More recently, supramolecular approaches were applied
to this photochirogenic reaction. Thus, the photoisome-
rization of 1ZZ was mediated by chiral sensitizing hosts,
naphthalene-appended molecular clips,6 and cyclodex-
trins,7 to give 1EZ in low 0.7% and 4.6% ee, respectively.
These results revealed that achieving high ee for 1EZ is an
equally difficult task even in supramolecular photochiro-
genesis. Nevertheless, this approach is worth expanding
from such simple clips and macrocycles to polymeric
biomolecules, where the host structures are anticipated to
be more dynamically manipulated by various environmen-
tal factors.
Curdlan (Cur) is a linear glucan composed of (1f3)-
linked β-D-glucose units and is known to form a triple
helical structure.8 The most intriguing feature of Cur is the
ability to reversibly denature/renature by simply changing
the solvent from water or aqueous acidic solution to
DMSO or aqueous alkaline solution.8,9 Thus, Cur host is
expected to provide an induced chiral environment fitted
to the size and shape of the target guest through reversible
complexation equilibrium with dynamic conformational
changes.
The chiroptical properties of Nap-Cur were examined in
DMSO and aqueous solutions. The CD spectrum of Nap-
Cur measured in DMSO showed an extremely weak
negative Cotton effect at the 1La band of Nap centered at
280 nm (Figure 1, black line; the wavelength region <260
nmwasmaskedbytheDMSO absorption), suggesting that
the Cur host is uncoiled to a single strand. In an aqueous
solution containing 10% DMSO, the negative Cotton
1
1
effects at the Bb and La bands were greatly enhanced,
while the 1La and 1Lb peaks were diffused with appreciable
bathochromic shifts (Figure 1, red line). The large negative
CD extremum observed at 246 nm is likely to be the first
half of a negative couplet caused by the exciton coupling
interaction of the adjacent Nap groups introduced to Cur.
This is because a clear bisignate CD pattern was observed
for a 5% DMSO solution of Nap-Cur in a thinner 1-mm
cell, and its crossover wavelength was very close to the
peak top (237 nm) of the 1Bb band of methyl 2-naphthoate
used as a reference compound. According to the exciton
chirality theory,11 the negative couplet observed for Nap-
Cur indicates that the Nap chromophores are helically
arranged in a counterclockwise (left-handed) fashion on
the triple helical Cur backbone. Such a dramatic chirop-
tical property change in DMSO versus aqueous solution
indicates that the original feature of Cur to reversibly
denature/renature in the two solvents is preserved even
after the Nap modification. It is of particular interest that
the hydrophobic Nap moieties in Nap-Cur are incorpo-
rated in the right-handed helical triplex Cur to give the
In this study, we newly synthesized a sensitizer-ap-
pended Cur, 6-O-(2-naphthoyl)Curdlan (Nap-Cur), as a
sensitizing host, elucidated its chiroptical properties and
complexation behavior with 1ZZ, and then investigated
the enantiodifferentiating photoisomerization of 1ZZ in-
cluded and sensitized by Nap-Cur.
Scheme 2. Synthesis of 6-O-(2-Naphthoyl)curdlan (Nap-Cur)
By using the synthetic procedure similar to that em-
ployed previously,10 Nap-Cur was prepared in 76% yield
in the reaction of 2-naphthoyl chloride with commercially
available native Cur, which was swollen overnight in N-
methyl-2-pyrrolidinone at 130 °C prior to the reaction
(Scheme2).ThedegreeofsubstitutionofNap-Curobtained
was determined as 0.08 from the integrated areas of aro-
matic versus sugar proton signals in the 1H NMR spectrum;
see Figure S1 in the Supporting Information (SI).
€
(6) Fukuhara, G.; Klarner, F.-G.; Mori, T.; Wada, T.; Inoue, Y.
Photochem. Photobiol. Sci. 2008, 1493.
(7) Yang, C.; Mori, T.; Wada, T.; Inoue, Y. New J. Chem. 2007, 31,
697.
Figure 1. UV/vis (top) and CD (bottom) spectra of 7.5 μM (in
chromophore unit) solutions of Nap-Cur in DMSO (black) and
in 1:9 (v/v) DMSO-H2O (red), both measured in a 1-cm cell at
25 °C. Note that the molar extinction coefficient (ε) and molar
ellipticity (Δε) were calculated by using the chromophore con-
centration (not monomer unit).
(8) For reviews, see: (a) Sakurai, K.; Uezu, K.; Numata, M.; Hase-
gawa, T.; Li, C.; Kaneko, K.; Shinkai, S. Chem. Commun. 2005, 4383.
(b) Numata, M.; Shinkai, S. Adv. Polym. Sci. 2008, 220, 65.
(9) (a) Yanaki, T.; Norisuye, T.; Fujita, H. Macromolecules 1980, 13,
1462. (b) Deslandes, Y.; Marchessault, R. H.; Sarko, A. Macromolecules
1980, 13, 1466.
(10) Fukuhara, G.; Inoue, Y. Chem. Commun. 2010, 46, 9128.
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