Self-assembly of diphenylalanine peptides into microtubes with "turn on" fluorescence using an aggregation-induced emission molecule

Article abstract of DOI:10.1039/c3cc45320k

The self-assembly of diphenylalanine peptides (l-Phe-l-Phe) into microtubes with "turn on" bright yellow green fluorescence was described, which was achieved using an aggregation-induced emission (AIE) molecule of 9,10-bis[4-(3-sulfonatopropoxyl)-styryl]

Full text of DOI:10.1039/c3cc45320k

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Self-Assembly of Diphenylalanine Peptide into Microtubes with “Turn
on” Fluorescence by an Aggregation-Induced Emission Molecule
a
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Na Na, Xiaoyan Mu, Qiuling Liu, Jiying Wen, Fangfang Wang, and Jin Ouyang*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
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DOI: 10.1039/b000000x
The self-assembly of diphenylalanine peptide (L-Phe-L-Phe)
In this work, the selfꢀassembly of PheꢀPhe peptides (FF) into
into microtubes with “turn on” bright yellowgreen 50 microtubes with “turn on” fluorescence by a new AIE molecule
fluorescence were described, which achieved by an
aggregation-induced emission (AIE) molecule of 9,10-bis[4-(3-
sulfonatopropoxyl)-styryl] anthracene (BSPSA) sodium.
of 9,10ꢀbis[4ꢀ(3ꢀsulfonatopropoxyl)ꢀstyryl] anthracene (BSPSA)
was reported (Molecular structure of FF and BSPSA is shown in
Figure S2, Supporting Information). With the presence of BSPSA
during the selfꢀassembly of FF, the obtained microtubes show the
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Selfꢀassembly of peptides, biological building blocks composed 55 yellowgreen fluorescence, and the BSPSA as well as the FF
of several to dozens of amino acids, is attracting more and more
attention owing to their structural simplicity and tunability,
functional versatility, cost effectiveness, and widespread
content will have effects on this fluorescence signal. As shown in
Figure 1, we obtained the microtubes by the selfꢀassembly of FF
in both water and BSPSA solution (The preparing procedures are
shown in Supporting Information). However, two kinds of
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applications. It is effectively achieved by interactions of
hydrogen bonds and amino acid residues, electrostatic 60 microtubes give the quite different fluorescence signals. For the
interactions, hydrophobic interactions and πꢀπ stacking
microtubes obtained in water, no remarkable fluorescence
response was observed, and therefore we only recorded the bright
field images (shown in Figure 1A). While the microtubes formed
by FF assembly in BSPSA solutions showed the remarkable
2, 3
interactions. Among these, diphenylalanine peptide (LꢀPheꢀLꢀ
Phe, FF), a core recognition motif of Alzheimer’s βꢀamyloid
polypeptide, has been extensively studied due to its simple
structure, versatile function and broad applications in biology as 65 yellowgreen fluorescence (Figure 1B). In addition, no remarkable
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well as in nanotechnology.
FFꢀbased building blocks are able
background was recorded for the same concentration of BSPSA
solution. Furthermore, if incubating formed nonꢀfluorescence
microtubes with BSPSA solution after the microtubes were
to selfꢀassemble into a variety of nanostructures including
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nanotubes, spheric vesicles, nanofibrils, nanwires, and ordered
molecular chains, which can be regarded as an assembly of
formed in water, the microtubes showed nonꢀhomogeneous
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, 9
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molecules into supramolecular systems. These selfꢀassemblied 70 fluorescence, which was due to the aggregation of BSPSA on
peptides have important roles for nanoꢀdevice development.
As we know, selfꢀassembly can provide an effective way
the surface of microtubes (Figure S3ꢀA). This signal was also
different from fluorescence emission of fluorescent microtubes
by FF selfꢀassembly in BSPSA aqueous solution (Figure S3ꢀB).
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towards the formation of nanofilbers with luminescent features.
However, the emissions would often be weaken largely owing to
the formation of detrimental excimers or exciplexes when the
luminophores are condensed and aggregated, which is known as
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aggregationꢀcaused quenching (ACQ).
Opposite to ACQ,
Tang’s group has found “aggregation induced emission (AIE)”ꢀ
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active molecules,
which are nonꢀfluorescent molecularly
dissolved in solvents, while they emits strong luminescence upon
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aggregating in solvents.
These molecules have been used as
fluorescence sensors, biological probes, immunoassay markers,
polyacrylamide gel electrophoresis (PAGE) visualization
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agents, organic lightꢀemitting diodes, and etc. Recently, the
fabrication of selfꢀassembled fluorescent nanofilbers utilizing
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AIEꢀactive molecules,
AIEꢀactive anionic dye, or guest 75 Figure 1. Images of fluorescence microscope for FF assemblages in water
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(A) and BSPSA solution (B). (C) and (D) are the corresponding statistical
dyes have been reported. The further studies on the selfꢀ
assembly of peptides with AIE molecules would be significant to
find a simple and efficiency way to construct microꢀoptical
devices for nano/microscale biomaterials in tissue engineering
and will have applications in regenerative medicine and
distribution of diameters for microtubes in (A) and (B). (E) and (F) are
the corresponding magnified SEM images. BSPSA concentration: 0.01
mg/mL.
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Then, a comparation on distribution and diameter of FF
assemblages synthesized either in water and BSPSA solution
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controlled drug release.
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were carried out. As demonstrated in Figure1C, the microtubes 50 in a certain BSPSA aqueous solution was also examined by the
obtained in water showed 0.5 to 6 ꢁm of diameters. In BSPSA
aqueous solution, the diameters of the nanotubes are in the range
of 2 to 8 ꢁm (Figure 1D). Furthermore, the microꢀstructures of
two kinds of microtubes were examined by scanning electron
selfꢀassembly of 1.0 to 4.0 mg/mL of FF in 0.01 mg/mL BSPSA
aqueous solution. As shown in Figure S5, with the increase of FF
concentration, the FL intensity increased accompanied a slightly
red shift (from 532 nm to 544 nm). This is in accordance with the
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microscope (SEM). As shown in Figure 1E, the microtubes 55 blue shift with the increase of BSPSA concentration
prepared in water show the hollow tubular structure with the
irregular diameter and the sharp tip. While the microtubes formed
by the selfꢀassembly of FF in BSPSA aqueous solution (Figure
1F) showed much larger diameter of hexagonal hollow
microtubes with flat tip of. Thus, the microtubes formed by the
selfꢀassembly of FF in BSPAS aqueous solution can give the
bright yellowgreen fluorescence with the wider diameters.
(demonstrated in Figure 3).
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Figure 3. Fluorescence spectra of FF assemblages in different
concentrations of BSPSA solutions. BSPSA concentration: 0.005, 0.01,
0.015, 0.02, 0.03, and 0.04 mg/mL, respectively. FF concentration: 0.2
mg/mL. Excitation wavelength: 404 nm.
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Simultaneously, the polarized emissions of microtubes
obtained by the selfꢀassembly of different concentrations of FF in
BSPSA solution were recorded. As demonstrated, all the obtained
microtubes showed emission anisotropy with a higher PL
intensity in the horizontal direction than the vertical one. Figure
S6ꢀA shows the emission of microtubes obtained by the selfꢀ
assembly of 0.12 mg/mL FF, which resulted the reproducible
polarization ratios (I /I ) of 0.641, and the degree of polarization
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Figure 2. Fluorescence microscope images of FF assemblages prepared
in different concentrations of BSPSA aqueous solution. BSPSA
concentration: 0 mg/mL (A), 0.006 mg/mL (B), 0.008 mg/mL (C), 0.01
mg/mL (D), 0.02 mg/mL (E), and 0.04 mg/mL (F).
v
h
To study the role of BSPSA on FF selfꢀassembly, different 70 [P=(I ꢀI )/(I +I )] of ꢀ0.218. Furthermore, different values of I /I
v h
v
h
v
h
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25
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concentrations of BSPSA solutions ranged from 0 to 0.04 mg/mL
acted as the media for the selfꢀassembly of 0.20 mg/mL FF. The
obtained microtubes were then examined by the fluorescence
microscope. As shown in Figure 2, we obtained different
and P were resulted for microtubes by selfꢀassembly of different
concentrations of FF in BSPSA. As shown in Figure S6ꢀB, with
the increase of FF concentration, both I /I and P increased. As
v
h
reported, the fluorescence polarization is inversely proportional to
microtubes by the selfꢀassembly of FF in different solutions. No 75 the rate of molecular rotation. Once the viscosity and temperature
significant FL signal was observed for the microtubes obtained in
water (Figure 2A). While for the assemblage in BSPSA solutions,
the FL responses increased with the increase of BSPSA
concentration (Figure 2BꢀF). In addition, the densities as well as
of solution are held constant, it will be dependent on the
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molecular volume, that is, the size of fluorescent molecule.
Therefore, the increased P and I /I indicated the larger size of
v
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fluorescent assemblage by the selfꢀassembly of FF in BSPSA.
the diameters of microtubes are increased with increasing BSPSA 80 This further confirmed the successful selfꢀassembly of FF to
concentration till 0.01 mg/mL (Figure 2C). But to further increase
concentration, the length and diameter of microtubes will
decrease, even lead to the aggregation in 0.04 mg/mL BSPSA
solution (Figure 2F). Furthermore, the colors of the microtubes
have slightly changed from yellowgreen (Figure 2B) to bright 85 circular dichroism (CD) spectra of the obtained selfꢀassembly
yellow (Figure 2F). Thus, the BSPSA concentration does have
effect on the FF selfꢀassembly.
Fluorescence spectra of these assemblages were collected for
the examination. As shown in Figure 3, with the increase of
BSPSA concentration, the FL intensity for characteristic peak of 90 mg/mL, the solution will show some aggregates or crystals, and
BSPSA increased slightly with a blue shift from 536 nm to 527
nm. The FL picture is shown in the inset of Figure 3. Furthermore,
emission peak of BSPSA at about 435 nm (Figure S4) also
confirmed the little influences of BSPSA on the fluorescence
emission of the microtubes (at about 530 nm). It can be 95 during the selfꢀassembly.
concluded that the BSPSA concentration will have effect on FF
selfꢀassembly, which will result in the different color of FL
signals as well as the different FL intensity. This result also
demonstrates the important role of water in generating FL
generate microtubes with fluorescent signals.
For preliminarily studying the mechanism of selfꢀassembly of
FF in BSPSA aqueous solution, ultraviolet and visible (UVꢀVis)
spectroscopy, fluorescence spectroscopy and the corresponding
system were recorded. As shown in Figure S7ꢀA, the UVꢀvis
spectra of FF microtubes show triple absorption peaks centered at
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257 nm attributed to the aromatic residues of FF molecules.
However, when the FF concentration increased to higher than 2.0
no obvious peak was observed (Figure S7ꢀB). As demonstrated,
for the selfꢀassembly of FF without aggregation, no obvious shift
or broadening was observed for four microtubes, which indicated
no strong πꢀπ interactions occur between the aromatic residues
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In addition, the characteristic
fluorescence emission of FF centered at 290 nm was also
recorded, and no peak shift was observed in different BSPSA
solutions (Figure S8ꢀA). It can be demonstrated that instead of πꢀ
π stacking, it is more likely that the hydrophobic collapse of FF
microtubes. The selfꢀassembly of different concentrations of FF 100 aromatic residues acts as a driving force to direct the formation of
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microtubes. To examine whether the selfꢀassembly will affect
the secondary structure of the peptide, the CD spectra in the far
UV region was recorded. As shown in Figure S8ꢀB, the
microtubes obtained in water, 0.01 mg/mL BSPSA and 0.04
mg/mL BSPSA all show the CD peak at about 218 nm (indicated
detections.
Acknowledgment
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The authors gratefully acknowledge the support from the
National Nature Science Foundation of China (91027034,
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of a typical βꢀsheet structure),
which demonstrated that the
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1005007, 21175014), and A Foundation for the Author of
selfꢀassembly has no effect on the secondary structure of the
assemblage. However, the heavily loaded BSPSA microtubes will
show the induced signals generated from the binding of achiral
molecule of BSPSA (In Figure S8ꢀB, BSPSA showed no intrinsic
CD signal due to the planar aromatic chromophores) to the chiral 65 College of Chemistry, Beijing Normal University, Beijing 100875, China
molecule of PheꢀPhe.
Therefore, the selfꢀassembly mechanism of FF in BSPSA
solution to generate fluorescent microtubes was proposed. The
soluble BSPSA solution is weakly luminescent because the
intramolecular rotation of these molecules in solution is active
and will form a relaxation channel for nonꢀradiative decay of the
excited state. In the selfꢀassembly of 2.0 mg/mL FF, the
solution was first ultrasonic treated at 65 °C for 30 min. While
during the annealed to room temperature, the FF tended to be
oversaturated in the solution. Then, the kinetic control of
nucleation and growth during the slow crystallization or
National Excellent Doctoral Dissertation of PR China (201221).
Notes and references
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Fax: 86 1062799838; Tel: 86 1058805373; E-mail:
jinoyang@bnu.edu.cn
†
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
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We have successfully applied an AIE molecule of BSPSA in
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fluorescence. Microtubes obtained by the selfꢀassembly of FF in
BSPSA aqueous solution emit the bright yellowgreen
fluorescence, and show the wider diameters of microtubes than
the ones obtained by the FF assembly in water. The BSPAS
content relative to FF amount has effects on the morphology of
microtubes, FL intensity and the emission wavelength. This work
provides a facile pathway for constructing fluorescence peptide
microtubes and possesses the capability for adjusting morphology
and FL signals of microtubes. This may expand the application of
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