hour (Figure 4A). After that, the clear solu-
[
5h,6,12]
peptides.
However, the formation of helical structures tion obtained above was irradiated by visible light, and an
in the assembly of diphenylalanine peptides has not been increased opalescence, which was visible by the naked eye,
reported thus far. It has been demonstrated that the trans- occurred. After irradiation for about fifteen minutes, the
azobenzene unit contains a planar molecular configuration whole system changed into a completely turbid suspension
that favors cooperative packing in a helical form because of again. Figure S1 in the Supporting Information shows the
the strong interactions and steric hindrance of the adjacent SEM images of the assembled structures in the above suspen-
[
13]
molecules.
formed here are the result of the existence of EPABS in the structure regenerated after being irradiated by visible light
supramolecular assemblies. again, which indicates that EPABS returned to its trans-form.
Thus, we propose that the helical structures sion with a different scale. It can be seen that the branched
Fourier-transform infrared spectroscopy (FTIR) was per- To prove the key role of light in the structural transition
formed to further clarify the formation of the as-prepared process, we put the sample (in its clear state, i.e., after UV
irradiation) in the dark; and the sample
remained almost completely clear even
after several hours (Figure S2, Supporting
Information). Moreover, we found that
the reversible transition can be repeated
for many times without significant attenu-
ation (Figure 4B). UV–vis spectroscopy
was performed to monitor the state transi-
tion of the co-assembly system. As shown
in Figure 5A, the UV-vis spectrum of the
co-assembly system after UV irradiation
was in accordance with that of the pure
EPABS under the same conditions. This
confirms that EPABS is in the cis-state
in the supramolecular assemblies after
UV irradiation, which indicates that the
Figure 2. A,B,D,E)SEM images and C,F)TEM images of microstructures formed by co-assembly
of CDP and EPABS under visible light.
photo-induced trans–cis isomerization of
2
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Photo-induced Reversible Structural Transition of Cationic Diphenylalanine Peptide Self-Assembly
by reducing the molecular ratio of EPABS and CDP to 0.5:1,
both branched structures and vesicle-like structures were
formed (Figure S4, Supporting Information). So it is reason-
able to assume that the vesicle-like structures were formed
by assembly of the CDP molecules. Previous reports have
demonstrated that azobenzene molecules in the cis-form are
more hydrophilic and show a higher steric hindrance than
[
17]
molecules in the trans-form. Huang et al. have investigated
supramolecular assemblies based on cetyltrimethylammo-
nium bromide (CTAB) and sodium (4-phenylazo-phenoxy)-
acetate (AzoNa) through electrostatic interactions, and they
have found that the cis-AzoNa escapes from the supra-
molecular assemblies after UV irradiation because of its
[
18]
stronger hydrophilic property and higher steric hindrance.
A similar phenomenon has also been reported by Gröhn
and co-workers in a supramolecular assembly system based
[
19]
on a dendrimeric macro-ion and diazo dyes. Moreover, to
clarify the importance of electrostatic interactions, t-butyloxy-
carbonyl (Boc)-FF (N-Boc-Phe-Phe-COOH), an anionic
Figure 3. A–C) FTIR spectrum of EPABS (A), the co-assembly structure
B), and CDP (C).
(
EPABS was the driving force for the state transition of the diphenylalanine peptide, was also co-assembled with EPABS.
co-assembly system. However, both the illumination experiments (Figure S5,
In order to clarify the structural transition of the supra- Supporting Information) and the SEM images (Figure S6,
molecular assembly system, TEM (transmission electron Supporting Information) indicate that the assembly system
microscopy) was performed to investigate the clear solution does not show any photoswitching property because of the
obtained under UV light irradiation. As shown in the TEM absence of electrostatic interactions. Thus it can be deduced
image (Figure 5B), vesicle-like structures were obtained after that the EPABS molecules escape from the co-assembly
UV irradiation. These vesicles, ranging in size from 100 nm structures after being irradiated by UV light, leading to the
to 300 nm, as confirmed by dynamic light scattering (DLS) formation of vesicle-like structures from the self-assembly of
measurements (Figure S3, Supporting Information), coin- the CDP molecules.
cide well with the vesicle-like structures formed by the self-
Combing the results described above, we propose a pos-
assembly of CDP in dilute aqueous solutions.[ Furthermore, sible mechanism of the assembly and transformation process,
16]
which is illustrated in Figure 4A. Firstly, before UV illumi-
nation, trans-EPABS is inserted into the CDP molecular
arrangement driven by electrostatic interactions and π–π
[
15a]
interactions.
According to the FTIR results (Figure 3B),
the aromatic rings of EPABS overlap with the CDP aromatic
rings, which results in the formation of branched structures.
After being irradiated by UV light, trans-EPABS in the
branched structure is gradually transformed into cis-EPABS.
The higher hydrophilic property and steric hindrance of cis-
EPABS may lead to cis-EPABS escaping from the branched
structures, indicating a disassembly of the co-assembled
structure. Free CDP molecules can then perform a self-
assembly procedure to form vesicle-like structures. When
this system was then exposed to visible light, EPABS would
return to its trans-form again, leading to the regeneration of
the co-assembled branched structures.
In conclusion, we have successfully manipulated the
self-assembly of CDP molecules using EPABS. Branched
structures composed of elongated nanoplates and helical
nanobelts were obtained by the co-assembly of CDP and
EPABS under visible light. After UV irradiation, a trans–cis
photoisomerization of EPABS results in the disassembly of
the structure, leading to a structural transition from branched
structures to vesicle-like structures. The photo-induced struc-
tural transition is reversible and can be repeated many times
without significant attenuation. This work may help to under-
stand and design smart assemblies to reversibly control the
action of biosystems.
Figure 4. A) The illustration of the photo-responsive property of the
supramolecular assembly. B) The turbidity of the supramolecular
assembly sample (absorbance at 600 nm) by alternate irradiation at UV
light (blue solid square) and vis light (red solid square), respectively.
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The obtained suspended solids were care-
fully picked up and transferred onto a silicon
substrate for SEM measurements. Then the
sample was dried under vacuum and coated
by a thin layer of platinum. The TEM images
of the samples were carried out using a
Philips CM200-FEG transmission electron
microscope. 1H-NMR spectra were recorded
on a Bruker AV400 (400 MHz) spectrometer.
The FTIR spectra were recorded on a Bruker
TENSOR-27 spectrophotometer. The samples
were dried under vacuum and then pressed
into KBr pellet for FTIR spectroscopy. A Hitachi
U-3010 spectrophotometer was used to
record the UV–vis spectra of the various sam-
Figure 5. A) UV–vis spectra of pure EPABS (a) and the assembly system (b) under UV light;
B) TEM images of nanostructures formed by supramolecular assembly under UV light.
Experimental Section
ples. The samples were irradiated with a Xe light using a filter with
a wavelength of 365 nm. Visible-light irradiation was achieved by
holding the samples under sun light for fifteen minutes.
Materials: The cationic dipeptide (CDP, H-Phe-Phe-NH ·HCl) was
2
purchased from Bachem (Budendorf, Switzerland). Boc-FF (N-Boc-
Phe-Phe-COOH) was purchased from Alfa Aesar. 1,1,3,3,6,6-hex-
afluoro-2-propanol (HFIP) was obtained from Sigma-Aldrich. EPABS
was synthesized according to the literature.[20]
Synthesis of 4-[(4-Hydroxy)phenylazo]benzenesulfonic Acid
(
HPABS): To synthesize HPABS, p-aminobenzene sulfonic acid
Supporting Information
(3.5 g, 20 mmol) was dissolved in a mixture of 3 mL of concen-
trated HCl and a small amount of water, which was cooled to 0 °C
using an ice bath. Then, another 3 mL of concentrated HCl was
Supporting Information is available from the Wiley Online Library
or from the author.
added under vigorous stirring. After that, a NaNO solution (1.5 g
2
NaNO dissolved in 10 mL water) was slowly added dropwise to
2
the mixture at 0 °C. Under strong stirring a phenol solution (1.9 g
Acknowledgements
(
20 mmol) of phenol dissolved in 50 mL of K CO aqueous solu-
2 3
tion) was slowly added dropwise to the above mixture at 0 °C. After
reacting for 2 h, the pH value of the reaction liquid was adjusted
to 2.0 using 0.1 M HCl solution. The yellow precipitate was fil-
trated out and dried under vacuum. The crude product was puri-
fied by recrystallization from water/ethanol (yield: 76%). 1H-NMR
The authors acknowledge the financial support of this research
from the National Nature Science Foundation of China (Project No.
9
1027045, 21320102004, 21321063) and the National Basic
Research Program of China (973 program, 2013CB932800).
(
400 MHz, [D6]-DMSO, 25 °C, 7.85 (d, 2H; Ar-H), 7.8 (s, 4H; Ar-H),
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