Compartmentalized host spaces accommodating guest aromatic molecules in a chiral way in a helix-peptide-aromatic framework

Article abstract of DOI:10.1039/c8cc07380e

A novel host molecular assembly of a free-standing flat nanosheet with compartmentalized spaces was prepared using a bolaamphiphilic peptide composed of two amphiphilic helical peptides and an oligo(naphthaleneethynylene) (ONE) unit at the center of the m

Full text of DOI:10.1039/c8cc07380e

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Compartmentalized host spaces accommodating
guest aromatic molecules in a chiral way in a
helix-peptide-aromatic framework†
Cite this: Chem. Commun., 2018,
54, 12483
Received 11th September 2018,
Accepted 11th October 2018
a
c
Hirotaka Uji,
Shunsaku Kimura
Junya Ogawa,a Kenji Itabashi,b Tomoya Imai
and
a
*
DOI: 10.1039/c8cc07380e
rsc.li/chemcomm
A novel host molecular assembly of a free-standing flat nanosheet which can be dispersed in a polymer membrane8 and self-
with compartmentalized spaces was prepared using a bolaamphi- assembled into supramolecular polymers.9 MOP can provide
philic peptide composed of two amphiphilic helical peptides and an cavities of ca. 1.5 nm and more,8,10 but channels composed of
oligo(naphthaleneethynylene) (ONE) unit at the center of the mole- continuous and relatively large cavities are still difficult to obtain.
cule. The nanosheet possesses void host spaces that can accom- One way to solve the instability problem of the large pore and
modate two mol-equivalent ONE groups to form columns of ONE channel structure generated by the self-organization of organic
groups in a right-handed helical way and ONE channels over a long linkers by adding a space-filled component to the organic linker-
distance. The present molecular system therefore can provide a frame structure is considered and proposed here. As an analogy,
chiral pore channel for relatively large molecules.
our idea can use a house construction method of frame wall
construction using walls and floors or ceilings as flat space-filled
Producing synthetic 2D materials on a length scale of 10 nm–10 mm components to support the house, which is in contrast to the axis
has been an attractive challenge,1 and free-standing nanosheets construction method using only pillars and beams as vertical and
and free-floating sheets can be prepared using self-assembly horizontal axes, respectively. With the aim of obtaining a large pore
methods.2,3 Functionalized ultrathin materials comprising aro- and long channel structure, we designed here the molecular
matic groups at the core region have also been reported.4,5 To assembly of a free-standing sub-mm sheet with ca. 10 nm thickness
the best of our knowledge, however, free-standing nanosheets with including the void host space at the center.
void spaces that have an ability to accommodate guest molecules
Another challenge here with regard to the pore and channel
have not been reported so far, which makes a vivid contrast to structure is to accommodate guest molecules in a chiral arrange-
metal–organic frameworks (MOFs). MOFs have been attracting ment. Chiral environments have been provided in MOFs using
increasing attention as functional porous materials applicable for chiral organic linkers to achieve an enantioselective reaction11
various purposes.6 MOFs are advantageous for their simple pre- and enantioselective recognition.12 Apart from these instances, we
paration and system designs with high versatility and tunability for report here the arrangement of guest aromatic molecules in a
relevant functions. One of the current challenges for porous spiral way in the void spaces. Thereby, achiral aromatic molecules
materials is achieving significant porosity characterized by large self-assemble in a one-handed helical array without any help from
intermetallic separation and wide accessibility.7 However, they are covalent joints. Further, the continuous pores make channels at a
generally poor in terms of thermal and mechanical stability. When long-length scale of mm, where the aromatic molecules are filled
the requirement of crystallinity of the porous material is removed, in with a retention of p-stacking. A homochiral form of the porous
large porosity is attainable using metal–organic polyhedra (MOP), organic cage was reported,13 however, which provided an isolated
space. Chirality control at a long-length scale, which is still a
challenging task,14–16 is targeted here. Homochiral porous mole-
a Department of Material Chemistry, Graduate School of Engineering,
cular networks were also reported, however, using substrate
Kyoto University, Kyoto-Daigaku-Katsura, Nishikyo-ku, Kyoto 615-8510, Japan.
surfaces to organize molecules into two-dimensional arrays.17
E-mail: shun@scl.kyoto-u.ac.jp
b Department of Polymer Chemistry, Graduate School of Engineering,
Kyoto University, Kyoto-Daigaku-Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
c Research Institute for Sustainable Humanosphere, Kyoto University, Uji,
Kyoto 611-0011, Japan
On the other hand, in the present paper, a free-standing two-
dimensional sheet was prepared for chiral channel environments
without the help of an interface.
The molecular structure of the free-standing nanosheet
with pore and channel structures is shown as S29L8ONE in
Fig. 1. Two chains of the AB-type amphiphilic polypeptide,
† Electronic supplementary information (ESI) available: Peptide synthesis, pre-
paration of molecular assemblies, TEM and electron diffraction, AFM measure-
ments, and conditions of spectroscopy. See DOI: 10.1039/c8cc07380e
This journal is ©The Royal Society of Chemistry 2018
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arrangement with ca. 5 Å intervals (Fig. 1E). In addition, the
naphthyl groups of the ONE moiety are expected to contribute
to the channel formation due to the kink structure via linkages
at the b position leading to successive connections of the ONE
cores through p-stacking of the naphthyl groups (Fig. 1F).20–22
The peptide and ONE components of S29L8ONE were synthe-
sized separately, and subsequently combined with one another
(synthetic details are available in the ESI,† Fig. S1–S3). The Leu-Aib
alternating sequence was synthesized using the conventional
liquid-phase method. The ONE component was synthesized using
Sonogashira coupling reactions starting from 2,5-dibromohydro-
quinone. After these components were combined, poly(sarcosine)
chains were grown by Sar-NCA polymerization from the Leu
N-terminus.
Molecular assemblies were prepared using the injection
method. A 10 mL DMF solution of S29L8ONE was injected into
1 mL MilliQ water (1% DMF) at 4 1C followed by heat treatment
at 50 1C for 1 h. Molecular assemblies as small as ca. 10 nm were
observed using TEM (Fig. 2A). Liquid AFM was used to obtain
the topological image, which clearly shows a flat nanosheet
morphology with a height of 10.0 Æ 2.0 nm (Fig. 2B and C,
and the height histogram is shown in Fig. S4, ESI†). The side-by-
side association of helices is considered to be the driving force of
the formation of the self-assemblies (Fig. 1D). Previously we have
Fig. 1 (A) Chemical structures of S29L8ONE and BocONE. (B) Schematic
illustration of S29L8ONE. (C) Molecular model of S29L8ONE without
poly(sarcosine) moieties. (D) Schematic illustration of the arrangement of
two adjacent Leu-Aib helices (left) and the molecular assembly structure
prepared from S29L8ONE (right). Black and grey balls of isobutyl moieties
are protruding over the front side and back side of the helices, respectively.
(E) Schematic illustration of the column structure of the ONE groups.
(F) Schematic illustration of the channel structure of the ONE groups.
poly(sarcosine)-b-(L-Leu-Aib)4, were symmetrically connected to
the oligo(naphthalene-ethynylene) (ONE) core. Previously, poly-
(sarcosine)-b-(L-Leu-Aib)6 was reported to self-assemble into a
nanotube morphology as a result of the association of hydro-
phobic helices in water.18 The favourable space filling of Leu-side
chains between neighboring helices in a knob-and-hole arrange-
ment is the origin of the robust self-assembled monolayer.19 The
blocks of poly(sarcosine)-b-(L-Leu-Aib)4 in S29L8ONE are therefore
Fig. 2 TEM and AFM measurements of molecular assemblies. (A–C) Negatively
stained TEM image with 2% uranyl acetate (A), liquid AFM image (B), and height
expected to self-assemble in water into two monolayers acting as profile (C) of molecular assemblies prepared from S29L8ONE (9.5 Â 10À2 mM)
with heating treatment at 50 1C for 1 h. (D) TEM image of molecular
walls and sandwiching the ONE cores as pillars (Fig. 1D and E).
assemblies prepared from S29L8ONE (9.5
Â
10À2 mM)
+ BocONE
Since the diameter of the helices is around 1.5 nm, which is
thicker than the flat ONE group, there will be a void space in the
immediate surroundings of the ONE cores where two additional
(1/2 mol/mol) with heating treatment at 50 1C for 1 h. (E–G) TEM image (E),
liquid AFM image (F), and height profiles (G) of molecular assemblies prepared
from S29L8ONE (9.5 Â 10À2 mM) + BocONE (1/2 mol/mol) with heating
aromatic groups can be accommodated in a face-to-face treatment at 90 1C for 24 h.
12484 | Chem. Commun., 2018, 54, 12483--12486
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analysed a nanosheet morphology prepared from a mixture of
poly(sarcosine)-b-(L-Leu-Aib)6 and poly(sarcosine)-b-(D-Leu-Aib)6
to possess a 7.4 nm thickness,23 where the thickness of the
poly(sarcosine) layer was estimated to be ca. 2.8 nm. Accordingly,
the total thickness of two poly(sarcosine) layers of 5.6 nm, two
helix-block layers of 2.4 nm, and the ONE layer of 2 nm is
estimated to be 10 nm, which is consistent with the thickness
observed with AFM. S29L8ONE is therefore suggested to self-
assemble into a monolayer with two hydrophobic helix layers
flanking the ONE layer with void spaces. Further heating at 90 1C
for 24 h, however, made the molecular assemblies transform
into irregular aggregates, suggesting that the tiny nanosheets are
not thermodynamically stable (Fig. S5, ESI†).
As described above, the molecular assemblies of S29L8ONE
will have void spaces to accommodate two flat guest molecules on
the basis of 5 Å intervals available for p-stacking. Accordingly, a
mixture of S29L8ONE and BocONE at a molar ratio of 1/2 was self-
assembled in water. After heat treatment at 50 1C for 1 h, some
aggregates of a few tens of nanometers, in addition to the tiny
molecular assemblies, were observed using TEM (Fig. 2D). The
hydrophobic guest molecules of BocONE may be difficult to
incorporate into the molecular assemblies. However, further heat-
ing at 90 1C for 24 h afforded molecular assemblies of a square-
sheet morphology with a length of ca. 1 mm, as indicated by the
TEM (Fig. 2E) and liquid AFM (Fig. 2F and Fig. S6, ESI†) observa-
tions, which were in contrast to the irregular aggregates observed
in the absence of BocONE (Fig. S5, ESI†). The AFM height profiles
clearly show the formation of nanosheets with a height of
ca. 10 nm (Fig. 2G), which is consistent with the dimension of
the S29L8ONE monolayer. The surface roughness decreased down
Fig. 3 Spectra of molecular assemblies prepared from S29L8ONE
(9.5 Â 10À2 mM) + BocONE. (A–D) Molar ratio dependent UV-vis spectra
(A and C) and fluorescence spectra (B and D) with heat treatment at 50 1C
for 1 h (A and B) and at 90 1C for 24 h (C and D) in MilliQ. (E and F) Heat
treatment time dependent UV-vis spectra (E) and CD spectra (F) of mole-
cular assemblies prepared from S29L8ONE (9.5 Â 10À2 mM) and BocONE at
a molar ratio of 1/2 in MilliQ.
It is notable that the absorption in the range from 250 nm to
to 0.5 nm, supporting the formation of the homogeneous nano- 400 nm decreases in intensity with the emergence of a new peak at
sheet structure. Structural evolution with different heating 425 nm, indicating that the long tailing over 600 nm is due to
treatments should follow a path which can be controlled by a elongated p-stacking with less contribution from the light scattering
combination of thermodynamics and kinetics.24–26 It is sug- of the large molecular assemblies. The hypochromism around
gested that the induced type of host–guest interaction stabilizes 250 nm to 400 nm also supports the idea that the ONE groups
the molecular assemblies thermodynamically, and the driving begin to take regular positions in close proximity to the H type in the
force is considered to be the combination of hydrophobic molecular assembly with heat treatment at 90 1C for 24 h.
interactions and p-stacking between the ONE groups.
Fluorescence spectra show monomer emission at 425 nm and
Molecular assemblies with varying amounts of the guest 445 nm and emission over 490 nm with vibrational structures
molecule BocONE were analysed using UV-vis, fluorescence, and (Fig. 3D). The latter is considered to be emitted from the partial
CD spectroscopy. BocONE alone shows an absorption tail over overlap species because the excitation spectrum indicates a con-
450 nm, which should be due to self-aggregation. The tailing tribution from the partial overlap species at 425 nm (Fig. S7H,
disappears in the presence of S29L8ONE, meaning that BocONE ESI†). These drastic changes in the UV-vis and fluorescence
should disassemble upon incorporation into the S29L8ONE assem- spectra indicate that the assembly evolution could be controlled
blies (Fig. 3A). Fluorescence spectra of the mixtures of S29L8ONE primarily by p-stacking interactions between the ONE groups.
and BocONE, however, show the additive spectra of S29L8ONE and
BocONE in water, suggesting that any specific emissive species are assemblies of a mixture of S29L8ONE and BocONE (1/2 mol/mol)
not formed with heat treatment at 50 1C for 1 h (Fig. 3B). was analysed using CD measurements (Fig. 3F). An induced Cotton
The spatial arrangement of the ONE groups in the molecular
With heat treatment at 90 1C for 24 h, drastic changes were effect with a large exciton coupling centered at 425 nm appeared
observed in the UV-vis and fluorescence spectra. A new absorption with heat treatment at 90 1C in a similar time scale to the
signal at 425 nm and a long tailing appeared in the UV-vis spectra appearance of the absorption signal at 425 nm (Fig. 3E). Since the
(Fig. 3C). The former absorption is ascribable to a partial overlap intensity of the Cotton effect due to the exciton coupling changed
of the ONE groups, and the latter may correspond to a successive the sign from positive to negative with a shift to shorter wave-
overlap of the ONE groups over a long-distance. The time-course lengths, four ONE groups, two from the cores and two from the
of the UV-vis spectral change revealed that the molecular assem- guests, take a right-handed helical arrangement to form columns
blies were converted into a new structure with time (Fig. 3E).
(Fig. 1F). It is speculated that these columns are successively aligned
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This research was supported partially by JSPS KAKENHI
Grant Numbers JP16H02279 and JP18K19000, and collaboration
with Prof. Junji Sugiyama, Analysis and Development System
for Advanced Materials, Research Institute for Sustainable
Humanosphere, Kyoto University.
Conflicts of interest
Fig. 4 Electron diffraction pattern with an electron beam perpendicular
to the axis of the molecular assembly prepared from S29L8ONE (9.5 Â
10À2 mM) + BocONE at a molar ratio of 1/2 in MilliQ with heat treatment at
90 1C for 24 h. Inset axes a* and c* correspond to the axes a and c in Fig. 2D.
There are no conflicts to declare.
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12486 | Chem. Commun., 2018, 54, 12483--12486
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