β-1,3-glucan polysaccharide (schizophyllan) acting as a one-dimensional host for creating supramolecular dye assemblies

Article abstract of DOI:10.1021/ol062229a

We have demonstrated that one-dimensional supramolecular dye assemblies with a uniform diameter can be created by utilizing schizophyllan (SPG) as a one-dimensional host. In the supramolecular nanofibers, the dye molecules are assembled into the different

Full text of DOI:10.1021/ol062229a

ORGANIC
LETTERS
2006
Vol. 8, No. 24
5533-5536
â
-1,3-Glucan Polysaccharide
(Schizophyllan) Acting as a
One-Dimensional Host for Creating
Supramolecular Dye Assemblies
Munenori Numata,† Shingo Tamesue,† Tomohisa Fujisawa,† Shuichi Haraguchi,†
|
Teruaki Hasegawa,† Ah-Hyun Bae,† Chun Li,†, Kazuo Sakurai,§ and
Seiji Shinkai*,†,‡
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu
UniVersity, Fukuoka 819-0395, Japan, Center for Future Chemistry, Kyushu
UniVersity, Fukuoka 819-0395, Japan, and Department of Chemical Processes and
EnVironments, Faculty of EnVironmental Engineering, The UniVersity of Kitakyushu,
1-1 Hibikino, Wakamatsu-ku, Kitakyushu, Fukuoka 808-0315, Japan
seijitcm@mbox.nc.kyushu-u.ac.jp
Received September 8, 2006
ABSTRACT
We have demonstrated that one-dimensional supramolecular dye assemblies with a uniform diameter can be created by utilizing schizophyllan
(SPG) as a one-dimensional host. In the supramolecular nanofibers, the dye molecules are assembled into the different aggregation modes
depending on the preparation procedures. The findings establish that SPG is useful for creating the supramolecular nanofibers, where temporal
superstructures can be stabilized by the SPG-specific helical higher-order structure.
Dipolar dyes and their one-dimensional assemblies have been
of much concern because of their strong absorption band
around the visible wavelength region due to the push-pull
mechanism of delocalized electrons. These dipolar dyes have
inherent properties to form self-assembling architectures
through intermolecular π-π stacking or dipolar-dipolar
interactions. So far, creating well-regulated supramolecular
assemblies from a rationally designed dye molecule has
attracted widespread interest in light of their potential
applications for nonlinear optical and photorefractive de-
vices.1 A particularly challenging aspect is to create a wide
variety of supramolecular assemblies from a single dye, for
which the rational control of intermolecular dye interactions
is considered to be a key process. Nevertheless, only a few
attempts have been reported to create a variety of dye
assemblies, and the rational control of dye interactions in
(1) (a) Wu¨rthner, F.; Yao, S.; Beginn, U. Angew. Chem., Int. Ed. 2003,
42, 3247. (b) Parins, L. J.; Thalacker, C.; Wu¨rthner, F.; Timmerman, P.;
Reinhoudt, D. N. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 10042. (c) Jones,
R. M.; Lu, L.; Helgeson, R.; Bergstedt, T. S.; McBranch, D. W.; Whitten,
D. G. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 14769. (d) Yao, S.; Beginn,
U.; Gress, T.; Lysetska, M.; Wu¨rthner, F. J. Am. Chem. Soc. 2004, 126,
8336. (e) Morikawa, M.-a.; Yoshihara, M.; Endo, T.; Kimizuka, N. J. Am.
Chem. Soc. 2005, 127, 1358.
* Fax: +81-92-802-2820. Phone: +81-92-802-2823.
† Department of Chemistry and Biochemistry, Kyushu University.
‡ Center for Future Chemistry.
§ The University of Kitakyushu.
| Present address: Department of Chemistry, Tsinghua University, Beijing
100084, China.
10.1021/ol062229a CCC: $33.50
© 2006 American Chemical Society
Published on Web 11/04/2006

the molecular level seems to still be very difficult. One of
the effective ways to overcome this difficulty is to use an
appropriate template,2 such as a biopolymer with a unique
higher-order structure, on which dye molecules can be
arranged in a well-regulated fashion to generate a wide
variety of colors as well as functions, reflecting their original
shapes and electronic properties.
Schizophyllan (SPG) is a natural polysaccharide produced
by the fungus Schizophyllum commun and adopts a triple
helix (t-SPG) in nature, which can be dissociated into a single
chain (s-SPG) by dissolving in dimethylsulfoxide (DMSO)
or alkaline media (Figure 1).3 The s-SPG chain can retrieve
supramolecular nanofibers with a uniform diameter and an
intrinsic helical supramolecular structure can be created.
Here, we report our novel findings that the creation of dye
assembling structures with a one-handed helical superstruc-
ture can be achieved in the unique one-dimensional SPG
cavity, where SPG acts not only as a one-dimensional host
for dye assemblies but also as a sheath to stabilize the
temporarily formed supramolecular polymers.
We designed here a dipolar dye 1 (Azo dye 1) having
pyridine and carboxylic acid terminals (Figure 1c).5 It is
expected that the intermolecular interactions among dye 1
molecules would involve cooperative π-π stacking and
dipolar-dipolar interactions and be mainly dominated by the
strong hydrogen-bonding interaction between the terminal
groups. This molecular design implies, therefore, that if the
composite formation is carried out in the presence of s-SPG
utilizing the different renaturing solvents, e.g., a DMSO or
NaOH solution, the dye assemblies with different molecular
arrangements would be entrapped in the SPG cavity; that is,
in DMSO solution, a self-assembling structure would be
more dominated by the hydrogen-bonding interaction, whereas
in an alkali solution, π-π stacking and dipolar-dipolar
interactions in addition to hydrophobic interactions would
become major driving forces.
First, to test the feasibility of this idea, we prepared the
DMSO solution containing s-SPG (10.0 mg mL-1, 100.0 µL,
MW ) 15 000) and mixed it with a DMSO solution of dye
1 (1.0 mg mL-1, 100.0 µL). To the resultant DMSO solution,
water (1800 µL) was gradually added to give a clear orange
solution. The final composition of water/DMSO (v/v) was
adjusted to 95:5 (v/v). During this procedure, the renaturing
of s-SPG and the self-assembling of dye 1 through the
intermolecular hydrogen-bonding interactions proceeded and
the supramolecular dye assemblies thus constructed would
be insulated in the SPG one-dimensional cavity. After leaving
the resultant solution for 2 days at room temperature, the
composite was characterized by spectroscopic and micro-
scopic measurements.
Figure 1. (a) Calculated models of an SPG triple helix and
repeating units of SPG. (b) Renature and denature processes of SPG.
(c) Designed dipolar dye with pyridine and carboxylic acid
terminals.
Figure 2a shows UV-vis spectra of the SPG/dye 1
solution. The absorption maximum of dye 1 itself appears
at 446 nm,6 whereas the peak of the composite is red shifted
to 468 nm. This significant red shift suggests that J-type
assemblies are mainly formed in the solution, promoted by
the original triple helix by exchanging DMSO for water or
by neutralization of an alkaline solution with an acid.
Recently, we have found that when this renaturing process
is carried out in the presence of a hydrophobic polymer, the
polymer can be incorporated into a one-dimensional cavity
created by the SPG helical structure with the aid of
hydrophobic interaction to give a clear aqueous solution. This
result suggests that SPG can act as a one-dimensional host
for a hydrophobic polymer to create a water-soluble nano-
composite, in which the individual guest polymer is isolated
and twisted by reflecting the original chiral motif of SPG.4
One may consider, therefore, that if dye molecules, which
are rationally designed to form the self-assembling structures,
can be entrapped in the SPG cavity it follows that insulated
(4) (a) Numata, M.; Asai, M.; Kaneko, K.; Hasegawa, T.; Fujita, N.;
Kitada, Y.; Sakurai, K.; Shikai, S. Chem. Lett. 2004, 33, 232. (b) Numata,
M.; Asai, M.; Kaneko, K.; Bae, A.-H.; Hasegawa, T.; Sakurai, K.; Shinkai,
S. J. Am. Chem. Soc. 2005, 127, 5875. (c) Numata, M.; Hasegawa, T.;
Fujisawa, T.; Sakurai, K.; Shinkai, S. Org. Lett. 2004, 6, 4447. (d) Bae,
A.-B.; Numata, M.; Hasegawa, T.; Li, C.; Sakurai, K.; Shinkai, S. Angew.
Chem., Int. Ed. 2005, 44, 2030. (e) Li, C.; Numata, M.; Bae, A.-H.; Sakurai,
K.; Shinkai, S. J. Am. Chem. Soc. 2005, 127, 4548. (f) Hasegawa, T.;
Haraguchi, S.; Numata, M.; Fujisawa, T.; Li, C.; Kaneko, K.; Sakurai, K.;
Shinkai, S. Chem. Lett. 2005, 34, 40. (g) Sakurai, K.; Uezu, K.; Numata,
M.; Hasegawa, T.; Li, C.; Kenji, K.; Shinkai, S. Chem. Commun. 2005,
4383.
(5) Similar molecular designs have already been reported: (a) Aoki, K.;
Nakagawa, M.; Ichimura, K. J. Am. Chem. Soc. 2000, 122, 10997. (b) Aoki,
K.; Nakagawa, M.; Seki, T.; Ichimura, K. Chem. Lett. 2002, 378. (c)
Nakagawa, M.; Ishii, D.; Aoki, K.; Seki, T.; Iyoda, T. AdV. Mater. 2005,
17, 200.
(2) (a) Hannah, K. C.; Armitage, B. A. Acc. Chem. Res. 2004, 37, 845
and references cited therein. (b) Amylose has been utilized as an effective
template for a dye assembly formation: Heuer, W. B.; Kim, O.-K. Chem.
Commun. 1998, 2649. (c) Clays, K.; Olbrechts, G.; Munters, T.; Persoons,
A.; Kim, O.-K.; Choi, L.-S. Chem. Phys. Lett. 1998, 293, 337.
(6) We took UV-vis spectra of Azo dye 1 as a function of concentration
(concentration range: 0-0.1 mg mL-1). From an LB plot at 446 nm, it is
confirmed that no aggregate is formed at the concentration range used.
(3) Yanaki, T.; Norisue, T.; Fujita, M. Macromolecules 1980, 13, 1462.
5534
Org. Lett., Vol. 8, No. 24, 2006

As an alternative strategy, we tried to prepare the
composite utilizing the neutralization process of an alkaline
solution containing SPG and dye 1, expecting that the
different dye arrangement may be achieved through π-π
stacking and dipolar-dipolar interactions. It is known that
the triple strand of SPG is dissociated into the single strand
at pH > 12, whereas it retrieves the original triple strand by
pH neutralization.3 Accordingly, we prepared an NaOH
solution containing s-SPG (10.0 mg mL-1, 100.0 µL, pH )
13) and mixed it with an NaOH solution of dye 1 (1.0 mg
mL-1, 100.0 µL, pH ) 13), where the carboxylic terminal
group should be dissociated into the carboxylate anion at
this stage. To the resultant solution, AcOH was gradually
added to give a clear yellow solution. The final pH was
adjusted to 7.0. UV-vis and CD spectroscopic measurements
of the resultant solution revealed that the self-assembling
architecture of dye 1 is created in the SPG one-dimensional
cavity. As shown in Figure 3a, one can recognize that the
Figure 2. (a) UV-vis and (b) CD spectra of the samples containing
the SPG/Azo dye 1 composite (blue lines), monomeric Azo dye 1
in DMSO (black line), and an Azo dye 1 aggregate in a water/
DMSO mixed solvent (red lines), prepared from a DMSO solution
(1.0 cm, rt). (c) Photo image of the solution containing the SPG/
Azo dye 1 composite. (d) Schematic illustration of the J-type
assembly formation during the renaturing of s-SPG: this type of
assembly would be created as a major component in the solution.
the intermolecular hydrogen bonding in addition to π-π
stacking interactions among dye 1 molecules.7 As a reference
experiment, a DMSO solution containing dye 1 itself was
diluted with water. Although the mixture provided the
precipitate after several minutes, one could measure the UV-
vis spectra of the temporarily “transparent” solution, which
gave a red shifted absorption maximum (468 nm) comparable
with that of the SPG/dye 1 composite. The result indicates
that dye 1 itself has the inherent property to form a J-type
assembly with the aid of hydrogen-bonding interactions even
in the absence of SPG. From these results, one may expect
that if the J-type assembly is entrapped in the SPG cavity it
adopts the one-dimensional and helical structure which is
enforced by the SPG-specific helical superstructure.
CD spectroscopy is helpful for monitoring the definitive
interaction between SPG and dye 1. Dye 1 itself is optically
inactive, so that no CD signal was detected even after
forming the J-type assembly in the mixed solvent. Upon
mixing with s-SPG, however, an intense split-type induced
CD (ICD) appeared in the π-π* transition region of the
dye 1 assembly (Figure 2b). The result strongly supports the
view that the J-type assembly is entrapped in the one-
dimensional cavity, in which the self-assembling nanofiber
structure with the twisted molecular arrangement is created
through the intermolecular hydrogen-bonding interaction
among dye 1 molecules.9
Figure 3. (a) UV-vis and (b) CD spectra of the samples containing
the SPG/Azo dye 1 composite (blue lines), monomeric Azo dye 1
in DMSO (black line), and the Azo dye 1 aggregate in a water/
DMSO mixed solvent (red lines), prepared from NaOH solution
(1.0 cm, rt). (c) Photo image of the solution containing the SPG/
Azo dye 1 composite. (d) Schematic illustration of the H-type
assembly formation during the renaturing of s-SPG: this type of
assembly would be created as a major component in the solution.
absorption maximum of dye 1 is blue shifted from 446 to
417 nm, accompanied by a slight peak broadening. This peak
broadening implies that the obtained yellow solution contains
several unidentified aggregates as minor components. How-
ever, taking the blue shift of the absorption maximum into
consideration, we can propose that the main structure created
by dye 1 is an H-type assembly, in which dye 1 adopts the
antiparallel orientation through dipolar-dipolar interactions.
(7) The formation of the intermolecular hydrogen-bonding interaction
is directly evidenced by IR spectroscopy (Supporting Information, Figure
(9) As the CD signal shows the negative Cotton effect, one may consider
that the left-handed helical structure is created inside the SPG cavity despite
the right-handed helical nature of SPG. The CD sign reflects the dye dipole
moments in the microscopic level, whereas the morphology reflects the
aggregation mode in the macroscopic level. These two factors are not
necessarily correlated. In fact, such a mismatch has been observed by
Wu¨rthner et al.1c
S1). The stretching band of the pyridine group appears at around 1371 cm-1
,
whereas after mixing SPG, the peak is shifted to 1379 cm-1. For related
papers, see: Tanaka, S.; Shirakawa, M.; Kaneko, K.; Takeuchi, M.; Shinkai,
S. Langmuir 2005, 21, 2163 and references cited therein.
(8) Similar morphologies were confirmed in TEM images (Supporting
Information, Figure S4).
Org. Lett., Vol. 8, No. 24, 2006
5535

As a reference experiment, we carried out the sample
preparation in the absence of SPG. When AcOH was added
to the NaOH solution containing dye 1, a yellow precipitate
was gradually formed with decreasing pH values from 13
to 7. Although the absorbance of dye 1 largely decreased
because of this precipitate formation, one could still estimate
the absorption maximum of this weak band in the supernatant
phase: the value, 414 nm, suggests that dye 1 results in the
H-type assembly even in the absence of SPG. This result
supports the view that if the self-assembling structure of dye
1 is prepared in the presence of s-SPG it will be entrapped
in the SPG cavity to give the water-soluble composite.
Furthermore, as shown in Figure 3b, ICD is also detected at
the π-π* transition region of the dye 1 assembly, indicating
that the one-dimensional H-type assembly is entrapped in
the helical SPG cavity. These results clearly support the view
that the intermolecular hydrogen-bonding interactions among
dye 1 molecules are indispensable for J-type assembly
formation. This means that the different dye assemblies can
be created from dye 1 depending on the preparation
procedures, which control the hydrogen-bonding capability
of dye 1. One may regard this phenomenon to be a sort of
“polymorphism” induced by the presence of SPG.
The intrinsic property of SPG as a one-dimensional host
can be well characterized by the morphological images of
the obtained composites. Figures 4a-d show the AFM
Figure 4. AFM images of the samples (bar ) 500 nm): (a)
composite with SPG, (b) only Azo dye 1 prepared from a DMSO
solution, (c) composite with SPG, and (d) only Azo dye 1 prepared
from a NaOH solution. The composite was casted on mica.
images of SPG/dye 1 composites. In Figures 4a,c, one can
recognize many fibrous structures with 10-20 nm height,
whereas in Figures 4b,d, dye 1 itself results in an amorphous
aggregate mass and did not give any fibrous structures.8
Taking the diameter of the composites into consideration,
somewhat larger J- or H-type assemblies must be entrapped
in the SPG cavity. It should be noted that the amorphous
aggregates formed in the bulk phase also contain J- or H-type
assemblies because their UV-vis spectral patterns are almost
the same as those of the composites with SPG. The findings
establish that SPG strongly restricts formation of the three-
dimensional amorphous mass during the self-assembling
process of dye 1 and forces the formed assemblies to adopt
the one-dimensional structure with a uniform diameter.
In summary, we have demonstrated that J- or H-type dye
assemblies can be selectively created in the SPG cavity by
utilizing the two different renaturation processes of s-SPG.
The findings clearly show that SPG can act as a unique one-
dimensional sheath not only for synthetic polymers as shown
in the preceding papers4c,e but also for the supramolecular
nanofibers as demonstrated in this study, where temporal
superstructures can be stabilized inside the cavity by the
wrapping effect of SPG. We believe that the present system
will provide a novel concept for creation of supramolecular
nanofibers and will play an important role in the design of
novel optical and photorefractive devices.
To obtain further reliable evidence that the hydrogen-
bonding interactions among dye 1 molecules are indispen-
sable for the creation of a J-type assembly, we carried out
the sample preparation utilizing dye 2 (Figure 1c), which
has no ability to form the intermolecular hydrogen bond due
to the lack of the carboxylic acid terminal group. The sample
was prepared from a DMSO solution according to the same
procedure as described above. As shown in Supporting
Information Figure S2a, the absorption maximum of the
resultant solution is blue shifted to 417 nm as expected for
the assembly formation through non-hydrogen-bonding
interactions. The result indicates that dye 2 forms H-type
assemblies even in the water/DMSO mixed solvent, which
is in sharp contrast to the J-type assembly formation observed
for the dye 1 system.
Here, one may consider that the observed spectral differ-
ence between J- and H-type assemblies is attributed to the
difference in solvent properties surrounding the composites;
that is, the J-type assembly is always created in the water/
DMSO mixed solvent, whereas the H-type assembly is
created in water containing AcONa salt formed through the
neutralization process. Thus, to compare the spectral features
of two composites under the same conditions, we removed
DMSO or AcONa utilizing an ultrafiltration technique. UV-
vis spectra of the obtained aqueous solutions are shown in
Supporting Information Figure S3. It is seen from Figure
S3 that the absorption maximum and the spectral features
of these samples are not changed even after exchanging the
solvents for pure water. Consequently, we can reasonably
conclude that the entrapped supramolecular structures in the
SPG cavity are not affected by the solvent properties,
suggesting that the SPG sheath effectively stabilizes the
entrapped one-dimensional supramolecular structures, re-
flecting their preparation processes.
Acknowledgment. We thank Mitsui Sugar Co., Japan,
for providing the schizophyllan samples. This work was
supported by The Japan Science and Technology Corpora-
tion, SORST Project, a Grant-in-Aid for the 21st Century
COE Program “Functional Innovation of Molecular Infor-
matics” from the MEXT, and a Grant-in-Aid for Scientific
Research S (15105004).
Supporting Information Available: IR spectra of the
SPG/Azo dye 1 composite. UV-vis and CD spectra of the
SPG/Azo dye 2 composite. Comparison of UV-vis spectra
after dialysis. TEM images of the SPG/Azo dye 1 composite.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OL062229A
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