Switching of a macromolecular helicity for visual distinction of molecular recognition events [22]

Full text of DOI:10.1021/ja016393z

J. Am. Chem. Soc. 2001, 123, 8159-8160
8159
Switching of a Macromolecular Helicity for Visual
Distinction of Molecular Recognition Events
Eiji Yashima,*,†,‡ Katsuhiro Maeda, and Osamu Sato†
†
Department of Molecular Design and Engineering
Graduate School of Engineering, Nagoya UniVersity
Form and Function, PRESTO, JST, Chikusa-ku
Nagoya 464-8603, Japan
Figure 1. Structures of â-CyD-NH
2
, poly-1, and poly-2 (A) and
ReceiVed June 12, 2001
ReVised Manuscript ReceiVed July 9, 2001
schematic illustration of interconvertible right- (red) and left-handed
yellow) helices of poly-1 (B).
(
Much attention has been recently focused on controlling and
1
switching the helicity of (macro)molecules. Such switchable
materials have potential applications in data storage, optical
devices, and liquid crystals for display, but still remain rare. Only
a few synthetic polymers as well as biopolymers can be reversibly
switched between right- and left-handed helical conformations
2
by changing the external conditions, such as solvent or temper-
ature, or by the irradiation of light. Here we report a unique
macromolecular helicity inversion accompanied by a color change
responding to molecular and chiral recognition events that
occurred at the remote side chain.
We designed and synthesized a stereoregular (cis-transoidal),
chromophoric polyphenylacetylene having an optically active,
bulky â-cyclodextrin (â-CyD) residue as the side group (poly-1;
3
Figure 1A). Poly-1 appears to have a predominantly one-handed
helical conformation induced by the â-CyD units so that it exhibits
Figure 2. CD (upper) and absorption (lower) spectral changes of poly-1
in DMSO (1 mg/mL) with temperature (the sample was kept for 5 min
at the desired temperature before measurement) (A), plots of the helicity
an intense circular dichroism (CD) in the long absorption region
of the conjugated polyene backbone in dimethyl sulfoxide
4
(
DMSO) at 25 °C. However, the CD pattern dramatically and
inversion temperature (T
mixtures of DMSO and H
DMSO at 25 and 80 °C (C).
m
) of poly-1 versus DMSO contents (%, v/v) in
sharply changed at high temperatures, and the sign inverted at
0 °C through a transition temperature ([θ] of the first Cotton
) 70 °C) (Figure 2A). These ICD changes were
2
O (B), and visible difference of poly-1 in
8
effect ≈ 0 at T
m
accompanied by remarkable changes in the absorption spectra;
the absorbance maxima (λmax) at 516 nm shifted to a shorter
wavelength by 33 nm with a clear isosbestic point at 487 nm
and the solution color changed from red to bright yellow (Figure
5
on the temperature could be excluded. These results suggest that
poly-1 may undergo a helix-helix transition from one helix to
another with a different helical pitch by changing the temperature,
and this conformational change leads to the thermochromism
2C). These CD and absorption spectral changes are reversible
(Figure 1B).
and independent of the poly-1 concentration (0.05-4.0 mg/mL)
and time, indicating that the formation of aggregates depending
The right- and left-handed helices of poly-1 are not exactly
enantiomers. They are diastereomers because of the presence of
chiral CyD residues of poly-1; therefore, their CD spectra differ
from one another. Although these CD changes together with the
calculation results (see below) strongly indicate the inversion of
helicity of the poly-1 backbone, there may be another possibility
to explain the changes in the CD patterns; that is a change in the
helical pitch of poly-1 with the same-handedness rather than the
†
Nagoya University.
‡
PRESTO, JST.
(
1) (a) Okamoto, Y.; Nakano, T. Chem. ReV. 1994, 94, 349. (b) Green, M.
M.; Peterson, N. C.; Sato, T.; Teramoto, A.; Cook, R.; Lifson, S. Science
1
995, 268, 1860. (c) Pu, L. Acta Polym. 1997, 48, 116. (d) Rowan, A. E.;
Nolte, R. J. M. Angew. Chem., Int. Ed. 1998, 37, 63. (e) Gellman, S. H. Acc.
Chem. Res. 1998, 31, 173. (f) Nielsen, P. E. Acc. Chem. Res. 1999, 32, 624.
(
g) Prince, R. B.; Barnes, S. A.; Moore, J. S. J. Am. Chem. Soc. 2000, 122,
2
758. (h) Berl, V.; Huc, I.; Khoury, R. G.; Krische, M. J.; Lehn, J.-M. Nature
000, 407, 720.
helix inversion, and this possibility could not be thoroughly ruled
2
out.6
(
2) (a) Pohl, F. M.; Jovin, T. M. J. Mol. Biol. 1972, 67, 375. (b) Toriumi,
On the other hand, the CD and absorption spectra of a DMSO
H.; Saso, N.; Yasumoto, Y.; Sasaki, S.; Uematsu, I. Polym. J. 1979, 11, 977.
c) Watanabe, J.; Okamoto, S.; Satoh, K.; Sakajiri, K.; Furuya, H.; Abe, A.
(
solution of the poly-2 bearing 91 mol % â-CyD residues (Figure
Macromolecules 1996, 29, 7084. (d) Maxein, G.; Zentel, R. Macromolecules
1A) hardly changed upon heating (no thermochromism); the
1
995, 28, 8438. (e) Yashima, E.; Maeda, Y.; Okamoto, Y. J. Am. Chem. Soc.
998, 120, 8895. (f) Maeda, K.; Okamoto, Y. Macromolecules 1998, 31, 5164.
solution remains yellow with a weak positive Cotton ([θ] ) 4 ×
1
3
(
g) Cheon, K. S.; Selinger, J. V.; Green, M. M. Angew. Chem., Int. Ed. 2000,
10 ) at 440 nm. The cooperative interaction between the â-CyD
3
9, 1482. (h) Fujiki, M. J. Am. Chem. Soc. 2000, 122, 3336 and references
residues at the side chains might be essential for the dramatic
conformational change as observed for the poly-1.
therein.
(
3) Poly-1 was prepared by the polymerization of [(6-â-CyD)]carbam-
oylphenyl]acetylene (1) with [Rh(nbd)Cl]
2
(nbd ) norbornadiene) in DMF at
The color changes of poly-1 can be ascribed to a change in
the twist angle of the conjugated double bonds. The blue-shift of
the absorption spectra of poly-1 in the high-temperature ranges
5
3
0 °C; the number average molecular weight (M
n
) was 1.63 × 10 as
determined by size exclusion chromatography (SEC). Poly-2 was prepared
by the macromolecular reaction of cis-transoidal poly((4-carboxyphenyl)-
4
acetylene) (its M
n
is 1.8 × 10 as its methyl ester) with â-CyD-NH
2
(Figure
1
%
A). The content of the â-CyD residue in poly-2 was estimated to be 91 mol
(5) SEC and static light-scattering analyses of the poly-1 in solution also
support this conclusion (see Supporting Information).
(6) Direct measurement of both helices of poly-1 by scanning tunneling
microscopy may be useful. Shinohara, K.; Yasuda, S.; Kato, G.; Fujita, M.;
Shigekawa, H. J. Am. Chem. Soc. 2001, 123, 3619.
by the anthrone reagent method.
4) (a) Yashima, E.; Huang, S.; Matsushima, T.; Okamoto, Y. Macromol-
(
ecules 1995, 28, 4184. (b) Yashima, E.; Maeda, K.; Okamoto, Y. Nature 1999,
99, 449.
3
1
0.1021/ja016393z CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/28/2001

8160 J. Am. Chem. Soc., Vol. 123, No. 33, 2001
Communications to the Editor
-
4
Figure 4. CD and absorption spectra of poly-1 (1 mg/mL, 7.9 × 10
M monomer units) in an alkaline water (pH 11.7)-DMSO (7/3, v/v) in
-
1
the presence of (R)-7 (7.8 × 10 M) (orange lines) and (S)-7 (7.8 ×
Figure 3. Visible color changes in poly-1 in DMSO-H
5 °C induced by the addition of guest molecules (3-6) (20 equiv to
monomer units of poly-1) and structures of the guests. λmax (nm), [θ]max
2
O (8/2, v/v) at
-
1
10
M) (red lines) (A) and visible difference of the poly-1s(R)-7 and
2
the poly-1-(S)-7 solutions at 25 °C (B).
4
2
-1
-1
of the first Cotton (10 degree cm dmol ), and K (M ) are also shown.
intensity at 479 nm of poly-1 versus the concentration of the guest
showed a sigmoidal curvature, indicating that the guest coopera-
tively enters in the â-CyD cavity of poly-1 to induce a confor-
mational change.
The helicity of poly-1 can also be switched by complexation
with chiral guest molecules. The enantiomers of 4 induced a
negligible color change in DMSO-water mixtures of poly-1.
However, in a mixture of DMSO and alkaline water (pH 11.7)
suggests that the poly-1 has a more tightly twisted helical
conformation at the high temperatures in DMSO, while the red-
colored poly-1 has an extended π-conjugation (Figure 1B).
Molecular mechanics calculations with the Dreiding force field
for the model polymer (20-mer) reveal that the averaged dihedral
angles of adjacent double bonds from planarity (φ) for the
favorable conformations in the right- and left-handed helices are
(
3/7, v/v) the poly-1 exhibited a color change (from yellow-orange
to red) with a negative first Cotton in the presence of excess (S)-
-phenylethylamine ((S)-7), whereas the poly-1-(R)-7 solution
10 ( 4° and -31 ( 5°, respectively (see Supporting Information).
It thus seems plausible that the red-colored poly-1 may have a
right-handed helix and the yellow-colored poly-1, a left-handed
helix.
1
remained yellow with a positive first Cotton (see Figure 4). The
chirality induced change in the sign of the CD spectra may result
from inversion of the helicity of poly-1, which can be observed
The poly-1 also exhibits a similar color change (from red to
yellow) in the presence of an increasing amount of water (0-
9
by the naked eye. Under the same conditions, the poly-2 did not
30% v/v) and alcohols accompanied by inversion of the Cotton
show any spectroscopic and color change. Although a number of
molecular sensors for the direct colorimetric detection of chemical
species10 including the enantiomers have been developed, they
have employed dyes or chromophoric macrocycles as color
indicators. The present system is conceptually new and based on
effects at 25 °C. Moreover, we could control the T
tuning the solvent compositions. For example, the T
m
values by
m
linearly
11
decreased with an increase in the content of water (Figure 2B).
This unique solvatochromism makes this system an effective
discriminator of structurally similar alcohols with the naked eye.
The DMSO solutions of poly-1 containing simple primary
alcohols such as methanol, ethanol, n-propanol, and n-butanol
12
a change in the tunable helical pitch (“helical spring”) probably
with the helicity inversion which may provide new approaches
not only for the construction of new chiral materials as chiral
sensors11 and chiral selectors but also for the rational design of
novel switchable helical architectures.
(
DMSO/alcohol ) 8/2, v/v) showed a color change, depending
on the compositions and temperatures as reflected by their
difference in hydrophobicity. The T s increased in the order of
9 (methanol), 43 (ethanol), 50 (n-propanol), and 55 °C (n-
13
m
2
Acknowledgment. We thank T. Nishimura for the molecular mechan-
ics calculations. This work was supported in part by Grant-in-Aid for
Scientific Research from the Ministry of Education, Culture, Sports,
Science and Technology, Japan and the Venture Business Laboratory
Project at Nagoya University.
butanol), and we were able to distinguish the alcohols with the
naked eye at 40 °C. On the other hand, the poly-2 showed no
solvatochromism in the presence of water and the alcohols.
It is well-known that â-CyD possesses a chiral hydrophobic
cavity to form inclusion complexes with a variety of organic
7
molecules that fit to the cavity size. It then became of interest
Supporting Information Available: Experimental procedures, CD
data, calculated helical conformations of poly-1, and the Hill plots (PDF).
This material is available free of charge via the Internet at http://pubs.acs.org.
to investigate whether the poly-1 could show a color change by
formation of such inclusion complexes. When 1-adamantanol (3)
or (-)-borneol (4) was added to the poly-1 solution in DMSO-
JA016393Z
H
2
O (8/2, v/v) at 25 °C, the solution color immediately changed
(
9) In this case, chiral solvation effect should be also taken into consid-
eration for the observed color change.
10) (a) Ueno, A.; Kuwabara, T.; Nakamura, A.; Toda, F. Nature 1992,
from yellow-orange to red accompanied by the inversion of the
4
4
Cotton effects (from [θ] ) +0.47 × 10 to -1.76 × 10 for 3)
and a large red-shift of λmax by 25 nm (Figure 3). In contrast,
cyclooctanol (5) and cyclohexanol (6) did not bring about such a
dramatic color change in the solution. The inclusion complex
formation should occur between the â-CyD residues of poly-1
and 3 and 4 as evidenced by the large association constants (Ks),
(
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8
obtained by the Hill plot analysis of the titration data in DMSO-
(
2
H O (8/2, v/v) at 25 °C (Figure 3). The changes in the absorbance
(
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