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Published on the web July 5, 2012
A Sulfonated Polyaramide: Stir-induced Chirality in Its Aqueous Solution
Kunihiko Okano,*1 Kanami Machida,2 and Takashi Yamashita*2
1Department of Applied Chemistry, Faculty of Science and Technology, Keio University,
3-14-1 Kohoku-ku, Yokohama, Kanagawa 223-8522
2Department of Pure and Applied Chemistry, Tokyo University of Science,
2641 Yamazaki, Noda, Chiba 278-8510
(Received April 3, 2012; CL-120292; E-mail: okano@applc.keio.ac.jp, yama@rs.noda.tus.ac.jp)
We demonstrated that aqueous solutions containing a
sulfonated polyaramide exhibited stir-induced chirality detected
by circular dichroism (CD) spectroscopy. This phenomenon can
be visualized as dynamics of supramolecular structure compris-
ing hydrogen-bonded polyaramides. We further show that the
sign of the CD response can be tuned using vortex direction,
stirring-rate, and concentration.
Chart 1.
Determination of the origin of the homochirality in naturally
occurring organic molecules is a problem that is yet to be solved
in molecular biology, organic chemistry, and supramolecular
chemistry.1 Previous studies on this subject have examined the
amplification of enantiomeric imbalances that occur as a result
of the existence of achiral precursors that are generalized by
circularly polarized light, chiral inorganic crystal, and so on.
Moreover, in 1976, Honda et al. reported the induction of CD
sign in a vortex flow of J-aggregated isocyanine chloride.2
However, this system was controversial as the combinations of
linear dichroism (LD) and birefringence can lead to artifactual
CD signals. In a recent study performed for reexamining these
results, Muller matrix spectroscopy, which is an advanced
measurement method, showed true CD signal excluded artificial
polarizations in a stirred sample.3 Moreover, several types of
porphyrin derivatives, which is a representative structure
showing stir-induced chirality, have been examined.4 A plau-
sible mechanism of the effect is twisting the supramolecular
structures in solution, and this gives the possibility that other
molecular structures also exhibit chirality in vortex flow. In fact,
an organo gelator forming 3D networks in solution exhibits stir-
induced chirality, and it can transfer the chiral information to
guest dyes.5
Herein, we show a polyaramide lyotropic liquid crystal
(LLC) solution exhibiting chirality in vortex flow. Polyaramide
derivatives are used in broad applications in industry owing to
their extremely high durability and strength. However, for
conventional polyaramides, the hydrophobic nature of their
aromatic backbones in combination with strong intermolecular
hydrogen bonding between chains reduces their solubility,
thereby restricting their application in solution. This problem is
solved by introducing an ionic sulfonyl group to a polyaramide
backbone (PPSA) (Chart 1), and this can be dissolved in
aqueous solution.6 The aqueous polyaramide solution exhibits a
LLC phase, and this means that supramolecular structure
formed. Thus, it is expected that the ionic polyaramide solutions
exhibit stir-induced chirality.
be dissolved in water because of the hydrophilic nature of its
sulfonic acid moiety. Self-assembly of PPSA in aqueous
solution results in H-bonding and gives rise to an LLC phase
of over 0.8 wt % at 20 °C. Moreover, PPSA is particularly useful
owing to its simple molecular structure, which can be obtained
by carrying out rudimentary chemical syntheses. We measured
the CD of this system in a 10 © 10 © 40 mm3 quartz optical cell
in which 4 mL of sample solutions were stirred mechanically
using a magnetic stirring bar (º2.0 © 5.0 mm3) placed at the
bottom of the cell, 13 mm below the center of an º8.0 mm wide
polarized beam of light.
Unstirred aqueous PPSA at 0.5 wt % exhibited a stagnant
CD sign, but it became optically active upon mechanical stirring
(Figure 1a). When we stirred the solution at 1000 rpm in a
clockwise (CW) direction, the sample solution showed negative
CD bands in the absorbance region at approximately 440 nm.
When the stirring direction was reversed the CD spectral sign
was inverted perfectly. Therefore, it is possible to select the
optical activity by specifying the stirring direction. Furthermore,
when the stirring was stopped, the optical activity disappeared.
As shown in Figure 1b, the sample solution underwent quick
chiroptical responses to changes in the stirring directions. We
also observed that the CD intensity increased in a sigmoidal
fashion with increasing stirrer rotation rate (Figure S3 of SI).10
From the data of Figure 1, one question that arises is whether or
not the CD that is observed during stirring is an artifact of
measurement instrumentation such as fiber alignment in the
vortex. In the next experiment, we measured linear dichroism
(LD) spectra detecting linearly artifactual elements in the sample
(Figure S4 of SI).10 The sample solution used in this study has
molecular order caused from an LLC phase, thereby LD signal
was observed even in a stagnant state and this sign was enhanced
to negative value from ¹0.017 to ¹0.046 by stirring. This
indicates that the stir-induced CD signal contains linear artifacts.
Analogous with other supramolecular systems showing chirality
in vortex flows, our system could also contains chiral elements
even in highly birefringent solution.
The PPSA that we used in this study was synthesized
by polycondensation between 2-sulfoterephthalic acid and 1,4-
phenylenediamine (see Supporting Information10). PPSA could
We also investigated the effect of the PPSA concentration
on the stir-induced optical activity (see Figure S5 of SI).10 The
intensity of the CD signal increased with increasing PPSA
Chem. Lett. 2012, 41, 750-752
© 2012 The Chemical Society of Japan
Okano, Kunihiko
