Thermo- and solvent-responsive polymer complex created from supramolecular complexation between a helix-forming polysaccharide and a cationic polythiophene

Article abstract of DOI:10.1021/ja1067349

The helical structure is one of key structural components for both biological systems and artificial chiral systems. So far, we have succeeded in fabricating "tight" insulated molecular wires consisting of a triple-stranded cohelical structure formed thro

Full text of DOI:10.1021/ja1067349

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Published on Web 09/13/2010
Thermo- and Solvent-Responsive Polymer Complex Created
from Supramolecular Complexation between a Helix-Forming
Polysaccharide and a Cationic Polythiophene
Tomohiro Shiraki,† Arnab Dawn,‡ Youichi Tsuchiya,† and Seiji Shinkai*,†,‡,§
Nanotechnology Laboratory, Institute of Systems, Information Technologies and
Nanotechnologies (ISIT), 203-1 Moto-oka, Nishi-ku, Fukuoka 819-0385 Japan, Institute for
AdVanced Study, Kyushu UniVersity, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan, and
Department of Nanoscience, Faculty of Engineering, Sojo UniVersity,
4-22-1 Ikeda, Kumamoto 860-0082, Japan
Received July 28, 2010; E-mail: shinkai_center@mail.cstm.kyushu-u.ac.jp
Abstract: The helical structure is one of key structural components for both biological systems and artificial
chiral systems. So far, we have succeeded in fabricating “tight” insulated molecular wires consisting of a
triple-stranded cohelical structure formed through supramolecular wrapping of synthetic polymers by a helix-
forming polysaccharide (schizophyllan). Herein, we have designed a new modified polysaccharide (Cur-
oeg) to form a “loose” macromolecular complex with a conjugated polymer (CP) that allows structural
changes in response to external stimuli. Cur-oeg forms a helical complex with an achiral cationic
polythiophene (PT1), and the effective conjugation length is changed by temperature, showing a large
absorption peak shift from 403 to 482 nm between 85 and 5 °C. According to the change in the conjugation
system, the fluorescence and the induced circular dichroism show the continuous spectral shifts under
temperature control. The color changes in the absorption and the fluorescence are detectable with
observation by the naked eye and are reversibly controlled under thermal cycles, indicating that this system
has the function of a “molecular thermometer”. It is shown that the induced thermoresponsiveness is
associated with structural rearrangement of the helical conformation of PT1 in the complex. Moreover,
another unique responsiveness is discovered for the film state: that is, the film color is varied when it is
exposed to the vapor of water or methanol (vaporchromism), resulting from the structural change of PT1
occurring even in the film state. These flexible molecular motions in both the solution state and the film
state can be applicable to the design of CP-based smart sensors, polarized materials, switching devices,
etc.
Introduction
helix-forming polysaccharides such as Schizophyllan (SPG) and
Curdlan (Cur) that are classified into a ꢀ-1,3-glucan family.3
The helical motif that is found in natural polymers including
DNAs, polypeptides, and polysaccharides plays important roles
in the biological activities. Beyond the biological systems, the
helical structure is also applied to artificial key technologies
such as chiral sensing, separation of natural compounds, and
design of polarized materials.1 Until now, a lot of chiral
complexes have been developed through templating methods
utilizing chiral self-assemblies and helical biopolymers.2 Re-
cently, we have succeeded in fabricating unique helical polymer
complexes through a supramolecular wrapping method using
First, we found that SPG forms cohelical complexes with certain
DNAs or RNAs that are constructed from two SPG chains and
one DNA or RNA chain.4 Subsequently, we noticed that a
modified polythiophene with a syn-type conformation has a
similar helical pitch length with SPG as well as DNA.4a-c We
(2) (a) Janssen, P. G. A.; Ruiz-Carretero, A.; Gonza´lez-Rodr´ıguez, D.;
Meijer, E. W.; Schenning, A. P. H. J. Angew. Chem., Int. Ed. 2009,
48, 8103–8106. (b) Ho, H.-A.; Najari, A.; Leclerc, M. Acc. Chem.
Res. 2008, 41, 168–178. (c) Hannah, K. C.; Armitage, B. A. Acc.
Chem. Res. 2004, 37, 845–853. (d) Woolfson, D. N.; Ryadnov, M. G.
Curr. Opin. Chem. Biol. 2006, 10, 559–567. (e) Kim, O.-K.; Je, J.;
Jernigan, G.; Buckley, L.; Whitten, D. J. Am. Chem. Soc. 2006, 128,
510–516.
† Nanotechnology Laboratory, Institute of Systems, Information Tech-
nologies and Nanotechnologies (ISIT).
‡ Institute for Advanced Study, Kyushu University.
(3) (a) Numata, M.; Shinkai, S. AdV. Polym. Sci. 2008, 220, 65–121. (b)
Sakurai, K.; Uezu, K.; Numata, M.; Hasegawa, T.; Li, C.; Kaneko,
K.; Shinkai, S. Chem. Commun. 2005, 4383–4398.
§ Department of Nanoscience, Faculty of Engineering, Sojo University.
(1) (a) Yashima, E.; Maeda, K.; Iida, H.; Furusho, Y.; Nagai, K. Chem.
ReV. 2009, 109, 6102–6211. (b) Palmans, A. R. A.; Meijer, E. W.
Angew. Chem., Int. Ed. 2007, 46, 8948–8968. (c) Tang, K.; Green,
M. M.; Cheon, K. S.; Selinger, J. V.; Garetz, B. A. J. Am. Chem. Soc.
2003, 125, 7313–7323. (d) Cornelissen, J. J. L. M.; Rowan, A. E.;
Nolte, R. J. M.; Sommerdijk, N. A. J. M. Chem. ReV. 2001, 101, 4039–
4070. (e) Hill, D. J.; Mio, M. J.; Prince, R. B.; Hughes, T. S.; Moore,
J. S. Chem. ReV. 2001, 101, 3893–4011. (f) Nakano, T.; Okamoto, Y.
Chem. ReV. 2001, 101, 4013–4038. (g) Goto, H.; Akagi, K. Angew.
Chem., Int. Ed. 2005, 44, 4322–4328.
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(b) Okobira, T.; Miyoshi, K.; Uezu, K.; Sakurai, K.; Shinkai, S.
Biomacromolecules 2008, 9, 783–788. (c) Miyoshi, K.; Uezu, K.;
Sakurai, K.; Shinkai, S. Biomacromolecules 2005, 6, 1540–1546. (d)
Sakurai, K.; Mizu, M.; Shinkai, S. Biomacromolecules 2001, 2, 641–
650. (e) Mizu, M.; Koumoto, K.; Anada, T.; Matsumoto, T.; Numata,
M.; Shinkai, S.; Nagasaki, T.; Sakurai, K. J. Am. Chem. Soc. 2004,
126, 8372–8373. (f) Kimura, T.; Beppu, A.; Sakurai, K.; Shinkai, S.
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9
13928 J. AM. CHEM. SOC. 2010, 132, 13928–13935
10.1021/ja1067349  2010 American Chemical Society

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Stimuli-Responsive Supramolecular Complex
A R T I C L E S
also to tune the induced helical structure of the isolated PT1
single chain in response to the environmental conditions because
of the decreased “tightness”, resulting in color changes in the
absorption and the fluorescence detectable by naked eye
observation. Moreover, a film made of PT1/Cur-oeg complex
shows a unique color change against the exposed vapors such
as water and methanol. In this paper, we thoroughly investigated
the mechanistic background of these color changes in solution
and tried to explain why some color changes are reversible while
some color changes are irreversible in the film state.
Figure 1. Chemical structures of a cationic polythiophene (PT1) and a
PEGylated Curdlan (Cur-oeg).
thus found that a cationic polythiophene (PT1, Figure 1A) and
amino acid-modified polythiophenes form stoichiometric cohe-
lical complexes with SPG, in which the composition of
polythiophene to SPG is also 1:2.5 In these SPG-wrapped
complexes, each polythiophene chain is insulated from any other
chains, and aggregation of PT1 is suppressed even in the solid
state.
Results and Discussion
Synthesis and Characterization of Cur-oeg. We have estab-
lished site-selective and quantitative modification of the 6-OH
group in native Curdlan using “Click Chemistry”.10 Cur-oeg
was synthesized according to the similar procedure (for the detail
see Supporting Information). In dimethyl sulfoxide (DMSO),
6-azido-6-deoxy Curdlan (Cur-N3) was treated with alkyne-
terminated triethylene glycol monomethyl ether in the presence
of propylamine as a base and CuBr2-ascorbic acid as a catalyst.
The reaction was monitored by FT-IR measurements until a
peak ascribable to the N3 group disappeared. The product was
purified by dialysis against distilled water and collected by
lyophilization. The molecular weight of the obtained Cur-oeg
was estimated by SEC to be weight-average molecular weight
Mw ) 7.1 × 104 (Mw/Mn ) 1.9, degree of polymerization (DP)
) ∼182).
By using this supramolecular wrapping technique, we have
succeeded in constructing insulated molecular wires of various
conjugated polymers (CPs) with SPG.6 It is known that the
solution properties of CPs are affected by both intermolecular
aggregation and single-chain conformational change. To dis-
tinguish these two factors therefore becomes essential for well-
defined applications of controlled CP structures.7 Since our
supramolecular wrapping method has achieved the insulation
of each CP chain, chemical and physical changes observed are
simply attributable to events occurring in the single chain. The
immobilization of the helical PT1 structure by the SPG wrapping
brought forth an advantage to create new functions arising from
the single chain; for example, PT1/SPG complex emits a circular
polarized light arising from the chirality of the helical complex.8
Moreover, the redox potential of modified polythiophenes can
be changed by the SPG wrapping because of the immobilization
in the helical structure; that is, the complexed polythiophenes
can acquire oxidation resistance.9 Through these studies, it has
firmly been established that SPG can form the “tight” triple-
stranded helical complexes with modified polythiophenes, the
helical structure of which features high stability and high
robustness.8,9
On the other hand, it is known that the biological systems
employ more dynamic higher-order structures to realize the
required functionalities. From this viewpoint, the “tight” SPG
complexes are not suitable to design “dynamic” functional
materials. It occurred to us that fabrication of a “loose” complex
would be important to realize stimuli-responsive functions
leading to sensors and devices. Herein, we report a new
PEGylated ꢀ-1,3-glucan polysaccharide, Cur (Cur-oeg, Figure
1B) through “Click Chemistry”. We have found that the Cur-
oeg wrapping is able not only to insulate each PT1 chain but
SPG and Cur form triple-stranded helices that can be
confirmed by a measurement of optical rotatory dispersion
(ORD). Optical rotation occurs when refractive index shows a
difference between right-handed and left-handed circularly
polarized lights. ORD measurements have been employed to
characterize helical conformation of polymers including polysac-
charides, polypeptides, and synthetic polymers.11,12 For the
ꢀ-1,3-glucan of SPG and Cur, a helix-forming triple-stranded
structure shows a positive signal,11 whereas a single-stranded
random coil structure shows a negative signal12 in the ultraviolet
to visible region. Figure 2A shows ORD spectra of Cur-oeg in
DMSO and aqueous solutions. An ORD signal of Cur-oeg was
characterized by a negative sign in DMSO, which acts as a
denaturing-solvent to give a single-stranded SPG and Cur. This
result is consistent with the signal of SPG itself in DMSO
(Figure 2B). Cur-oeg also showed a negative sign even in
aqueous solution in contrast to a positive sign of triple-stranded
aqueous SPG. Consequently, one may consider that Cur-oeg
adopts a single-stranded random coil in aqueous media. Previ-
ously, we reported that modified Curdlan tethering a quaternized
amino group also shows a negative sign in water, taking a single-
stranded form.10c Therein, electrostatic repulsion is a major
factor to destabilize the triple-stranded form. The present results
(5) (a) Haraguchi, S.; Tsuchiya, Y.; Shiraki, T.; Sugikawa, K.; Sada, K.;
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A R T I C L E S
Shiraki et al.
One can conclude, therefore, that PT1 entrapped in the helix is
also twisted into the right-handed helical structure.
According to a change in the effective conjugation length,
an emission peak of PT1 was also shifted from 529 to 571 nm
in the presence of Cur-oeg (Figure 3C). As shown in Figure
3D, TEM observation revealed that a fibrous structure was
formed from a mixture of PT1 and Cur-oeg. On the other hand,
PT1 itself or Cur-oeg itself gave random or spherical aggregates
with various sizes under the TEM observation (Figure S2,
Supporting Information). From these results, it is clear that the
supramolecular complexation between PT1 and Cur-oeg creates
the one dimensional (1D)-shaped fibrous structure. We previ-
ously reported that the characteristic nanowire form is created
from the complexes of SPG with various conjugated polymers.3,6
Therefore, the inherent helix-forming nature of ꢀ-1,3-glucans
is also present in Cur-oeg and does configurate the helical
nanofibrous structure through the wrapping action on PT1. As
shown in an inset in Figure 3D, the 1D-shaped fibrous structure
was also confirmed by AFM. The height of the fibers was
estimated to be 0.82-1.2 nm (the average value of 33 fibers
was 1.1 nm), which is comparable with that of conjugated
polymer/SPG complexes where each conjugated polymer chain
is wrapped separately.5,6a,b One can conclude, therefore, that
Cur-oeg can insulate each PT1 chain by the wrapping action.
From the above-described results, it is undoubted that a chiral
macromolecular complex (PT1/Cur-oeg complex) including
single PT1 chain is formed by simply mixing PT1 and Cur-oeg
in aqueous media.
Figure 2. ORD curves of (A) Cur-oeg and (B) SPG in DMSO or in water
containing 5 vol % DMSO. Concentrations of polysaccharides were 5 mg/
mL in a 1-cm optical cell at 25 °C.
indicate that the increase in the hydrophilicity due to the
“neutral” side chains can be another factor to destabilize the
triple-stranded form.
Supramolecular Complexation between Cur-oeg and PT1.
PT1 was synthesized according to a published paper,5b and the
13C NMR measurement (Figure S1, Supporting Information)
revealed that the head-to-tail regioregularity was over 95%.13
Mw of the PT1 was estimated by SEC to be 4.1 × 104 (Mw/Mn
) 3.4, DP ) ∼165), which was comparable with the reported
values for other water-soluble poly(alkoxythiophene)s.14
Complexation between PT1 and SPG can occur only through
a denaturation-renaturation process of SPG, because complex-
ation can take place only when SPG is denatured.5b,8,9 On the
other hand, Cur-oeg already exists as a single-stranded form in
DMSO and water. When PT1 was mixed with Cur-oeg in
aqueous media, the mixed solution displayed the red-orange
color, whereas aqueous PT1 itself gave a yellow solution. In
UV/vis spectroscopy, an absorption peak of PT1 ascribable to
the π-π* transition was largely red-shifted from 414 to 458
nm by the interaction with Cur-oeg (Figure 3A). This result
clearly indicates that an interaction between PT1 and Cur-oeg
causes elongation of the effective conjugation length, followed
by a conformational change of PT1 to a more planar structure.13,15
Although PT1 itself is an achiral polymer, the mixture of PT1
and Cur-oeg showed a splitting type of induced circular
dichroism (ICD) with a first positive and a second negative
Cotton effect at 500 and 420 nm, respectively, in the π-π*
transition region (Figure 3B). A similar ICD pattern has been
observed for the PT1/SPG complex that possesses a right-handed
helical conformation derived from a nature of right-handed
helix-forming ꢀ-1,3-glucans.5b In addition, this type of CD
pattern can be observed for higher-order structures with a right-
handed helical motif of polymers and molecular assemblies.14c,16,17
A stoichiometric proportion between PT1 and Cur-oeg in the
complex was investigated through a continuous variation method
(Job’s plot) in CD spectroscopy. The plot for the ICD intensity
at 500 nm gave a maximum at [PT1]unit/([PT1]unit + [Cur-
oeg]unit) ) 0.5 (Figure 4). This value corresponds to the 1:1
stoichiometric interaction between PT1 and Cur-oeg with respect
to each monomer unit. In addition, a CD titration of PT1 varying
the Cur-oeg concentration was performed. From the titration
curve in Figure 5A, the composition was also determined to be
1:1.
Assuming the 1:1 complexation between PT1 and Cur-oeg,
an apparent binding constant (K) was estimated to be K25 °C
)
3.7 × 104 M-1 through an analysis of the plot according to the
least-squares method. Figure 5B compares UV/vis absorption
spectra of PT1 with increasing Cur-oeg concentration, in which
an isosbestic point is confirmed at 435 nm. It is clear, therefore,
that one energetically stable species is yielded in the complex.
Previously, we reported that a stoichiometry of PT1/SPG
complex is 1:2 and the complex consists of a triple-stranded
cohelical structure because of the coincidence of the helix pitch
length between the syn-type helical polythiophene chain and
the helical SPG chain.4a-c,18 Since the ICD pattern of PT1/
Cur-oeg complex also showed a right-handed helical conforma-
tion, the helical pitch of PT1 should be also close to that of the
syn-type polythiophene. However, the stoichiometry is 1:1, but
not 1:2. We consider, therefore, that the difference in the
stoichiometry would be associated with some essential difference
in their solution properties: for example, PT1 and Cur-oeg may
form a more dynamic, “loose” double-stranded helical structure,
(13) (a) Fa¨ıd, K.; Fre´chette, M.; Ranger, M.; Mazerolle, L.; Le´vesque, I.;
Leclerc, M. Chem. Mater. 1995, 7, 1390–1396. (b) Le´vesque, I.;
Leclerc, M. Chem. Mater. 1996, 8, 2843–2849.
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Plesu, R.; Leclerc, M. AdV. Mater. 2005, 17, 1251–1254. (b) Ho, H.-
A.; Boissinot, M.; Bergeron, M. G.; Corbeil, G.; Dore´, K.; Boudreau,
D.; Leclerc, M. Angew. Chem., Int. Ed. 2002, 41, 1548–1551. (c)
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Lukkari, J.; Saloma¨ki, M.; Viinikanoja, A.; Aa¨ritalo, T.; Paukkunen,
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Macromolecules 2004, 37, 6316–6321.
(18) (a) Chuah, C. T.; Sarko, A.; Deslandes, Y.; Marchessault, R. H.
Macromolecules 1983, 16, 1375–1382. (b) Deslandes, Y.; Marches-
sault, R. H.; Sarko, A. Macromolecules 1980, 13, 1466–1471. (c)
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Stimuli-Responsive Supramolecular Complex
A R T I C L E S
Figure 3. (A) UV/vis absorption, (B) CD, and (C) fluorescence spectra of PT1/Cur-oeg complex (solid line) and PT1 itself (dashed line) in water containing
5 vol % DMSO at 25 °C; [PT1]unit ) 0.2 mM, [Cur-oeg]unit ) 0.6 mM, λex ) 458 nm. These data were collected after once heating to 85 °C. (D) TEM image
of PT1/Cur-oeg complex; negative stain with 1 wt % phosphotungstic acid aqueous solution adjusted to pH 7 with NaOH. An inset image is an AFM image
of the complex.
(Figure 3), suggesting the formation of right-handed helix with
PT1. The stoichiometry of PT1/Cur-oeg complex was 1:1, which
is different from the 1:2 stoichiometry of PT1/SPG complex. It
is difficult to explain why the stoichiometry is different between
these two complexes. Two possible explanations come to our
mind; first, Cur-oeg bearing one oligoethylene glycol chain on
every glucose unit is more sterically crowded than SPG bearing
one glucose side group on every three glucose units. Second,
the PT1/SPG complex is a fully packed triple-stranded helix
and shows a “rigid” nature, whereas the PT1/Cur-oeg complex
is a double-stranded helix having “grooves” as in the case of
double-stranded DNAs and can show a “dynamic” nature (vide
infra).
Figure 4. Job’s plot using the peak intensity of the first positive Cotton
effect at 500 nm (θ500 nm) of the ICD signal in PT1/Cur-oeg complex;
[PT1]unit + [Cur-oeg]unit ) 0.2 mM, 25 °C, in water containing 5 vol %
DMSO.
Thermoresponsive Properties of the PT1/Cur-oeg Com-
plex. Interestingly, we have found that the PT1/Cur-oeg complex
shows novel thermoresponsiveness in the optical properties. As
shown in Figure 6, the solution color of the PT1/Cur-oeg
complex continuously changed from reddish orange to yellow
by changing the temperature from 5 to 85 °C. In addition, the
fluorescence also displayed a color change from orange to green
with the increase in the temperature.
Parts A-C of Figure 7 show temperature dependencies of
the UV/vis absorption, CD, and fluorescence spectra, respec-
tively. The absorption spectrum at 25 °C gave a peak at 458
nm. Upon increasing temperature, the peak was blue-shifted to
403 nm at 85 °C. On the other hand, when the temperature was
decreased to 5 °C, the peak was red-shifted to 482 nm: that is,
the absorption peak of the PT1/Cur-oeg complex arising from
the πsπ* transition of PT1 was reversibly shifted in response
to the temperature change with an 80 nm wide range. As we
have confirmed that the spectral change should be induced by
the conformational change of the PT1 single chain (but not by
the aggregation of PT1), this remarkable spectral shift is
ascribable to a change in the effective conjugation length of
PT1 in the complex. A fluorescence peak of the PT1/Cur-oeg
complex was varied from 533 to 569 nm by a change in the
Figure 5. (A) Peak intensity change at 500 nm (θ500 nm) for ICD of PT1/
Cur-oeg complex as a function of the molar ratio of [Cur-oeg]unit to [PT1]unit
.
(B) Spectral changes of UV/vis absorption of PT1 upon adding Cur-oeg;
[PT1]unit ) 0.2 mM, [Cur-oeg]unit ) 0-1 mM, 25 °C, in water containing
5 vol % DMSO.
which is different in the stoichiometry from the “tight” triple-
stranded helical structure of PT1/SPG complex.
PT1/Cur-oeg complex showed the splitting-type ICD that
displayed a similar pattern with that of PT1/SPG complex
9
J. AM. CHEM. SOC. VOL. 132, NO. 39, 2010 13931