Detection and amplification of a small enantiomeric imbalance in α-amino acids by a helical poly(phenylacetylene) with crown ether pendants

Article abstract of DOI:10.1021/ja028348c

We have designed a novel stereoregular poly(phenylacetylene) bearing the bulky crown ether as the pendant (poly-1) for the amino acid binding site. The polymer forms a one-handed helix upon complexation with L-amino acid perchlorates, and the complexes exhibit an induced circular dichroism (ICD) with the same Cotton effect signs in the polymer backbone region through a significant cooperative interaction. Poly-1 is highly sensitive to the amino acid chirality and can detect an extremely small enantiomeric imbalance in α-amino acids (less than 0.005% enantiomeric excess of alanine, for example).

Full text of DOI:10.1021/ja028348c

Published on Web 12/28/2002
Detection and Amplification of a Small Enantiomeric
Imbalance in r-Amino Acids by a Helical
Poly(phenylacetylene) with Crown Ether Pendants
Ryuji Nonokawa and Eiji Yashima*
Contribution from the Department of Molecular Design and Engineering,
Graduate School of Engineering, Nagoya UniVersity, Chikusa-ku,
Nagoya 464-8603, Japan
Received August 30, 2002 ; E-mail: yashima@apchem.nagoya-u.ac.jp
Abstract: We have designed a novel stereoregular poly(phenylacetylene) bearing the bulky crown ether
as the pendant (poly-1) for the amino acid binding site. The polymer forms a one-handed helix upon
complexation with L-amino acid perchlorates, and the complexes exhibit an induced circular dichroism (ICD)
with the same Cotton effect signs in the polymer backbone region through a significant cooperative
interaction. Poly-1 is highly sensitive to the amino acid chirality and can detect an extremely small
enantiomeric imbalance in R-amino acids (less than 0.005% enantiomeric excess of alanine, for example).
Introduction
the presence of optically active compounds capable of interact-
ing with the polymer’s functional groups, a dynamic, one-handed
The origin of biomolecular homochirality such as L-amino
acids and D-sugars in living systems1 has been postulated to be
caused by physical forces such as magnetochiral anisotropy,2
circular polarized light (CPL),3 and an electroweak interaction.1,4
However, the enantiomeric excess (ee) of the amino acids
induced by these forces is quite small, and the detection is very
difficult by conventional analytical methods, so that the
development of detecting methods5 that are highly sensitive to
a small imbalance of the amino acids chirality, that also require
universal availability to all chiral amino acids, is valuable in
the area of chemistry, physics, astronomy, and geology.
macromolecular helicity is induced in the polymers, resulting
in a characteristic, induced circular dichroism (ICD) in the UV-
visible region.10 A similar drastic CD change was also observed
in an optically active poly(phenylacetylene) having a â-cyclo-
dextrin upon inclusion complexation with chiral and achiral
guest molecules.11 We now designed and synthesized a
cis-transoidal poly(phenylacetylene) consisting of a chro-
mophoric polyacetylene backbone and a bulky crown ether as
the pendant for the amino acid binding site (poly-1; Figure 1).12
Crown ethers are known to form stable complexes with amino
acids,13 and, as shown below, the amino acid derived chiral
information is transmitted to the polymer backbone leading to
an excess of the one-handed helical sense, which is the source
of the observed optical activity (ICD) (Figure 1).
In earlier studies, we reported the induction of helicity in
achiral poly(phenylacetylene)s bearing functional groups, such
as carboxy,6 amino,7 boronate,8 and phosphonate groups.9 In
We further report that an extremely small enantiomeric
imbalance in R-amino acids can be detected by a significant
amplification of the amino acid chirality using poly-1, through
a cooperative nonbonding interaction. In the literature, a high
amplification chirality has been reported by various means, for
example, via polymerization,14 supramolecular aggregation,15
(1) Reviews: (a) Bonner, W. A. Top. Stereochem. 1988, 18, 1-96. (b)
Keszthelyi, L. Q. ReV. Biophys. 1995, 28, 473-507. (c) Avalos, M.;
Babiano, R.; Cintas, P.; Jime´nez, J. L.; Palacios, J. C. Tetrahedron:
Asymmetry 2000, 11, 2845-2874.
(2) Rikken, G. L. J. A.; Raupach, E. Nature 2000, 405, 932-935.
(3) (a) Rau, H. Chem. ReV. 1983, 83, 535-547. (b) Inoue, Y. Chem. ReV.
1992, 92, 741-770. (c) Bailey, J.; Chrysostomou, A.; Hough, J. H.; Gledhill,
T. M.; McCall, A.; Clark, S.; Me´nard, F.; Tamura, M. Science 1998, 281,
672-674. (d) Green, M.; Selinger, J. V. Science 1998, 282, 880-881. (e)
Bailey, J. Origins Life EVol. Biosphere 2001, 31, 167-183.
(4) (a) Mason, S. F. Nature 1984, 311, 19-23. (b) Avalos, M.; Babiano, R.;
Cintas, P.; Jime´nez, J. L.; Palacios, J. C.; Barron, L. D. Chem. ReV. 1998,
98, 2391-2404. (c) Service, R. F. Science 1999, 286, 1282-1283. (d)
Berger, R.; Quack, M. ChemPhysChem 2000, 57-60. (e) Berger, R.;
Gottselig, M.; Quack, M.; Willeke, M. Angew. Chem., Int. Ed. 2001, 40,
4195-4198.
(8) (a) Yashima, E.; Nimura, T.; Matsushima, T.; Okamoto, Y. J. Am. Chem.
Soc. 1996, 118, 9800-9801. (b) Kawamura, H.; Maeda, K.; Okamoto, Y.;
Yashima, E. Chem. Lett. 2001, 58-59.
(9) Onouchi, H.; Maeda, K.; Yashima, E. J. Am. Chem. Soc. 2001, 123, 7441-
7442.
(10) Yashima, E. Anal. Sci. 2002, 18, 3-6.
(11) Yashima, E.; Maeda, K.; Sato, O. J. Am. Chem. Soc. 2001, 123, 8159-
8160.
(5) Pizzarello, S.; Cronin, J. R. Nature 1998, 394, 236.
(6) (a) Yashima, E.; Matsushima, T.; Okamoto, Y. J. Am. Chem. Soc. 1995,
117, 11596-11597. (b) Yashima, E.; Matsushima, T.; Okamoto, Y. J. Am.
Chem. Soc. 1997, 119, 6345-6359. (c) Yashima, E.; Goto, H.; Okamoto,
Y. Polym. J. 1998, 30, 69-71. (d) Yashima, E.; Maeda, K.; Okamoto, Y.
Nature 1999, 399, 449-451. (e) Saito, M. A.; Maeda, K.; Onouchi, H.;
Yashima, E. Macromolecules 2000, 33, 4616-4618. (f) Maeda, K.; Goto,
H.; Yashima, E. Macromolecules 2001, 34, 1160-1164.
(7) (a) Yashima, E.; Maeda, Y.; Okamoto, Y. Chem. Lett. 1996, 955-956. (b)
Yashima, E.; Maeda, Y.; Matsushima, T.; Okamoto, Y. Chirality 1997, 9,
593-600.
(12) Polyacetylenes bearing a crown ether moiety have been prepared, and their
metal-binding properties and morphology were investigated. (a) Kakuchi,
T.; Matsunami, S.; Kamimura, H.; Ishii, F.; Uesaka, T.; Yokota, K. J. Polym.
Sci., Part A: Polym. Chem. 1995, 33, 1431-1436. (b) Cheuk, K. K. L.;
Lam, J. W. Y.; Cha, J. A. K.; Tang, B. Z. Polym. Prepr. 2000, 41(1),
131-132.
(13) (a) Cram, D. J. Angew. Chem., Int. Ed. Engl. 1988, 27, 1009-1020. (b)
Lehn, J.-M. Angew. Chem., Int. Ed. Engl. 1988, 27, 89-112.
(14) Jha, S. K.; Cheon, K.-S.; Green, M. M.; Selinger, J. V. J. Am. Chem. Soc.
1999, 121, 1665-1673.
9
1278
J. AM. CHEM. SOC. 2003, 125, 1278-1283
10.1021/ja028348c CCC: $25.00 © 2003 American Chemical Society

Poly(phenylacetylene) with Crown Ether Pendants
A R T I C L E S
The typical CD and absorption spectra of poly-1 in the
presence of the simple amino acid, L- and D-alanine (Ala; 2
equiv to monomer units of poly-1) complexed with HClO4 in
pure acetonitrile, are shown in Figure 2A. The complexes
showed mirror images of split-type intense ICDs. The ICD
magnitude slightly increased with the decreasing temperature.
The CD titration using L-Ala showed that the CD intensity
increased with the increasing concentration of L-Ala and reached
an almost constant value at 1 equiv of L-Ala at -10 °C (Figure
2B). Plots of the CD intensities of the second Cotton (∆ꢀ2nd) of
poly-1 as a function of concentration of L-Ala gave a saturation
binding isotherm. The Hill plot analysis of the data resulted in
the apparent binding constants (Ks) of 1.8 × 104 and 2.6 × 104
at 25 and -10 °C, respectively.21 We note that at -10 °C, 0.1
equiv of L-Ala induced an almost one-handed helix, and poly-1
exhibited an apparent ICD even with 0.01 equiv of L-Ala,
indicative of a strong chiral amplification with cooperative
interaction in the pendants. That is, a very small chiral bias in
the monomeric crown ether units of poly-1 complexed with
L-Ala is amplified to induce the same helix on the major free
monomeric crown ether units. This result indicates that the
poly-1 may be acting analogously to the polyisocyanates in their
stiff helical character with alternating left- and right-handed
helical segments separated by rarely occurring helix reversals.22
Application of chiral information is then acted on cooperatively
to induce an excess helical sense upon complexation with L- or
D-Ala.23 Similar helicity induction on optically inactive polymers
and oligomers through intermolecular chiral interactions has
been reported.24
Figure 1. Schematic representation of the macromolecular helicity induction
on poly-1 upon complexation with L-alanine. The crown ether pendants,
represented by yellow and blue rings for clarity, arrange in a helical array
with a predominant screw-sense along the polymer backbone (bottom). The
helix-sense of poly-1 is tentative.
autocatalytic asymmetric synthesis,16 or by asymmetric photo-
reaction using CPL.17 Yet the present system differs in a
fundamental manner from this prior work in requiring no further
chemical reactions and derivatization and can provide a reliable
methodology for the rapid detection and identification of small
enantiomeric imbalances in amino acids and related chiral
molecules such as amino alcohols and even isovaline extracted
from meteorites18 and also those produced by CPL.19
The assay of 19 of the common L-amino acids (Ala, Arg,
Asn, Asp, Cys, Gln, Glu, His, Ile, Leu, Lys, Met, Phe, Pro,
Ser, Thr, Tyr, Trp, Val) produced intense ICDs with the same
Cotton effect signs (∆ꢀ2nd ) -10 to -17) even at 25 °C; only
the secondary amino acid (L-Pro) showed a weak ICD (∆ꢀ2nd
) -0.29 at 25 °C and -0.79 at 0 °C) (Table 1). The lower
limit of detection of L-Ala with concentrated poly-1 solution
(61 µg, 5 mg/mL) was examined, and it was found that only
Results and Discussion
Synthesis and Helicity Induction of Poly-1 with Chiral
Amino Acids and Amino Alcohols. Cis-transoidal poly-1 was
prepared by polymerization of the corresponding monomer with
a rhodium catalyst in a method similar to that previously
reported.6-9 The number average molecular weight (Mn) was
estimated to be 19.7 × 104 (Mw/Mn ) 2.6; degree of polym-
erization (DP) ) 503) as determined by size exclusion chro-
matography (SEC) with polystyrene standards using tetrahy-
drofuran (THF) containing 0.1 wt % tetra-n-butylammonium
(21) Connors, K. A. Binding Constants; John Wiley: New York, 1987.
(22) (a) Li, J.; Schuster, G. B.; Cheon, K.-S.; Green, M. M.; Selinger, J. V. J.
Am. Chem. Soc. 2000, 122, 2603-2612. For reviews, see: (b) Green, M.
M.; Peterson, N. C.; Sato, T.; Teramoto, A.; Cook, R.; Lifson, S. Science
1995, 268, 1860-1866. (c) Green, M. M.; Park, J.-W.; Sato, T.; Teramoto,
A.; Lifson, S.; Selinger, R. L. B.; Selinger, J. V. Angew. Chem., Int. Ed.
1999, 38, 3138-3154.
(23) We measured the viscosity of poly-1 with or without L-Ala to investigate
a change in rigidity of the polymer upon complexation with L-Ala. Poly-1
showed a linear relationship in the Huggins plot, and the intrinsic viscosity
was estimated to be 2.00 dL/g. However, in the presence of L-Ala, the
intrinsic viscosity increased to 3.56, which was 1.8 times larger than that
of the poly-1. Under the present experimental conditions, the poly-1
solutions containing L-Ala showed a full ICD at 25 °C. These results suggest
that poly-1 becomes more stiff than the poly-1 upon complexation with
L-Ala (see Supporting Information).
(24) (a) Schlitzer, D. S.; Novak, B. M. J. Am. Chem. Soc. 1998, 120, 2196-
2197. (b) Norris, I. D.; Kane-Maguire, L. A. P.; Wallace, G. G.
Macromolecules 1998, 31, 6529-6533 (c) Maeda, K.; Yamamoto, N.;
Okamoto, Y. Macromolecules 1998, 31, 5924-5926. (d) Nielsen, P. E.
Acc. Chem. Res. 1999, 32, 624-630. (e) Yashima, E.; Maeda, K.;
Yamanaka, T. J. Am. Chem. Soc. 2000, 122, 7813-7814. (f) Prince, R.
B.; Barnes, S. A.; Moore, J. S. J. Am. Chem. Soc. 2000, 122, 2758-2762.
(g) Ishikawa, M.; Maeda, K.; Yashima, E. J. Am. Chem. Soc. 2002, 124,
7448-7458. For reviews of helical polymers and oligomers (foldamers),
see: (h) Okamoto, Y.; Nakano, T. Chem. ReV. 1994, 94, 349-372. (i)
Gellman, S. H. Acc. Chem. Res. 1998, 31, 173-180. (j) Srinivasarao, M.
Curr. Opin. Colloid Interface Sci. 1999, 4, 370-376. (k) Fujiki, M.
Macromol. Rapid Commun. 2001, 22, 539-563. (l) Hill, D. J.; Mio, M. J.;
Prince, R. B.; Hughes, T. S.; Moore, J. S. Chem. ReV. 2001, 101, 3893-
4011. (m) Okamoto, Y.; Nakano, T. Chem. ReV. 2001, 101, 4013-4038.
(n) Conrnelissen, J. J. L. M.; Rowan, A. E.; Nolte, R. J. M.; Sommerdijk,
N. A. J. M. Chem. ReV. 2001, 101, 4039-4070. (o) Brunsveld, L.; Folmer,
B. J. B.; Meijer, E. W.; Sijbesma, R. P. Chem. ReV. 2001, 101, 4071-
4097.
1
bromide as the eluent. The H NMR spectrum of poly-1 in
CDCl3 showed a sharp singlet centered at 5.84 ppm, due to the
main chain protons, indicating that the polymer possesses a
highly cis-transoidal, stereoregular structure.20
(15) (a) Hirschberg, J. H. K. K.; Brunsveld, L.; Ramzi, A.; Vekemans, J. A. J.
M.; Sijbesma, R. P.; Meijer, E. W. Nature 2000, 407, 167-170. (b) Prins,
L. J.; Timmerman, P.; Reinhoudt, D. N. J. Am. Chem. Soc. 2001, 123,
10153-10163.
(16) (a) Shibata, T.; Yamamoto, J.; Matsumoto, N.; Yonekubo, S.; Osanai, S.;
Soai, K. J. Am. Chem. Soc. 1998, 120, 12157-12158. (b) Soai, K.; Shibata,
T.; Sato, I. Acc. Chem. Res. 2000, 33, 382-390.
(17) (a) Huck, N. P. M.; Jager, W. F.; de Lange, B.; Feringa, B. L. Science
1996, 273, 1686-1688. (b) Feringa, B. L.; van Delden, R. A. Angew. Chem.,
Int. Ed. 1999, 38, 3418-3438.
(18) (a) Kvenvolden, K.; Lawless, J.; Pering, K.; Peterson, E.; Flores, J.;
Ponnamperuma, C.; Kaplan, I. R.; Moore, C. Nature 1970, 228, 923-926.
(b) Bada, J. L. Philos. Trans. R. Soc. London, Ser. B 1991, 333, 349-358.
(c) Cronin, J. R.; Pizzarello, S. Science 1997, 275, 951-954. (d) Engel,
M. H.; Macko, S. A. Nature 1997, 389, 265-268. (e) Bada, J. L.; Glavin,
D. P.; McDonald, G. D.; Becker, L. Science 1998, 279, 362-365. (f)
Podlech, J. Angew. Chem., Int. Ed. 1999, 38, 477-478.
(19) Nishino, H.; Kosaka, A.; Hembury, G. A.; Shitomi, H.; Onuki, H.; Inoue,
Y. Org. Lett. 2001, 3, 921-924.
(20) (a) Simionescu, C.; Dumitrescu, S.; Percec, V. J. Polym. Sci., Polym. Symp.
1978, 64, 209-227. (b) Furlani, A.; Napoletano, C.; Russo, M. V.; Feast,
W. J. Polym. Bull. 1986, 16, 311-317. (c) Kishimoto, Y.; Eckerle, P.;
Miyatake, T.; Kainosho, M.; Ono, A.; Ikariya, T.; Noyori, R. J. Am. Chem.
Soc. 1999, 121, 12035-12044.
9
J. AM. CHEM. SOC. VOL. 125, NO. 5, 2003 1279

A R T I C L E S
Nonokawa and Yashima
Figure 2. Helicity induction on poly-1 with Ala. (A) CD spectra (molar ellipticity, ∆ꢀ) of poly-1 with L- and D-Ala‚HClO4 in acetonitrile at 25, 0, and -10
°C. Absorption spectrum (molar absorptivity, ꢀ) of poly-1 with L-Ala‚HClO4 at 25 °C is also shown. The concentration of poly-1 is 1.0 mg (2.6 µmol
monomer units)/mL. (B) Titration curves of poly-1 (∆ꢀ2nd) with L-Ala‚HClO4 in acetonitrile at 25 and -10 °C. Here ∆ꢀ2nd indicates the ICD intensity at the
second Cotton (365 nm). Inset shows expanded detail of the titration curves. Curves in the plots were the calculated ones using K ) 1.8 × 104 and 2.6 ×
104 at 25 and -10 °C, respectively.
Table 1. Signs and Difference in Exciton Coefficient of the
Second Cotton (∆ꢀ2nd) for the Complexes of Poly-1 with Amino
have been prepared, most of these receptor molecules showed
Acids or Amino Alcohols in CH3CN-1 N HClO4 (97.2/2.8, v/v)a
Although a number of receptor molecules for amino acids
molecular and chiral recognition for protected amino acids, and
host-guest systems capable of recognizing free amino acids
are still rare. This indicates that for detecting amino acid
chirality, poly-1 is indeed among the most sensitive and useful
synthetic receptors.27
Poly-1 also responded to chiral amino alcohols (Chart 1), and
the complexes exhibited similar ICDs in their patterns (Table
1). Here also the ICD intensity considerably increased with the
decreasing temperature. The amino alcohols ((S)-2-(S)-5)
derived from the L-amino acids, L-phenylalanine, L-phenylgly-
cine, L-leucine, and L-alanine, respectively, exhibited the same
Cotton effect signs as the L-amino acids, indicating that poly-1
will show ICDs with the same Cotton effect signs even if the
amino acids contain the amino alcohols derived from the amino
acids with the same configuration as an impurity.
second Cotton [λ (nm) and ∆ꢀ2nd (M-1 cm-1)]
guest
sign
25 °C λ (∆ꢀ)
amino acid
10°C λ (∆ꢀ)
0°C λ (∆ꢀ)
L-Ala
-
-
+
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
368 (17.39)
364 (19.50)
366 (17.25)
370 (10.69)
369 (16.83)
370 (16.33)
370 (16.72)
370 (17.43)
369 (17.72)
370 (16.96)
373 (0.29)
369 (16.63)
371 (16.03)
370 (17.38)
371 (15.04)
370 (16.63)
370 (14.83)
369 (17.34)
371 (12.16)
371 (10.08)
371 (12.06)
371 (1.87)
367 (18.55)
363 (20.69)
367 (18.36)
371 (15.05)
369 (18.17)
370 (17.62)
370 (18.08)
369 (18.64)
368 (18.95)
370 (18.18)
372 (0.65)
368 (17.89)
371 (17.57)
370 (18.75)
371 (16.25)
370 (18.01)
370 (16.88)
369 (18.88)
371 (14.79)
372 (13.60)
371 (14.81)
371 (3.27)
367 (19.16)
363 (21.40)
367 (19.03)
371 (16.30)
369 (18.85)
370 (18.30)
369 (18.80)
369 (19.27)
368 (19.58)
370 (18.81)
372 (0.79)
368 (18.49)
372 (18.35)
370 (19.53)
371 (16.88)
370 (18.74)
370 (17.69)
369 (19.65)
e
L-Alab
D-Ala
L-Asnc
L-Cys
L-Gln
L-Ile
L-Leu
L-Met
L-Phe
L-Pro
L-Ser
L-Thr
L-Trp
L-Tyr
L-Val
Chiral Amplification. We then investigated the changes in
the ICD intensity with respect to the ee of Ala (Figure 3) and
L-Aspc
L-Gluc
L-Argd
L-Hisd
L-Lysd
L-Isovaline
(25) (a) Green, M. M.; Reidy, M. P.; Johnson, R. D.; Darling, G.; O’Leary, D.
J.; Wilson, G. J. Am. Chem. Soc. 1989, 111, 6452-6454. (b) Yashima, E.;
Huang, S.; Matsushima, T.; Okamoto, Y. Macromolecules 1995, 28, 4184-
4193. (c) Maeda, K.; Okamoto, Y. Macromolecules 1998, 31, 1046-1052.
(d) Takei, F.; Onitsuka, K.; Takahashi, S. Polym. J. 2000, 32, 524-526.
(e) Nomura, R.; Fukushima, Y.; Nakako, H.; Masuda, T. J. Am. Chem.
Soc. 2000, 122, 8830-8836. (f) Toyoda, S.; Fujiki, M. Macromolecules
2001, 34, 640-644.
(26) For intermolecular sergeants and soldiers effects in supramolecular helical
arrays, see: (a) Palmans, A. R. A.; Vekemans, J. A. J. M.; Havinga, E. E.;
Meijer, E. W. Angew. Chem., Int. Ed. Engl. 1997, 36, 2648-2651. (b)
Langeved-Voss, B. M. W.; Waterval, R. J. M.; Janssen, R. A. J.; Meijer,
E. W. Macromolecules 1999, 32, 227-230. (c) Brunsveld, L.; Meijer, E.
W.; Prince, R. B.; Moore, J. S. J. Am. Chem. Soc. 2001, 123, 7978-7984.
(d) Schenning, A. P. H. J.; Kilbinger, A. F. M.; Biscarini, F.; Cavallini,
M.; Cooper, H. J.; Derrick, P. J.; Feast, W. J.; Lazzaroni, R.; Lecle`re, P.;
McDonell, L. A.; Meijer, E. W.; Meskers, S. C. J. J. Am. Chem. Soc. 2002,
124, 1269-1275. For intramolecular sergeants and soldiers effects in
supramolecular helical arrays, see: (e) Fiesel, R.; Scherf, U. Acta Polym.
1998, 49, 445-449. (f) Prince, R. B.; Moore, J. S.; Brunsveld, L.; Meijer,
E. W. Chem.-Eur. J. 2001, 7, 4150-4154.
e
e
370 (3.93)
amino alcohol
(S)-2
(S)-3
(R)-3
(S)-4
(S)-5
-
-
+
-
-
371 (6.46)
371 (5.55)
371 (5.75)
371 (4.87)
371 (7.05)
371 (13.32)
371 (11.39)
371 (11.43)
371 (11.96)
371 (13.51)
371 (15.76)
371 (14.09)
371 (13.90)
371 (15.00)
371 (15.47)
a The concentration of poly-1 is 1.0 mg/mL, and the molar ratio of a
guest to monomer units of poly-1 is 10; [HClO4]/[guest] ) 1.1. b In CH3CN.
c In CH3CN-1 N HClO4-water (95.0/2.8/2.2, v/v) and [amino acid]/[poly-
1] ) 5. d In CH3CN-1 N HClO4-water (92.5/2.8/4.7, v/v) and [amino
acid]/[poly-1] ) 5. e It could not be measured because the solution became
turbid.
(27) For recent examples of chirality recognition of protected amino acids in
organic media, see: (a) Huang, X.; Rickman, B. H.; Borhan, B.; Berova,
N.; Nakanishi, K. J. Am. Chem. Soc. 1998, 120, 6185-6186. (b) Zahn, S.;
Canary, J. W. Org. Lett. 1999, 1, 861-864. (c) Mizutani, T.; Sakai, N.;
Yagi, S.; Takagishi, T.; Kitagawa, S.; Ogoshi, H. J. Am. Chem. Soc. 2000,
122, 748-749. (d) Shinkai, S.; Ikeda, M.; Sugasaki, A.; Takeuchi, M. Acc.
Chem. Res. 2001, 34, 494-503. (e) Canary, J. W.; Holmes, A. E.; Liu, J.
Enantiomer 2001, 6, 181-188. For recent examples of chiral or chirality
recognition of free amino acids, see ref 9 and: (f) Tsukube, H.; Shinoda,
S.; Uenishi, J.; Kanatani, T.; Itoh, H.; Shiode, M.; Iwachido, T.; Yonemitsu,
O. Inorg. Chem. 1998, 37, 1585-1591. (g) Leipert, D.; Nopper, D.; Bauser,
M.; Gauglitz, G.; Jung, G. Angew. Chem., Int. Ed. 1999, 38, 3308-3311.
70 ng of L-Ala, corresponding to 0.005 equiv of the monomer
units of poly-1, was enough to produce an ICD at 25 °C. These
results belong to the class of sergeants and soldiers effects
widely observed in helical polymers14,22,25 and supramolecular
helical arrays, with the amino acids acting as the sergeants in
the present system.15,26
9
1280 J. AM. CHEM. SOC. VOL. 125, NO. 5, 2003

Poly(phenylacetylene) with Crown Ether Pendants
A R T I C L E S
Chart 1. Structures of L-Isovaline and Chiral Amino Alcohols
observed an extremely strong chiral amplification (positive
nonlinear effect) despite the noncovalent bonding interaction
in this system. Even a 5% ee of Ala gave rise to the full ICD
as induced by pure L-Ala. Complexation of a slight excess of
one enantiomeric Ala with the pendant groups of the polymer
led to an excess of the helical sense preferred by the majority
enantiomer.22 Similar strong chiral amplifications were also
observed for Leu and Trp (see Supporting Information). This
striking nonlinear effect of poly-1 enabled the detection of the
chirality of Ala with a very small ee less than 0.005% with
accuracy and reproducibility (Figure 3B and C). We found a
good linear relationship between the ee (from 0.1 to 0.005%)
and the ICD values of poly-1. These results are qualitatively
consistent with a theoretical analysis applied to analogous
majority rule effects observed in the polyisocyanates with further
experiments required for testing the quantitative correspon-
dence.22,28 We note that Ala is a typical amino acid frequently
found in several meteorites18 including the Murchison meteorite.
Other chiral R-amino acids (0.01% ee of Leu and Ser) and
unnatural amino acids such as isovaline (1% ee) (also found in
the Murchison meteorite18c) can be detected by poly-1 (see
Supporting Information). It is clear from the results reported
here that poly-1 has the potential to detect very small enantio-
meric imbalances in a wide variety of amino acids and related
molecules of interest in studies of meteorites and elsewhere.
Figure 3. (A) Changes in ICD intensity (∆ꢀ2nd) of poly-1 versus the % ee
of Ala (10 equiv) during the complexation with poly-1 in acetonitrile
containing 2.8 vol % 1 N HClO4 in water at 25 and -10 °C. The
concentration of poly-1 is 1.0 mg/mL. Inset shows expanded detail of the
ICD intensity. (B) Plots between the ICD intensity and 0.1-0.005% ee of
Ala at 25 and -10 °C. (C) CD spectra of poly-1 in the presence of 0.01
and 0.005% ee of L- and D-Ala (10 equiv) at -10 °C.
clooctadecane (1-aza-18-crown-6) was purchased from Aldrich (Mil-
waukee, WI). Dry acetonitrile (water content < 0.005 vol %) and
dichloromethane (water content < 0.005 vol %) were obtained from
Kanto Kagaku (Tokyo, Japan) and Aldrich, respectively. THF was dried
over sodium benzophenone ketyl and distilled onto LiAlH4 under
nitrogen. Triethylamine was dried over KOH pellet and distilled under
nitrogen. These solvents were distilled again under high vacuum just
before use. [(Norbornadiene) rhodium (I) chloride]2 ([Rh(nbd)Cl]2) was
purchased from Aldrich. L-Alanine and D-alanine (>99.9% ee) were
obtained from Peptide Institute, Inc. (Osaka, Japan). DL-Alanine and
aqueous perchloric acid solution (60 wt %) were from Kanto Kagaku.
L-Leucine, D-leucine, and DL-leucine and tryptophan (purity > 98%)
were purchased from Tokyo Kasei (TCI, Tokyo, Japan). Other L-, D-,
and DL-amino acids were available from Aldrich. The enantiomeric
excesses of these amino acids are not determined by suppliers. (S)-
Alaninol was purchased from Aldrich, and other amino alcohols used
in this study were from TCI. (S)-Isovaline (99% purity and ee > 99%)
was purchased from Acros Organics (Geel, Belgium). Racemic isovaline
was prepared according to the literature procedure.30
Conclusions
We have developed a novel helical poly(phenylacetylene) that
is highly sensitive to amino acid chirality via formation of
complexes that exhibit intense ICDs. The advantage of the
present helical polymer is not only its high sensitivity to chiral
amino acids and amino alcohols on a nanoscale without
derivatization, but also its response to all chiral R-amino acids
with the same Cotton effect sign if the configurations are the
same. The polymer described here may have wide application
in the study of nearly racemic amino acids and related molecules
of interest, including for extraterrestrial studies.29
Experimental Section
(4-Ethynylbenzoyl)monoaza-18-crown-6 (1). This new compound
was synthesized according to Scheme 1. 4-Ethynylbenzoyl chloride was
prepared according to the previously reported method.9 To a mixture
of 1-aza-18-crown-6 (1.1 g, 4.2 mmol) in triethylamine (1.6 mL) and
dichloromethane (50 mL) was added 4-ethynylbenzoyl chloride (0.97
g, 5.9 mmol) at 0 °C. The mixture was stirred under nitrogen at 25 °C
for 7 h. After the solvent was evaporated, the residue was purified by
silica gel chromatography with chloroform-methanol (9/1, v/v) as the
Materials. All starting materials were obtained from commercial
suppliers and were used as received. 1,4,7,10,13-Pentaoxa-16-azacy-
(28) (a) Selinger, J. V.; Selinger, R. L. B. Phys. ReV. Lett. 1996, 76, 58-61.
For other references of majority rule in helical polymers, see: (b) Green,
M. M.; Garetz, B. A.; Munoz, B.; Chang, H.; Hoke, S.; Cooks, R. G. J.
Am. Chem. Soc. 1995, 117, 4181-4182. (c) Okamoto, Y.; Nishikawa, M.;
Nakano, T.; Yashima, E.; Hatada, K. Macromolecules 1995, 28, 5135-
5138. (d) Takei, F.; Onitsuka, K.; Takahashi, S. Polym. J. 1999, 31, 1029-
1032. (e) Tabei, J.; Nomura, R.; Masuda, T. Macromolecules 2002, 35,
5405-5409.
(30) Ogrel, A.; Shvets, V. I.; Kaptein, B.; Broxterman, Q. B.; Raap, J. Eur. J.
Org. Chem. 2000, 857-859.
(29) Welch, C. J.; Lunine, J. I. Enantiomer 2001, 6, 69-81.
9
J. AM. CHEM. SOC. VOL. 125, NO. 5, 2003 1281

A R T I C L E S
Nonokawa and Yashima
Scheme 1
CD Measurements. Deionized, distilled water and dry acetonitrile
were distilled again just before use and stored under nitrogen. The
concentration of poly-1 was calculated on the basis of the monomer
units and was 1.0 mg/mL (2.6 mM monomer units) unless otherwise
stated. In the complexation of poly-1 with D- or L-amino acids, stock
solutions of poly-1 (2.0 mg/mL) in acetonitrile and 1 N HClO4 in water
were prepared in 5 mL and 50 mL flasks equipped with a stopcock,
respectively. Next 0.026 mmol of D- or L-amino acid (2.28 mg of Ala,
for example) was placed in a 1 mL flask equipped with a stopcock.
Twenty-eight microliters of 1 N HClO4 ([HClO4]/[amino acid] ) 1.1
(mol/mol)) followed by 0.50 mL of the poly-1 solution were transferred
to the 1 mL flask, and the solution was diluted with acetonitrile to
keep the poly-1 concentration at 1.0 mg/mL, and the molar ratio of
amino acids to monomer units of poly-1 was 10. The solution was
thoroughly mixed with a vibrator (Iuchi, Osaka, Japan) before measuring
the absorption and CD spectra. The same procedure was performed
for all of the D- or L-amino acids. The volumetric ratio of water to
acetonitrile was held constant at acetonitrile/water ) 97.2/2.8 (v/v) for
these ICD experiments, but water affects the amino acid-crown ether
complexation in aprotic organic solvents,31 and, therefore, the effect
of water on the ICD magnitude of the complex of poly-1 with L-Ala
was investigated on the basis of the CD titrations. We found that the
ICD signals of the poly-1-L-Ala complex decreased with an increase
in the amount of water, so we used pure acetonitrile in the CD titrations
of poly-1 with L-Ala (Figure 2). In the CD titrations in acetonitrile, the
L-Ala complexed with HClO4 was prepared according to the reported
method.32 In the CD measurements of poly-1 with nonracemic Ala
(mixtures of L- and D-Ala), a mixture of acetonitrile and aqueous 1 N
HClO4 (97.2/2.8, v/v) was used because of difficulty in accurately
preparing pure acetonitrile solutions of the Ala-HClO4 complexes with
different ee values.
CD Titrations of Poly-1 with L-Ala in Acetonitrile. A stock
solution of poly-1 (2.0 mg/mL) in acetonitrile was prepared in a 10
mL flask equipped with a stopcock. Stock solutions of L-Ala‚HClO4
complexes (61.5, 4.56, and 0.41 mM) in acetonitrile were also prepared
in 2, 10, and 50 mL flasks, respectively. The 0.50 mL aliquots of the
poly-1 solution were transferred to ten 1 mL flasks equipped with a
stopcock. Increasing amounts of the stock solution of the L-Ala‚HClO4
complexes were added to the flasks; the molar ratios of L-Ala‚HClO4
to poly-1 were 0.01, 0.02, 0.03 (0.41 mM L-Ala‚HClO4 was used),
0.05, 0.1 (4.56 mM L-Ala‚HClO4), and 0.5, 1.0, 2.0, 5.0, 10 (61.5 mM
L-Ala‚HClO4), and the resulting solutions were diluted with acetonitrile
to keep the poly-1 concentrations at 1.0 mg/mL (2.6 mM). The
absorption and CD spectra were then taken for each flask to determine
the changes in the CD spectra (Figure 2B).
eluent to give 1.5 g of 1 as a white solid (91% yield). Mp (81.4-81.8
°C). IR (KBr, cm-1): 3267 (νCtC), 1623 (νCdO), 1138 (νC-O). 1H NMR
(CDCl3, 500 MHz, TMS): δ 7.51 (d, J ) 8.5 Hz, aromatic, 2H), 7.38
(d, J ) 8.5 Hz, aromatic, 2H), 3.79-3.56 (m, CH2, 24H), 3.13 (s, t
CH, 1H). 13C NMR (CDCl3, 125 MHz, TMS): δ 171.4, 137.1, 132.1,
126.9, 123.0, 83.0, 78.3, 70.8-70.4 (br), 69.6, 69.3, 50.0, 46.0. MS
(FAB+): Calcd for C21H30NO6 (M + H), 392. Found: 392. Anal. Calcd
for C21H29NO6‚1/3H2O: C, 63.46; H, 7.52; N, 3.52. Found: C, 63.40;
H, 7.49; N, 3.54.
Polymerization. Polymerization was carried out in a dry glass
ampule under a dry nitrogen atmosphere using [Rh(nbd)Cl]2 as a catalyst
in a way similar to one previously reported.6-9 Monomer 1 (0.80 g,
2.1 mmol) was placed in a dry ampule, which was then evacuated on
a vacuum line and flushed with dry nitrogen. After this evacuation-
flush procedure was repeated three times, a three-way stopcock was
attached to the ampule, and dry THF (7.3 mL) and triethylamine (0.3
mL) were added with a syringe. To this was added a solution of [Rh-
(nbd)Cl]2 in THF (2.0 mL) at 30 °C. The concentrations of the monomer
and the rhodium catalyst were 0.2 and 0.002 M, respectively. After 24
h, the resulting poly-1 was precipitated into a large amount of ether,
collected by centrifugation, and dried in vacuo at 50 °C for 2 h (0.76
g, 95% yield). Poly-1 was soluble in water, methanol, chloroform,
acetonitrile, THF, acetone, and toluene. The number average molecular
weight (Mn) and molecular weight distribution were 19.7 × 104 and
2.6, respectively, as determined by SEC with polystyrene standards in
THF containing 0.1 wt % tetra-n-butylammonium bromide as the eluent.
Spectroscopic data of poly-1. IR (KBr, cm-1): 1637 (νCdO), 1107
(νC-O). 1H NMR (CDCl3, 60 °C, 300 MHz, TMS): δ 7.11 (s, aromatic,
2H), 6.70 (s, aromatic, 2H), 5.84 (s, dCH, 1H), 3.62 (br, CH2, 24H).
Anal. Calcd for (C21H29NO6‚H2O)n: C, 61.60; H, 7.63; N, 3.42.
Found: C, 61.86; H, 7.53; N, 3.43.
Instruments. Melting points were measured on a Bu¨chi melting point
Nonlinear Effects. The changes in the ICD intensity of poly-1 with
respect to the ee of the amino acids (nonlinear effects) were investigated.
A typical experimental procedure using Ala is described below. In this
experiment, Ala samples were separately prepared for large (1% < ee
< 100%) and small (0.001% < ee < 2%) ee values before the CD
measurements. The molar ratios of Ala to the monomer units of poly-1
and HClO4 were held constant at 10 and 0.91, respectively, and the
volumetric ratio of water to acetonitrile was held constant at acetonitrile/
water ) 97.2/2.8 (v/v) unless otherwise stated. The purity of the L-
and D-Ala (purchased from Peptide Institute Inc.) was confirmed by
high-performance liquid chromatography (HPLC) using a chiral column
(Crownpak CR, Daicel, Tokyo, Japan; 25 cm × 0.46 cm (i.d.)) at
ambient temperature. Aqueous HClO4 (pH 1.5) was used as the eluent
at a flow rate of 0.4 mL/min according to a guidebook supplied from
Daicel. In the chromatographic separation of the L- and D-Ala, opposite
enantiomers could not be detected, while DL-Ala was completely
separated into enantiomers under the same conditions (D- and L-Ala
eluted at 3.75 and 4.90 min, respectively), indicating that the enantio-
apparatus and are uncorrected. NMR spectra were taken on a Varian
Mercury 300 operating at 300 MHz for H or a Varian VXR-500S
1
spectrometer operating at 500 MHz for 1H and 125 MHz for 13C.
Chemical shifts are reported in parts per million (δ) downfield from
tetramethylsilane (TMS) as the internal standard. Fast atom bombard-
ment (FAB) mass spectra were obtained on a JEOL JMS-AX505HA
spectrometer (Akishima, Japan). Elemental analyses were performed
by the Nagoya University Analytical Laboratory in School of Engineer-
ing. SEC measurements were performed with a Jasco PU-980 liquid
chromatograph (Jasco, Hachioji, Japan) equipped with a UV (254 nm;
Jasco UV-970) detector. A Tosoh (Tokyo, Japan) TSKgel Multipore-
HXL-M SEC column (30 cm) was connected, and THF containing 0.1
wt % tetra-n-butylammonium bromide was used as the eluent at a flow
rate of 0.5 mL/min. The molecular weight calibration curve was
obtained with polystyrene standards (Tosoh). IR spectra were recorded
using a Jasco Fourier Transform IR-620 spectrophotometer. Absorption
and CD spectra were measured in a 1.0-mm quartz cell on a Jasco
V-570 spectrophotometer and a Jasco J-820 spectropolarimeter, re-
spectively. The temperature was controlled with a Jasco PTC-423L
apparatus (-10-25 °C). Dilute solution viscosities were measured using
an Ubbelohde microviscometer at 25 °C in a homemade thermostat.
(31) Zhang, X. X.; Bradshaw, J. S.; Izatt, R. M. Chem. ReV. 1997, 97, 3313-
3361.
(32) Sogah, G. D. Y.; Cram, D. J. J. Am. Chem. Soc. 1979, 101, 3035-3042.
9
1282 J. AM. CHEM. SOC. VOL. 125, NO. 5, 2003

Poly(phenylacetylene) with Crown Ether Pendants
A R T I C L E S
Supporting Information Available: 1H NMR spectrum of
poly-1, viscosity measurement results of poly-1 with or without
L-Ala, nonlinear effects between the ICD values for the second
Cotton and % ee of Leu and Trp in the complexation with poly-
1, CD spectra for the complexes of poly-1 with 0.1, 0.05, 0.01,
and 0.005% ee of L- and D-Ala, 0.01% ee of L-Ser and L-Leu,
and 5 and 1% ee of L-isovaline, and experimental procedures
for the preparation of Ala solutions with large and small
enantiomeric excesses (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
meric excesses of the L- and D-Ala were greater than 99.9%. Therefore,
we assumed that the L- and D-Ala are pure enantiomers and DL-Ala is
a 50:50 mixture of L- and D-Ala for preparing the Ala solutions with
different ee values. For the preparation of Ala solutions with large and
small ee and CD measurements with poly-1, see the Supporting
Information.
Acknowledgment. This work was partially supported by a
Grant-in-Aid for Scientific Research from Japan Society for the
Promotion of Science and the Ministry of Education, Culture,
Sports, Science, and Technology, Japan. R.N. expresses thanks
for a JSPS Research Fellowship (No. 01428) for Young
Scientists.
JA028348C
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J. AM. CHEM. SOC. VOL. 125, NO. 5, 2003 1283