Morphology change from nanotube to vesicle and monolayer/bilayer alteration by amphiphilic block polypeptides having aromatic groups at c terminal

Article abstract of DOI:10.1246/bcsj.20170044

Various amphiphilic block polypeptides having aromatic groups at the C terminal of a Leu-based hydrophobic helical block, poly(sarcosine)-b-(L-Leu-Aib)6-ethylcarbazole, poly- (sarcosine)-b-(D-Leu-Aib)6-naphthalimide, and poly(sarcosine)- b-(D-Leu-Aib)6-po

Full text of DOI:10.1246/bcsj.20170044

Morphology Change from Nanotube to Vesicle and Monolayer/Bilayer
Alteration by Amphiphilic Block Polypeptides Having Aromatic Groups
at C Terminal
Cheol Joo Kim, Saki Kurauchi, Tsuguaki Uebayashi, Ai Fujisaki, and Shunsaku Kimura*
Department of Material Chemistry, Graduate School of Engineering, Kyoto University,
Kyoto-Daigaku-Katsura, Nishikyo-ku, Kyoto 615-8510
E-mail: shun@scl.kyoto-u.ac.jp
Received: February 2, 2017; Accepted: February 23, 2017; Web Released: March 2, 2017
Shunsaku Kimura
Shunsaku Kimura, received his Ph.D. from Kyoto University in 1982. He was an instructor 1981–1992, a
lecturer 1992–1993, and associate professor 1993–1999. Since 1999, he has been a professor at Kyoto
University.
Abstract
philes.1,2 This parameter was originally applied for relatively
small lipid-like molecules, but was extended to amphiphilic
block polymers to confirm the applicability of the parameter
Various amphiphilic block polypeptides having aromatic
groups at the C terminal of a Leu-based hydrophobic helical
block, poly(sarcosine)-b-(L-Leu-Aib) -ethylcarbazole, poly-
3
,4
even for macromolecules. There are, however, several diffi-
cult tasks to be studied on control of molecular assemblies about
functionalization and preparation of complex morphologies
6
(sarcosine)-b-(D-Leu-Aib) -naphthalimide, and poly(sarco-
6
sine)-b-(D-Leu-Aib) -porphyrin, were synthesized and self-
6
5
assembled in water. Molecular assemblies of distorted sheet,
saddle-like sheet, and micelle were identified, which were dif-
ferent from the curved sheet of poly(sarcosine)-b-(L-Leu-Aib)6
without any aromatic groups, indicating that the attachment of
the aromatic groups to the C terminal significantly influenced
the morphology. Upon heating at 90 °C, vesicles were generated
with the three amphiphilic polypeptides with aromatic groups as
opposed to the nanotubes of poly(sarcosine)-b-(L-Leu-Aib)6.
The precise observations of these vesicles by AFM revealed
formation of monolayer for poly(sarcosine)-b-(L-Leu-Aib)6-
ethylcarbazole and bilayers for poly(sarcosine)-b-(D-Leu-Aib)6-
which are commonly found in nature as organelles in cells.
When we even limit the discussion on molecular assembly of
membrane or sheet morphology, it is still difficult to control
formation of monolayer or bilayer and to convert morphology
4
,6­8
from one to the other via membrane fusion and fission.
Further, functionalization of molecular assemblies by introduc-
tion of chromophores into amphiphiles generally makes it
difficult to predict the morphology of the molecular assemblies
on the basis of the amphiphiles prior to the introduction because
9
of change in hydrophobic/hydrophilic balance.
Since molecular assembly in water is driven by hydrophobic
interaction, the precise structural control of hydrophobic blocks
naphthalimide and poly(sarcosine)-b-(D-Leu-Aib) -porphyrin.
6
1
0
Attachment of aromatic groups at the C terminal therefore influ-
ences morphology and monolayer/bilayer alteration in accord-
ance with the type of aromatic groups.
is a first step for resolving several challenging tasks. We have
been using hydrophobic helical peptides for a hydrophobic
11­13
block,
which has advantages for control of molecular
packing by a wide range of elements of varying helical rod
lengths, availability of side chains displaying on the surface of
the helical rods, and tuning of membrane stiffness by stereo-
complex formation between right- or left-handed helices. For
example, with using the amphiphilic block polypeptides having
a hydrophobic helical block and a hydrophilic block of poly-
(sarcosine), a chimeric morphology of a round-bottomed flask
1
. Introduction
Amphiphilic molecules self-assemble into a wide range
of morphologies such as micelle, fiber, sheet, and vesicle in
accordance with their molecular-packing properties. The criti-
cal packing parameter is an established index which predicts
morphology of molecular assembly on the basis of molecular
shape and hydrophobic/hydrophilic balance of the amphi-
1
4
shape was attained by using nanotubes and vesicles compris-
ing the amphiphilic block polypeptides through membrane
1
2,15
fusion and fission.
568 | Bull. Chem. Soc. Jpn. 2017, 90, 568–573 | doi:10.1246/bcsj.20170044
© 2017 The Chemical Society of Japan

In the present study, we focused our attention on studying
the effect on morphology and membrane structure of intro-
duction of functional aromatic groups at the C terminal of
amphiphilic block polypeptide, poly(sarcosine)-b-(Leu-Aib)6,
which was reported to self-assemble into curved sheets and
upon heating to be converted into nanotubes.16 The curved
sheets and nanotubes were generated on the basis of the tight
helix packing in a monolayer membrane with interdigitated
helices. When the amphiphilic polypeptide is modified with
aromatic groups for functionalization, the tight helix packing
may be distorted to generate different morphology and mem-
brane structure due to the hydrophobic and π­π interactions
among aromatic groups. Herein, aromatic groups of N-ethyl-
carbazole, naphthalimide, and porphyrin were introduced to the
amphiphilic polypeptide, and their self-assemblies in water
were studied.
Cu grid and negative-stained with 2% uranyl acetate, followed
by suction of the excess fluid with filter paper.
2.5 Dynamic Light Scattering (DLS) Measurement. The
hydrodynamic diameter of dispersion was analyzed with a
DLS-8000KS (Photal Otsuka Electronics) using He-Ne laser.
Each self-assembly solution was filtered with a 0.80 ¯m PVDF
(polyvinylidene fluoride) syringe filter (GE Healthcare UK
limited) prior to measurements.
2.6 Atomic Force Microscopy (AFM). AFM imaging was
measured with an Agilent 5500 AFM (Agilent Technologies
Inc., CO, USA) in acoustic AC mode (AAC mode, so-called
tapping mode) using silicon nitride cantilevers (BL-AC40TS,
0.1 N/m, Olympus Corp.) in water and silicon cantilevers
(OMCL-AC160TN, 26 N/m, Olympus Corp., Tokyo, Japan) in
air for peptide-sheet and vesicle observation, respectively.
The Si-wafers for self-assembly immobilization were modi-
fied with 3-aminopropyltriethoxysilane (APTES). The freshly
prepared self-assembly dispersion was incubated on an APTES-
modified Si-wafer at room temperature for 1 h in a liquid cell.
The specimens for AFM imaging in air were dried in a vacuum
chamber for 12 h.
2
. Experimental
2
.1 Materials.
Boc-L-leucine, Boc-D-leucine, 2-amino-
isobutyric acid (aib) and Z-sarcosine were purchased from
Watanabe Chemical Industries, Ltd. (Japan). All other reagents
and solvents were purchased from commercial suppliers and
used as received.
3
. Results and Discussion
2
.2 Synthesis of Amphiphilic Block Polypeptides. Five
3.1 Morphology of Molecular Assemblies. Five kinds of
amphiphilic block polypeptides having commonly a hydro-
kinds of amphiphilic block polypeptides were prepared by the
liquid-phase method (Figure 1). Boc-(L-Leu-Aib) -OMe, Boc-
phobic helical block of (Leu-Aib) were synthesized (Figure 1).
6
6
(
D-Leu-Aib) -OMe, and two kinds of amphiphilic block poly-
Self-assemblies of these polypeptides in milliQ water were
prepared by the ethanol injection method. As previously report-
6
peptides of poly(sarcosine)-b-(L-Leu-Aib) -OCH (SLL), and
6
3
poly(sarcosine)-b-(D-Leu-Aib) -OCH (SDL) were synthesize
ed, poly(sarcosine)-b-(L-Leu-Aib) -OMe (SLL) self-assembled
6
3
6
as previously described.12 Other amphiphilic block polypep-
tides with functional groups were synthesized according to the
schemes in the supporting information.
into curved sheets, which were converted into nanotubes of
1
2
80 nm diameter upon heating at 90 °C for 1 hour (Figure 2).
The origin of the curving of the sheet morphology is ascribed to
the tight molecular packing of helices leading to consecutive tilt
of helix axis upon association. Three kinds of aromatic groups
2
.3 Preparation of Self-Assemblies. Amphiphilic block
polypeptides (10 mg) were dissolved in ethanol or methanol
200 ¯L or 300 ¯L). The polymer solution was injected into
(
are introduced at the C terminal of the (Leu-Aib) helix block,
6
distilled water (1 mL) with stirring at 4 °C for 30 min at the
final concentration of 1 mg/mL. All self-assemblies were pre-
pared with heat treatment at 90 °C for 48 hours.
which may affect sterically the helix packing or strengthen the
association by hydrophobic and π­π interactions. Molecular
assemblies formed just after the injection were observed by
TEM measurements to show distorted sheets of poly(sarcosine)-
2
.4 Transmission Electron Microscopy (TEM). TEM
images of self-assemblies were analyzed with a JEOL JEM-
000EXII at an accelerating voltage of 100 kV. For specimen
b-(L-Leu-Aib) -ethylcarbazole (SLL-ECz) and poly(sarcosine)-
6
2
b-(D-Leu-Aib) -naphthalimide (SDL-NapIm), and spherical
6
preparation, a drop of solution was mounted on a carbon-coated
micelles of poly(sarcosine)-b-(D-Leu-Aib) -porphyrin (SDL-
6
Por). The distorted sheets prepared from SLL-ECz and SDL-
NapIm take mostly a saddle-like shape, which is clearly differ-
ent from the curved sheet of SLL. The observed curving of the
sheets, however, suggests that the knobs-into-holes arrangement
Figure 1. Molecular structures of amphiphilc block poly-
peptides with various aromatic groups at the C terminal.
The most frequent residue numbers of poly(sarcosine)
blocks in TOF Mass spectra are 27, 28, 28, 27, and 20
for SLL, SLL-ECz, SDL, SDL-NapIm, and SDL-Por,
respectively.
Figure 2. Transmission electron microscopy (TEM) images
of SLL self-assemblies in milliQ water: a) just after
injection into water, b) with heating at 90 °C for 1 hour.
Negatively stained by uranyl acetate.
Bull. Chem. Soc. Jpn. 2017, 90, 568–573 | doi:10.1246/bcsj.20170044
© 2017 The Chemical Society of Japan | 569

Figure 3. TEM images of self-assemblies in milliQ water of a), e), i) a mixture of SLL and SDL (1:1), b), f), j) SLL-ECz, c), g), k)
SDL-NapIm, and d), h), l) SDL-Por. e)­h) Upon heat treatment at 90 °C for 48 hours. i)­l) Hydrodynamic diameters of vesicles
determined by DLS measurements. Negatively stained by uranyl acetate.
created by isobutyl groups on the helix surfaces still operates
dominantly for the helix association in SLL-ECz and SDL-
NapIm irrespective of attachment of ECz or NapIm at the C
terminal of the helix. The hydrophobic ECz and NapIm groups,
however, may increase the line tension to induce the morphol-
ogy of the saddle-like sheet.17 On the other hand, SDL-Por
molecular assemblies took micelle, small sheet, planar sheet,
and vesicle morphology but no saddle-like morphology by
time-lapse TEM images (supplementary information (SI),
Figures S3a and S3b). The observation of coexistence of small
sheets, planar sheets, and vesicles (Figure S3b) strongly support
our interpretation that small sheets were fused together to
grow into planar sheets, which were directly transformed into
vesicles. Especially, micelle formation (Figure S3a) suggests
that the hydrophobic interaction among porphyrins should
exceed the effect of the knobs-into-holes arrangement on the
helix association to result in the symmetric spherical morphol-
tified by TEM observation (Figure 3a), suggesting that the
knobs-into-holes arrangement brings a staggered association
(X shape) of right- and left-handed helices in the sheet into
the most sterically stable state. The sheet formation either with
a flat or distorted shape suggest the helix association and their
recognition should operate predominantly for molecular as-
sembling processes. On the other hand, the spherical micelle
morphology of SDL-Por means the effect of porphyrin is
significant on the molecular assembling probably due to the
large size of porphyrin.
With heat treatment at 90 °C for 48 hours, molecular assem-
blies in water convert morphology into a thermodynamically
stable one. Interestingly, SLL-ECz, SDL-NapIm, and SDL-
Por generated vesicles upon heating (Figures 3f, 3g, 3h),
which make a vivid contrast with nanotubes generated by SLL.
The mean sizes of vesicles were determined by DLS to be 151
nm (PDI = 0.10) for SLL-ECz, 165 nm (PDI = 0.64) for SDL-
NapIm, and 118 nm (PDI = 0.82) for SDL-Por (Figures 3j,
3k, 3l), which are relatively smaller than that prepared from a
mixture of SLL and SDL of 193 nm (PDI = 0.190) (Figure 3i).
1
8
ogy of micelles.
In the case of a binary system of SLL and poly(sarcosine)-b-
(
D-Leu-Aib) -OMe (SDL), flat sheet formation was iden-
6
570 | Bull. Chem. Soc. Jpn. 2017, 90, 568–573 | doi:10.1246/bcsj.20170044
© 2017 The Chemical Society of Japan

In regard to thermodynamic stability of morphologies, molec-
ular assemblies are more stabilized in the order of sheet <
nanotube < vesicle under an assumption of the hydrophobic
edges of morphologies being the primary factor for destabili-
zation of the molecular assembly in water. Since the stability
of nanotube is just between vesicle and sheet, TEM observa-
tions of samples with varying duration of heat treatment were
examined to see whether nanotube morphology appears as a
transient morphology from sheet or micelle to vesicle upon heat
treatment. Interestingly, SLL-ECz took nanotube morphology
at 6 hours after heating as an intermediate morphology between
distorted sheet and vesicle (Figure S2). On the other hand,
neither SDL-NapIm nor SDL-Por took nanotube morphology
at any moment during the heating process. Taken together,
nanotube morphology to prepare from the helical polypeptides
specifically requires strict and tight helix association to cause
consecutive helix tilting in molecular assembly, which prevents
further conversion into vesicle upon heating. Previously, we
reported a similar observation with molecular assembly pre-
pared from (poly(sarcosine)) -b-(L-Leu-Aib) having the same
a)
b)
c)
Figure 4. a) Atomic force microscopy (AFM) in water
image and b) a height profile along the line in a) of molec-
ular assembly of a mixture of SLL and SDL. c) Schematic
illustration of the interdigitated monolayer of a mixture of
SLL and SDL.
2
6
helical hydrophobic block. This A B-type polypeptide took a
2
rolled-sheet morphology, which could not be converted even
into nanotube morphology upon heating.19 The tightness of
molecular packing should therefore decrease in the order of
SLL > SDL-ECz > SDL-NapIm­SDL-Por in light of the
most stable morphology.
Polydispersity indices of SDL-NapIm and SDL-Por vesi-
cles by DLS measurements were 0.64 and 0.82, respectively,
while a mixture of SLL and SDL (SLLSDL) and SLL-ECz
vesicles showed significantly smaller values of 0.19 and 0.1,
respectively, meaning that SDL-NapIm and SDL-Por gener-
ated vesicles of variable sizes. One reason for the polydisper-
sity may be the presence of tiny sheets or spherical micelles
prior to heating, which could fuse with each other to grow into
various sizes. On the other hand, SLL-ECz generated relatively
large distorted sheets of 100 nm size prior to heating, which
size may determine the following vesicle size upon heating.
by AFM in water, because the self-assemblies took distorted or
saddle-like sheet morphology. Vesicles are therefore subjected
to analysis by AFM in air to examine the membrane structure.
3.3 Dimensions of Peptide Vesicle by AFM in Air. AFM
images and mean heights of the four vesicles in air are sum-
marized in Figures 5 and 6. The SLLSDL vesicle shows an
average height of 13.0 nm, which is considered to be the sum of
two monolayers keeping the vesicle structure on the substrate
upon drying. The membrane thickness of one monolayer is
therefore 6.5 nm, which is a little bit less than 7.4 nm when
measured in water, probably because the hydrophilic layer was
dehydrated and became thinner. In the case of the SLL-ECz
vesicle, vesicles seemed to be ruptured on the APTES-modified
Si-wafer substrate to be transformed into a sheet shape, because
the observed height of 5.3 nm is close to the monolayer thick-
ness of the SLLSDL vesicle of 6.5 nm. On the other hand,
SDL-NapIm and SDL-Por showed average thicknesses of
20.2 nm and 18.7 nm, respectively, which seem that two mem-
branes should pile up because the vesicle structure was retained
on the substrate upon drying. The membrane thicknesses are
therefore 10.1 nm and 9.4 nm, respectively, which are still
thicker than the interdigitated monolayer of the SLLSDL
vesicle of 6.5 nm. 1,8-Naphthalimide and porphyrin chromo-
phores are large hydrophobic aromatics, and π­π stacking
interaction is also available to stabilize molecular associa-
3
.2 Dimensions of Peptide Sheet by AFM in Water.
Whether vesicle membranes take monolayer or bilayer struc-
ture, membrane thicknesses were evaluated by AFM in water
(Figure 4). The flat sheet of a mixture of SLL and SDL prior to
heating by TEM observation was observed similarly by AFM.
The average height of the flat sheet was found to be 7.4 « 0.79
nm. The molecular arrangement in the sheet is explained by
alternating SLL and SDL helices like a molecular checkerboard
pattern on the basis of electron diffraction pattern leading to
formation of interdigitated monolayer.12 The monolayer is thus
composed of two hydrophilic poly(sarcosine) layers flanking a
middle hydrophobic (Leu-Aib) layer (Figure 4c). Because the
6
2
0,21
helix length of the 12 residue is calculated to be 1.8 nm, one
poly(sarcosine) layer should be 2.8 nm thick on average. In the
interdigitated structure, a poly(sarcosine) chain at the N terminal
has a free space aside because the neighboring C terminals do
not have a poly(sarcosine) chain. Each poly(sarcosine) chain
tion.
Due to the large size of the chromophore, the critical
packing parameter of the amphiphilic molecules may change
the molecular shape relatively from a cone-shape to a cylin-
drical shape. SDL-NapIm and SDL-Por are therefore consid-
ered to take a bilayer structure, with which the membranes will
become thicker by the sum of one helix of 1.8 nm and aromatic
layer of 1­2 nm than the interdigitated monolayer. The ob-
served differences in thicknesses taking the SLLSDL mono-
layer as a reference are 3.4 nm and 2.9 nm, respectively for
SDL-NapIm and SDL-Por, which are thus in consistent with
(the most frequent residue numbers in the TOF Mass are 25 and
2
7 of SLL and SDL, respectively) therefore have a free space to
allow the chain taking a mushroom structure of 2.8 nm thick.
Unfortunately, other peptides of SDL-ECz, SDL-NapIm,
and SLL-Por could not be evaluated for membrane thickness
Bull. Chem. Soc. Jpn. 2017, 90, 568–573 | doi:10.1246/bcsj.20170044
© 2017 The Chemical Society of Japan | 571

Figure 5. a), d), g), j) AFM in air images, b), e), h), k) height profiles, and c), f), i), l) histograms of membrane thicknesses of
a), b), c) SLLSDL, d), e), f) SLL-ECz, g), h), i) SDL-NapIm, and j), k), l) SDL-Por vesicles on APTES-modified Si-wafers.
residues. The existence of the aromatic layer in the middle of
the bilayer structure is also supported by the absorption spectra
(
SI, Figure S4). Bathochromic shifts and hypochromic effects
were observed with the absorption spectra of SDL-NapIm and
SDL-Por vesicles compared with those in methanol. Especial-
ly, vesicle of SDL-Por showed a small red-shift of 4 nm and a
strong hypochromic effect, suggesting formation of porphyrin
2
2,23
aggregate according to previous reports.
In order to confirm the interdigitated monolayer of SLL-
ECz, a fluorescence quenching experiment was carried out
(
Figure 7). The fluorescence quenching rates with varying
solvent and morphologies with the addition of acrylamide de-
crease in the order of in methanol > sheet­nanotube > vesicle.
The fluorescence was quenched the most in methanol because
SLL-ECz was molecularly dispersed in methanol. The quench-
ing rate of vesicle was the smallest, however, still showing
partial quenching. This observation is consistent with the
interpretation of the interdigitated monolayer of the SLL-ECz
vesicle, because only ECz groups, which are exposed to the
outer surface of vesicle, are subjected to acrylamide quenching
leading to the partial quenching.
Figure 6. Statistical significance of differences of average
membrane thicknesses of vesicles determined by the two-
tailed Student t test. p values; *** for p < 0.001.
the interpretation of the bilayer formation. The slightly thinner
membrane of SDL-Por than SDL-NapIm may be due to
the shorter poly(sarcosine) chain length of SDL-Por of 20
572 | Bull. Chem. Soc. Jpn. 2017, 90, 568–573 | doi:10.1246/bcsj.20170044
© 2017 The Chemical Society of Japan

TEM image of SLL-ECz self-assemblies upon heating at 90 °C
for 6 hours. This material is available on http://dx.doi.org/
1
0.1246/bcsj.20170044.
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This work is partly supported by Grants-in-Aid for Scientific
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2
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Supporting Information
2
3
Synthetic scheme of amphiphilic block polypeptides,
MALDI-TOF spectra of each amphiphilic block polypeptides,
Bull. Chem. Soc. Jpn. 2017, 90, 568–573 | doi:10.1246/bcsj.20170044
© 2017 The Chemical Society of Japan | 573