Regulation of saccharide binding with basic poly(ethynylpyridine)s by H+-induced helix formation

Article abstract of DOI:10.1021/ja054134u

A basic host polymer exhibiting pH-regulatable saccharide recognition has been investigated. Poly(m-ethynylpyridine) bearing dialkylamino groups forms helical complexes with saccharides to show induced circular dichroism (ICD). When trifluoroacetic acid was titrated on these complexes, the ICD was gradually enhanced until the amount of the acid reached ca. 0.5 molar equivalence versus the pyridine rings in the polymer, and further addition of the acid suppressed the ICD. The proper addition of the acid also increased the binding constants between the polymer and saccharides. These findings would be due to stabilization of the helical structure consisting of cisoid conformations for each of the adjacent pyridine pairs, which were caused by half-protonation of the pyridine rings. Computational analyses indicated that the pyridinium-pyridine dimeric structure prefers its cisoid conformation to its transoid one. copy; 2005 American Chemical Society.

Full text of DOI:10.1021/ja054134u

Published on Web 10/27/2005
Regulation of Saccharide Binding with Basic
Poly(ethynylpyridine)s by H+-Induced Helix Formation
Hajime Abe,* Nozomi Masuda, Minoru Waki, and Masahiko Inouye*
Contribution from the Faculty of Pharmaceutical Sciences, Toyama Medical and Pharmaceutical
UniVersity, Toyama 930-0194, and PRESTO, Japan Science and Technology Agency (JST),
Saitama 332-0012, Japan
Received June 22, 2005; E-mail: abeh@ms.toyama-mpu.ac.jp (H.A.)
Abstract: A basic host polymer exhibiting pH-regulatable saccharide recognition has been investigated.
Poly(m-ethynylpyridine) bearing dialkylamino groups forms helical complexes with saccharides to show
induced circular dichroism (ICD). When trifluoroacetic acid was titrated on these complexes, the ICD was
gradually enhanced until the amount of the acid reached ca. 0.5 molar equivalence versus the pyridine
rings in the polymer, and further addition of the acid suppressed the ICD. The proper addition of the acid
also increased the binding constants between the polymer and saccharides. These findings would be due
to stabilization of the helical structure consisting of cisoid conformations for each of the adjacent pyridine
pairs, which were caused by half-protonation of the pyridine rings. Computational analyses indicated that
the pyridinium-pyridine dimeric structure prefers its cisoid conformation to its transoid one.
host molecules.7 So far, however, rare are such artificial host
molecules that possess abilities to be regulated by their
Introduction
As seen in many enzymes, recognition of substrates is often
regulated by interaction with some chemical species (effectors)
other than the substrates. Various kinds of effectors such as ions,
ligands, and cofactors have been known to be involved in the
working of enzymes.1 In the chemistry of supramolecules, the
regulation of their structures and functions has been regarded
as one important subject. For mimicking biological processes
by means of host-guest chemistry, it is essential to regulate
the recognition ability and the structure of host molecules by
external stimuli.2-4 A proton is one of the most simple and
ubiquitous chemical stimuli, and actually many kinds of
enzymes are affected by a change of pH.1 To regulate the higher
order structure of artificial supramolecules, protonation has been
utilized for conformational changes of hydrogen-bonding fol-
damers.5,6 Protonation has also been applied to control the
translocation of metal cations in ditopic azacrown and related
recognition strengths and higher order structures together with
one kind of chemical stimulus.
Saccharides are one class of abundant substances in nature,
and their recognition is still an interesting challenge in the field
of molecular recognition.8 Recently, during the course of our
study about oligo- and polypyridine host molecules,9,10 we found
that m-ethynylpyridine polymers such as 1 associate with various
saccharides to form helical complexes (Figure 1).10-12 On those
polymers, the pyridine nitrogen atoms behave as hydrogen-
bonding acceptors to interact with the hydroxy groups of the
guest saccharides. The rigid acetylene bridges arrange the
pyridine rings at intervals fitting to the hydroxy groups of the
guests, and the resulting multipoint hydrogen bondings induce
the helix formation. The sense of the helices is biased by the
chirality of the guest saccharides to show characteristic induced
circular dichroism (ICD) at the wavelength range where the host
(1) Fersht, A. Enzyme Structure and Mechanism, 2nd ed.; W. H. Freeman and
Co.: New York, 1985. Dixon, M.; Webb, E. C. Enzymes, 3rd ed.; Longman
Group: London, 1979.
(2) Balzani, V.; Venturi, M.; Credi, A. Molecular DeVices and Machines s A
Journey into the Nanoworld; Wiley-VCH: Weinheim, Germany, 2003; pp
288-328. Lehn, J.-M. Supramolecular Chemistry, Concepts and Perspec-
tiVes; VCH: Weinheim, Germany, 1995.
(3) A representive class of stimuli-regulatable host molecules is photoresponsive
crown ethers. For reviews, see: Morrison, H., Ed. Biological Applications
of Photochemical Switches; John Wiley & Sons: New York, 1993. Shinkai,
S.; Manabe, O. Top. Curr. Chem. 1984, 121, 67-104. Shinkai, S. Pure
Appl. Chem. 1987, 59, 425-430. Kimura, K.; Sakamoto, H.; Nakamura,
M. Bull. Chem. Soc. Jpn. 2003, 76, 225-245. Cooke, G. Angew. Chem.,
Int. Ed. 2003, 42, 4860-4870.
(4) For recent examples of photoresponsive host molecules, see: Molard, Y.;
Bassani, D. M.; Desvergne, J.-P.; Horton, P. N.; Hursthouse, M. B.; Tucker,
J. H. R. Angew. Chem., Int. Ed. 2005, 44, 1072-75. Mulder, A.; Jukoviæ,
A.; Huskens, J.; Reinhoudt, D. N. Org. Biomol. Chem. 2004, 2, 1748-
1755. Mulder, A.; Jukoviæ, A.; van Leeuwen, F. W. B.; Kooijman, H.;
Spek, A. L.; Huskens, J.; Reinhoudt, D. N. Chem.sEur. J. 2004, 10, 1114-
1123. Hunter, C. A.; Togrul, M.; Tomas, S. Chem. Commun. 2004, 108-
109. Goodman, A.; Breinlinger, E.; Ober, M.; Rotello, V. M. J. Am. Chem.
Soc. 2001, 123, 6123-6214.
(5) (a) Dolain, C.; Maurizot, V.; Huc, I. Angew. Chem., Int. Ed. 2003, 42,
2737-2740. (b) Barboiu, M.; Lehn, J.-M. Proc. Natl. Acad. Sci. U.S.A.
2002, 99, 5201-5206.
(6) For a review introducing aromatic oligoamide foldamers including topics
about their conformational control, see: Huc, I. Eur. J. Org. Chem. 2004,
17-29.
(7) Lodeiro, C.; Parola, A. J.; Pina, F.; Bazzicalupi, C.; Bencini, A.; Bianchi,
A.; Giorgi, C.; Masotti, A.; Valtancoli, B. Inorg. Chem. 2001, 40, 2968-
2975. Amendola, V.; Fabbrizzi, L.; Mangano, C.; Pallavicini, P.; Perotti,
A.; Taglietti, A. J. Chem. Soc., Dalton Trans. 2000, 185-189. Ji, H.-F.;
Dabestani, R.; Brown, G. M. J. Am. Chem. Soc. 2000, 122, 9306-9307.
Ikeda, A.; Tsudera, T.; Shinkai, S. J. Org. Chem. 1997, 62, 3568-3574.
(8) Penade´s, S. Ed. Host-Guest ChemistrysMimetic Approaches to Study
Carbohydrate Recognition; Topics in Current Chemistry 218; Springer-
Verlag: Berlin, 2002. Davis, A. P.; Wareham, R. S. Angew. Chem., Int.
Ed. 1999, 38, 2978-2996.
(9) Inouye, M.; Chiba, J.; Nakazumi, H. J. Org. Chem. 1999, 64, 8170-8176.
Inouye, M.; Takahashi, K.; Nakazumi, H. J. Am. Chem. Soc. 1999, 121,
341-345. Inouye, M.; Miyake, T.; Furusyo, M.; Nakazumi, H. J. Am. Chem.
Soc. 1995, 117, 12416-12425.
(10) Inouye, M.; Waki, M.; Abe, H. J. Am. Chem. Soc. 2004, 126, 2022-2027.
9
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J. AM. CHEM. SOC. 2005, 127, 16189-16196
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A R T I C L E S
Abe et al.
To make clear the influences of the protonation on poly-
(m-ethynylpyridine)s, rather higher basicity is required for the
pyridine moieties. Thus, we decided to develop a highly basic
poly(m-ethynylpyridine) by introducing dialkylamino groups to
each pyridine ring on the polymer.
Among a variety of pyridine derivatives, one of the well-
known compounds for its high basicity is 4-(dimethylamino)-
pyridine (DMAP), which is often utilized as a catalyst in organic
syntheses.13 This utility is largely due to the virtue of the
dialkylamino group that can stabilize the cationic conjugate acids
and electrophile adducts of the pyridine ring. For the design of
new highly basic polymer 2, we anticipated that the adoption
of a DMAP-like unit structure gives new properties and
functions of the polymer. Recently, Heemstra and Moore et al.
studied pyridine-containing m-phenylene ethynylene oligomers,
which showed interesting features due to the helix stabilization
by N-protonation or -methylation.14
Figure 1. Structure of poly(m-ethynylpyridine) 1 and its conformation
change induced by complexation with a saccharide guest (butoxy groups
were omitted).
In the molecular structure of 2, the DMAP-like 4-(N-methyl-
N-octylamino)pyridine units work as the centers for hydrogen
bonding and the octyl chains improve the solubility of the
polymer in organic media. The DMAP-like units are expected
to be quantitatively protonated by treatment with the proper acid
to alter the characteristics of 2. The protonation of pyridine into
pyridinium (1) converts the hydrogen-bonding mode of the ring
from acceptor to donor, (2) gives a cationic nature to the ring,
and (3) inverts the direction of the local dipole moment on the
ring. As a result, the supramolecular characteristics of polymer
2, e.g., the recognition ability for saccharides and the higher
order structure of the complex, will be affected.
Preparation of Basic Polymer 2. The targeted basic polymer
2 was prepared mainly by repeating the Sonogashira reaction
as shown in Scheme 1. To obtain a clean polymer of enough
length, trimer 11 was chosen as the synthetic unit for polym-
erization. First, commercially available 2,6-dibromopyridine (3)
was converted into 2,6-dibromo-4-nitropyridine (4) by the
procedure in the literature.15 The electron-deficient 4 was treated
with N-methyloctylamine to give 2,6-dibromo-4-(N-methyl-N-
octylamino)pyridine (5). Sonogashira reaction of 5 with excess
(trimethylsilyl)acetylene yielded 6, and the following protiode-
silylation afforded diethynylpyridine 7. On the other hand,
copper-mediated halogen exchange16 of 5 gave diiodopyridine
8. Diiodo trimer 9 was obtained by a coupling reaction using 7
and an excess amount of 8. Sonogashira reaction using (trim-
ethylsilyl)acetylene and an excess amount of 9 gave 10, and
the following protiodesilylation yielded 11, which was applied
to the final polymerization to 2. Before the protiodesilylation
of 10, the copper salt must surely be removed by washing with
aqueous 1,2-diaminoethane; otherwise Glaser coupling leads to
a byproduct diyne. For the polymerization to 2, a diluted solution
of 11 in DMF/i-Pr2NH was treated under the Sonogashira
reaction conditions to afford a crude mixture containing 2 after
Figure 2. Structures of basic poly(m-ethynylpyridine) 2 and DMAP.
polymers absorb. At this point, considering the basicity of the
pyridine moieties in the polymers, one may remark on the
possibility that protonation on the nitrogen atoms can alter the
nature of the ring moieties. The conversion of pyridine into
pyridinium will locally change the hydrogen-bonding and
electric properties of the rings and eventually influence the
higher order structures of the polymers. Herein we report the
development of (4-dialkylamino-2,6-pyridylene)ethynylene poly-
mer 2, saccharide binding of which is regulated by H+-induced
stabilization of the helical conformation (Figure 2).
Results
Molecular Design for Basic Poly(m-ethynylpyridine) 2. We
have expected that protonation would be effective for the
regulation of the properties and the structures for poly-
(m-ethynylpyridine)s. However, we were afraid that the basicity
of polymer 1 was not high enough to capture many protons on
the pyridine sites quantitatively. As protons are added one by
one to the polymer, the basicity of the intermediate conjugate
acids of the polymer becomes weaker. Therefore, one must use
a large excess amount of acid, which may behave as an inhibitor
for the hydrogen bonding between the polymer and saccharides.
(11) For reviews on helical foldamers and polymers, see: Hill, D. J.; Mio, M.
J.; Prince, R. B.; Hughes, T. S.; Moore, J. S. Chem. ReV. 2001, 101, 3893-
4011. Nakano, T.; Okamoto, Y. Chem. ReV. 2001, 101, 4013-4038.
Cornelissen, J. J. L. M.; Rowan, A. E.; Nolte, R. J. M.; Sommerdijk, N. A.
J. M. Chem. ReV. 2001, 101, 4039-4070. Schmuck, C. Angew. Chem.,
Int. Ed. 2003, 42, 2448-2452. See also ref 6. For artificial oligohydrazide
foldamers which recognize saccharides, see: Hou, J.-L.; Shao, X.-B.; Chen,
G.-J.; Zhou, Y.-X.; Jiang, X.-K.; Li, Z.-T. J. Am. Chem. Soc. 2004, 126,
12386-12394.
(12) Oligo(m-ethynylpyridine) derivatives are known to form double- or triple-
stranded helicates by complexing with a Cu(I) or Ag(I) ion. See: Kawano,
T.; Nakanishi, M.; Kato, T.; Ueda, I. Chem. Lett. 2005, 34, 350-351.
Kawano, T.; Kato, T.; Du, C.-X.; Ueda, I. Tetrahedron Lett. 2002, 43,
6697-6700. Orita, A.; Nakano, T.; An, D. L.; Tanikawa, K.; Wakamatsu,
K.; Otera, J. J. Am. Chem. Soc. 2004, 126, 10389-10396. Orita, A.;
Nakano, T.; Yokoyama, T.; Babu, G.; Otera, J. Chem. Lett. 2004, 1298-
1299. Potts, K. T.; Horwitz, C. P.; Fessak, A.; Keshavarz-K, M.; Nash, K.
E.; Toscano, P. J. J. Am. Chem. Soc. 1993, 115, 10444-10445.
(13) Spivey, A. C.; Arseniyadis, S. Angew. Chem., Int. Ed. 2004, 43, 5436-
5441. Murugan, R.; Scriven, E. F. V. Aldrichimica Acta 2003, 36, 21-27.
Grondal, C. Synlett 2003, 1568-1569. Scriven, E. F. V. Chem. Soc. ReV.
1983, 12, 129-162. Ho¨fle, G.; Steglich, W.; Vorbru¨ggen, H. Angew. Chem.,
Int. Ed. Engl. 1978, 17, 569-583. Shinkai, S.; Tsuji, H.; Hara, Y.; Manabe,
O. Bull. Chem. Soc. Jpn. 1981, 54, 631-632.
(14) (a) Heemstra, J. M.; Moore, J. S. J. Org. Chem. 2004, 69, 9234-9237. (b)
Heemstra, J. M.; Moore, J. S. Chem. Commun. 2004, 1480-1481. (c)
Heemstra, J. M.; Moore, J. S. J. Am. Chem. Soc. 2004, 126, 1648-1649.
(d) Goto, H.; Heemstra, J. M.; Hill, D. J.; Moore, J. S. Org. Lett. 2004, 6,
889-892. (e) Heemstra, J. M.; Moore, J. S. Org. Lett. 2004, 6, 659-662.
(15) Neumann, U.; Vo¨gtle, F. Chem. Ber. 1989, 122, 589-591.
(16) Suzuki, H.; Kondo, A.; Inouye, M.; Ogawa, T. Synthesis 1986, 121-122.
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H+-Induced Saccharide Binding by a Basic Polymer
A R T I C L E S
Scheme 1. Preparation of Basic Poly(m-ethynylpyridine) 2
evaporation. The basicity of 2 was too high to be purified by
preparative GPC; therefore, a considerable amount of 2 was
adsorbed on the polystyrene gel. After several attempts, the
centrifugal procedure was chosen for the purification of 2. Thus,
the crude mixture was dispersed into AcOEt, and the resulting
brown precipitate was collected by centrifugation. By repeating
this washing, the impurities were removed, judging from the
1H NMR observation. The purified precipitate of 2 was applied
to the experiments described below.
Molecular Weight, Optical Properties, and Concentration
Dependency of Polymer 2. Vapor pressure osmometry (VPO)
analyses for CHCl3 solutions of 2 (ca. 1.2 × 10-2 to 2.0 ×
10-3 mmol kg-1, unit concentration) at 40 °C gave an Mn value
of 1.1 × 104, which corresponds to the weight of the 45-mer,
approximately.
The optical properties of 2 were studied by using CH2Cl2 or
CHCl3 as a solvent. The UV-vis spectrum of 2 in CH2Cl2
(Figure S1A in the Supporting Information; see also the red-
colored spectrum in Figure 3A) exhibited a broad absorption
band at λmax ) 291 nm with a shoulder around 310 nm. When
the concentration of 2 was varied from 1.0 × 10-3 to 5.0 ×
10-5 M (unit concentration), the shape of the spectrum did not
show any meaningful change, and the absorbance at 291 nm
obeyed Beer’s law (Figure S1B). To examine the VPO condi-
tions described above, absorbance at 363 nm was similarly
plotted versus the concentration of 2 ranging from 1.0 × 10-1
to 3.9 × 10-4 M (unit concentration) in CHCl3 at 40 °C, and
obedience to Beer’s law was also observed (Figure S1C). These
linearities mean that the contribution of intermolecular π-in-
teraction is negligible between two or more ground-state
Figure 3. (A) Change of the UV-vis spectrum of 2 on titration with TFA. Conditions: [2] ) 1.0 × 10-4 M (unit concentration), [TFA] ) 0.0 to 1.6 ×
10-4 M, CH2Cl2, 26 °C. Light path length 10 mm. (B) Change of the UV-vis spectrum of DMAP on titration with TFA. Conditions: [DMAP] ) 1.0 ×
10-4 M, [TFA] ) 0 to 1.8 × 10-4 M, CH2Cl2, 26 °C. Light path length 10 mm. Red-colored lines represent the spectra in the absence of TFA.
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A R T I C L E S
Abe et al.
Figure 4. CD spectra of the complex between 2 (1.0 mM, unit concentration) and hexoses (2.5 mM) in CH2Cl2 in the (A) absence or (B) presence of TFA
(0.5 mM) at 26 °C. Light path length 1 mm.
molecules of 2 up to 1.0 × 10-1 M. Thus, the relatively large
Mn value based on the VPO analyses would not be due to self-
association.
the Supporting Information. Although the shape of the UV-
vis spectrum changed in a manner similar to the case of 2, a
rather excess amount of TFA, about 1.0 × 10-3 M, was needed
to reach the end of this spectral change. This difference of
sensitivities for TFA obviously comes from the difference of
the Brønsted basicities of 1 and 2. These findings demonstrated
that 2 is more favorable to study the influences of protonation
on the natures of poly(m-ethynylpyridine)s, especially on the
association with saccharides by hydrogen bonds, which will be
disturbed in the presence of a large excess of acid. Moreover,
alkyl glycosides used as soluble guest saccharides in organic
media might be decomposed by the extra acid.
Fluorescence spectra of 2 in CH2Cl2 showed a structureless
emission band at Fmax ) 406 nm accompanied by a broad
weaker band around 580 nm on excitation at 315 nm (Figure
S2 in the Supporting Information). A similar broad emission at
a longer wavelength has also been observed in the case of 1.10
In the analyses of 2 under various concentrations, linear
relationships were observed below 2.5 × 10-5 M (unit con-
centration) between the concentration and the fluorescence
intensity at both 406 and 580 nm. Over that concentration, the
intensities at both of the wavelengths deviated from the linearity
probably because of intermolecular interaction between excited-
and gound-state molecules of 2. Considering this linearity, the
emissive band at longer wavelength would be attributed to
intramolecular excimer-like interaction, at least under a con-
centration below 2.5 × 10-5 M.
When 1H NMR spectra were measured for 2 (1.0 × 10-3 M)
with and without TFA (0, 0.5 × 10-3, and 1.0 × 10-3 M) in
CDCl3, it was found that protonation caused broadening and a
downfield shift of the signals in 2 (see the Supporting Informa-
tion). On the other hand, the changes of the chemical shifts
were relatively small during further addition of TFA (up to 2.0
× 10-3 M), probably because the protonation on DMAP-like
units in 2 was almost saturated by the addition of 1.0 × 10-3
Spectral Changes of Polymer 2 on Addition of TFA. To
study the characteristics of 2 as a strong polybase, the polymer
was titrated with trifluoroacetic acid (TFA). As shown in Figure
3A, the UV-vis spectrum of 2 (1.0 × 10-4 M in CH2Cl2, unit
concentration) significantly changed shape with the addition of
TFA, and this spectral change almost ceased when the amount
of TFA reached a small excess (1.5 × 10-4 M) relative to the
amount of the pyridine units on 2. During the titration, an
isosbestic point was observed at 315 nm. A titration experiment
was also carried out for DMAP itself (Figure 3B), and the UV-
vis spectrum changed in a manner similar to that for 2. In Figure
3B, the existence of an isosbestic point at 264 nm means the
participation of just two kinds of absorptive species during the
titration, DMAP and its conjugate acid. However, for polybase
2, the observation of an isosbestic point in Figure 3A seems
strange because there are many basic pyridine units in 2 and
uncountable kinds of possible conjugate acids. It can be
rationalized by the postulation that the π-conjugated systems
in 2 and its conjugate acids are not linked smoothly through
the “meta”-connection on the pyridine and pyridinium rings,
so that each of the rings apparently behaves as an isolated
chromophore to show the isosbestic point during the titration.
A titration experiment with TFA was also examined for 1
(1.0 × 10-4 M, unit concentration) as shown in Figure S3 in
1
M TFA. The H NMR data also illustrated that the basicity of
2 is high enough for 2 to be a H+-responsive poly(ethynylpy-
ridine).
Association of Polymer 2 with Hexoses and Induced CD.
As mentioned in our previous paper,10 the butoxy-derived
polymer 1 can associate with various saccharides to form helical
complexes which show characteristic ICDs. Also for the cases
of the basic polymer 2 (1.0 mM, unit concentration) in CH2Cl2
(Figure 4A), similar ICDs appeared when the polymer was
treated with four kinds of hexoses (2.5 mM; see Chart 1), octyl
â-D-glucopyranoside (â-D-Glc), octyl â-D-fructopyranoside (â-
D-Fru), octyl â-D-mannopyranoside (â-D-Man), and octyl â-D-
galactopyranoside (â-D-Gal). Under the presence of 0.5 mM
TFA (the reason for this amount will be discussed later), the
ICDs were somewhat enhanced (Figure 4B). These ICDs are
similar to those for 110 in that a main characteristic ICD band
was observed around 300-350 nm due to the helical complex
between the polymer and the guest hexose. Regardless of the
presence or absence of TFA, 2 gave the main ICD band of the
same sign as that for 110 by the complexation with the same
hexose except the case with â-D-Gal. The similarity of ICDs
between 1 and 2 would reflect the similarity of the helical
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H+-Induced Saccharide Binding by a Basic Polymer
A R T I C L E S
Chart 1. Guest Hexoses
2.0 mM TFA. Considering these results, when the unit
concentration of 2 was 1.0 mM, we chose 0.5 mM as the amount
of the additive TFA in the following experiments using hexoses
as a titrant (see also the above and Figure 4B). This amount
corresponds to a 0.5 molar equivalence for the pyridine rings
on 2, and approximately half-protonation is expected.
The butoxy-derived polymer 1 (1.0 mM, unit concentration)
in CH2Cl2 was also subjected to the titration with TFA in the
presence of â-D-Glc (2.5 mM). However, the metamorphosis
of the ICD was too complicated to make quantitative analyses,
and the ICD did not disappear until 13 mM TFA was added
(Figure S5 in the Supporting Information). In comparison with
the case of 2, this confusing observation for the ICD of 1 with
â-D-Glc would be due to the weaker basicity of 1, which was
once found by UV-vis observation as shown in Figure S3.
Thus, the higher basicity of 2 was corroborated to be favorable
for quantitative studies on the influence of protonation on its
higher order structure and saccharide recognition ability.
structures of the complexes. In the presence of TFA, it seems
that a new shoulder of ICD appeared over 350 nm in the cases
of â-D-Glc and â-D-Gal, but detailed reasons are obscured. It
might suggest the contribution of two or more styles of helical
structures.17
Titration with TFA on the Complex of Polymer 2 and
Hexoses. As described above, addition of TFA affects the
various optical natures of the basic polymer 2 and its complexes
with hexoses. Quantitative studies were performed to shed light
on the relation between the ICDs and the equivalency of added
TFA. When TFA was titrated into a mixture of 2 (1.0 mM,
unit concentration) and â-D-Glc (2.5 mM) in CH2Cl2, interesting
transitions were observed in the CD spectra (Figure 5). At the
initial stage of the titration, the ICD was gradually enhanced
until 0.3 mM TFA was added. While a further amount of TFA
was added up to 0.6 mM, only small structural changes were
observed in the ICD. When the amount of TFA increased to
2.0 mM, the ICD began to diminish and finally disappeared.
The highest ellipticity was observed at 326 nm in the presence
of 0.3 mM TFA. For â-D-Fru, â-D-Man, and â-D-Gal as a guest
hexose, the corresponding titration experiments with TFA
exhibited similar behaviors of the ICD as shown in Figure S4
in the Supporting Information. Also in those cases, the highest
ellipticities were observed when 0.3-0.5 mM TFA was added,
and these ICDs were finally quenched by the addition of 1.3-
Binding Constants of Polymer 2 with Hexoses and the
Additive Effects of TFA. To precisely evaluate the recognition
abilities and effects of protonation, titrations for 2 with guest
hexoses were performed. At first, the changes in the CD spectra
of 2 in CH2Cl2 were pursued during the titration with â-D-Glc
(Figure 6A), â-D-Fru (Figure S6A in the Supporting Informa-
tion), or â-D-Man (Figure S7A in the Supporting Information)
in the absence of TFA. In each of these titrations, growth of
the ICDs was observed according to the addition of the hexose.
The titration curves were drawn for the ellipticity at the
corresponding wavelength of maximal ICDs as a function of
the concentration of the hexoses added (Figures 6C, S6C, and
S7C). From those curves, the formal binding constants were
obtained as shown in Table 1 by iterative curve-fitting analyses
based on the assumptions of 1:1 complexation between 2 and
the hexoses18 and a molecular weight of 2 of 1.1 × 104 (Mn
from VPO analyses; see above). Among the three hexoses, â-D-
Glc, â-D-Fru, and â-D-Man, the association of 2 with â-D-
Man gave the strongest binding constant, Ka ) (3.3 ( 0.5) ×
103 M-1. Since the deviations between the experimental and
theoretical titration curves were small, the complexation in a
1:1 molar ratio of 2 and the guest hexose seems to be likely.
Unfortunately, the titration with â-D-Gal gave only an insuf-
ficient result because of the weakness of the ICD as shown in
Figure 4A.
As mentioned above, addition of TFA remarkably affected
the association between 2 and hexose. In the presence of 0.5
mM TFA, the changes in the CD spectra of 2 (1.0 mM, unit
concentration) were pursued during the titration with â-D-Glc
(Figure 6B), â-D-Fru (Figure S6B), â-D-Man (Figure S7B), or
â-D-Gal (Figure S8A in the Supporting Information) in CH2Cl2.
The enhancement of the intensities of the ICDs by the presence
of TFA was also observed in these quantitative experiments.
The formal binding constants were obtained by curve-fitting
analyses of the titration curves under the same assumptions as
in the cases for the absence of TFA (Figures 6C, S6D, S7C,
and S8B) and are summarized in Table 1. The formal binding
constants of 2 with â-D-Glc, â-D-Fru, and â-D-Man in the
presence of TFA are approximately 2-200 times as large as
Figure 5. Change of the CD spectrum of the complex between 2 and â-D-
Glc on titration with TFA. Conditions: [2] ) 1.0 mM (unit concentration),
[â-D-Glc] ) 2.5 mM, [TFA] ) 0-0.3 mM (black), 0.4-0.6 mM (red),
0.7-2.0 mM (blue), CH2Cl2, 26 °C. Light path length 1 mm.
(17) Stone, M. T.; Fox, J. M.; Moore, J. S. Org. Lett. 2004, 6, 3317-3320.
(18) For 1 (n ) 24), the Job plot analysis indicated the 1:1 complexation with
â-D-Glc.10
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A R T I C L E S
Abe et al.
Figure 6. Change of the CD spectrum on the titration of 2 (1.0 mM, unit concentration) with â-D-Glc in CH2Cl2 at 26 °C in the (A) absence or (B) presence
of TFA (0.5 mM). Light path length 1 mm. (C) Titration curves in the absence (circles; 315 nm was observed) or presence (squares; 324 nm was observed)
of TFA. The lines are theoretical curves given by the iterative curve-fitting analyses assuming 1:1 complexation between 2 and â-D-Glc and a molecular
weight of 2 of 1.1 × 104.
Table 1. Formal Binding Constants and -∆G Values for the
Association of Polymer 2 with Hexosesa
equivalence versus the pyridine rings), a helical structure will
spontaneously form. The resulting helix will be favorable for
encompassing guest hexoses by multipoint hydrogen bondings.
On the other hand, a sequence of the transoid conformation
yields a zigzag structure, and its multipoint interaction with
hexoses may suffer energetic loss resulting from the transforma-
tion of the transoid conformation into the cisoid conformation.
Dipole-induced helical structures of heterocyclic strands were
also reported by Lehn et al. for pyridine-pyrimidine and
pyridine-pyridazine oligomers.5b,19
[TFA]
(mM)
guest
binding Constant
−∆G
(kJ mol-
)
1
1
hexose
Kab (M-
)
none
0.5
none
0.5
none
0.5
0.5
â-D-Glc
â-D-Glc
â-D-Fru
â-D-Fru
â-D-Man
â-D-Man
â-D-Gal
(1.7 ( 0.3) × 102
(1.8 ( 0.1) × 103
(1.0 ( 0.1) × 102
(2.0 ( 0.5) × 104
(3.3 ( 0.5) × 103
(7.2 ( 1.3) × 103
(2.2 ( 0.4) × 103
13
19
11
25
20
22
19
a Conditions: [2] ) 1.0 mM (unit concentration), [TFA] ) 0 or 0.5 mM,
CH2Cl2, 26 °C. See also the Figure 6 caption. b Obtained by curve-fitting
analyses on the titration curves shown in Figures 6 and S6-S8.
Apart from the local dipole moment, the hydrogen-bonding
natures of pyridine and pyridinium moieties are different in that
the former is a neutral hydrogen acceptor and the latter is a
cationic donor. The electrostatic interaction of a cationic pyrid-
inium proton with a hexose OH will be stronger than that of a
neutral pyridine nitrogen with a hexose OH. The half-protonated
helical complex between 2 and a hexose could be further stabil-
ized by enforced stacking interactions as represented in Scheme
2 (right): the protonated pyridinium ring will interact with a
those in the absence of TFA; thus, the addition of a proper
amount of acid was found to improve the recognition abilities
of 2 for saccharides.
Discussion
Equivalency of Added TFA. Scheme 2 shows one explana-
tion for the observed additive effects of TFA on the association
of 2 with saccharides. The direction of the local dipole moment
on the pyridinium ring in 2 is postulated to be opposite that of
the local dipole moment on the pyridine ring (see below). Taking
the dipolar repulsion into account, the transoid conformation
will be preferred in pyridine-pyridine and pyridinium-pyri-
dinium pairs and the cisoid one will be preferred in the
pyridinium-pyridine pair (Scheme 2, left). When the cisoid
conformation lines up in 2 by the addition of TFA (0.5 molar
(19) For pyridine-pyrimidine oligomers, see: Barboiu, M.; Vaughan, G.;
Kyritsakas, N.; Lehn, J.-M. Chem.sEur. J. 2003, 9, 763-769. Gardinier,
K. M.; Khoury, R. G.; Lehn, J.-M. Chem.sEur. J. 2000, 6, 4124-4131.
Ohkita, M.; Lehn, J.-M.; Baum, G.; Fenske, D. Heterocycles 2000, 52,
103-109. Ohkita, M.; Lehn, J.-M.; Baum, G.; Fenske, D. Chem.sEur. J.
1999, 5, 3471-3481. Bassani, D. M.; Lehn, J.-M.; Baum, G.; Fenske, D.
Angew. Chem., Int. Ed. Engl. 1997, 36, 1845-1847. Bassani, D. M.; Lehn,
J.-M. Bull. Chem. Soc. Fr. 1997, 134, 897-906. Hanan, G. S.; Lehn, J.-
M.; Kyritsakas, N.; Fischer, J. J. Chem. Soc., Chem. Commun. 1995, 765-
766. For pyridine-pyridazine oligomers: Cuccia, L. A.; Ruiz, E.; Lehn,
J.-M.; Homo, J.-C.; Schmutz, M. Chem.sEur. J. 2002, 8, 3448-3457.
Cuccia, L. A.; Lehn, J.-M.; Homo, J.-C.; Schmutz, M. Angew. Chem., Int.
Ed. 2000, 39, 233-237.
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16194 J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005

H+-Induced Saccharide Binding by a Basic Polymer
A R T I C L E S
Scheme 2. Influences of Protonation on the Local Conformation (Left) and the Helical Structure (Right) of 2
Chart 2. Calculated Dipole Moments for 12 and Its Conjugate
Acids 12-H+-ring and 12-H+-amino and the Relative Stability
pyridine ring at an interval of one pitch by (cationic π)-π stack-
ing interaction,14,20,21 which is more effective than (neutral π)-π
stacking expected in nonprotonated 2. When most of the pyrid-
ine rings in 2 come to be protonated, electrostatic repulsion be-
tween the cationic centers may destabilize the helix. These hy-
potheses explain the observation that half-protonation of 2 is
favorable for hexose recognition accompanying increased ICDs.
Computational Analyses for the Conformation of Polymer
2. The relative stabilities were computationally evaluated for
Comparing the Two Acids
(20) For examples of π-interaction of pyridinium moieties, see: Hunter, C. A.;
Low, C. M. R.; Vinter, J. G.; Zonta, C. J. Am. Chem. Soc. 2003, 125,
9936-9937. Priem, G.; Pelotier, B.; Macdonald, S. J. F.; Anson, M. S.;
Campbell, I. B. J. Org. Chem. 2003, 68, 3844-3848. Hunter, C. A.; Low,
C. M. R.; Rotger, C.; Vinter, J. G.; Zonta, C. Proc. Natl. Acad. U.S.A.,
2002, 99, 4873-4876. Yamada, S.; Morita, C. J. Am. Chem. Soc. 2002,
124, 8184-8185. Wisner, J. A.; Beer, P. D.; Drew, M. G. B. Angew. Chem.,
Int. Ed. 2001, 40, 3606-3609. Kawabata, T.; Nagato, M.; Takasu, K.; Fuji,
K. J. Am. Chem. Soc. 1997, 119, 3169-3170. Kearney, P. C.; Mizoue, L.
S.; Kumpf, R. A.; Forman, J. E.; McCurdy, A.; Dougherty, D. A. J. Am.
Chem. Soc. 1993, 115, 9907-9919. McCurdy, A.; Jimenez, L.; Stauffer,
D. A.; Dougherty, D. A. J. Am. Chem. Soc. 1992, 114, 10314-10321.
Philp, D.; Slawin, A. M. Z.; Spencer, N.; Stoddart, J. F.; Williams, D. J. J.
Chem. Soc., Chem. Commun. 1991, 1584-1586. Ortholand, J.-Y.; Slawin,
A. M. Z.; Spencer, N.; Stoddart, J. F.; Williams, D. J. Angew. Chem., Int.
Ed. Engl. 1989, 28, 1394-1395. Petti, M. A.; Shepodd, T. J.; Barrans, R.
E., Jr.; Dougherty, D. A. J. Am. Chem. Soc. 1988, 110, 6825-6840.
(21) For a review on cation-π interaction, see: Ma, J. C.; Dougherty, D. A.
Chem. ReV. 1997, 97, 1303-1324. See also: Kla¨rner, F.-G.; Kahlert, B.
Acc. Chem. Res. 2003, 36, 919-932.
cisoid and transoid conformations in the local structure in 2
and its conjugate acids. In advance, to select the proper structural
formula of the conjugate acid, the stabilities were calculated
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A R T I C L E S
Abe et al.
Chart 3. Comparison of the Calculated Relative Stabilities for the
Transoid (Left) and Cisoid (Right) Conformers of Dimer 13 and Its
Conjugate Acids 13-1H+ and 13-2H+
structure. Both the transoid and cisoid conformers of 13 and its
monoprotonated 13-1H+ and diprotonated 13-2H+ were submit-
ted to DFT calculations using 6-31(d) basis sets for geometrical
optimization and zero-point energy correction and 6-311+(2d,p)
basis sets for single point analyses. The results are summarized
in Chart 3. The calculation indicated that the cisoid conformer
13-1H+-cis is more stable than the transoid conformer 13-1H+-
trans in the case of the pyridinium-pyridine dimer 13-1H+
whereas the transoid conformers are more stable for the
pyridine-pyridine 13 and the pyridinium-pyridinium 13-2H+.
These data support the discussion described above, and one may
imagine that the preferable conformation of a pyridine-
acetylene-pyridine repeat could be controlled by the degree
of protonation. For polymer 2, alternate half-protonation on its
pyridine rings would stabilize the cisoid conformation at the
local structure to help the formation of a helical higher order
entity.
Conclusion
By repeating the Sonogashira reaction, m-ethynylpyridine
polymer 2 bearing dialkylamino groups was synthesized.
Polymer 2 has high basicity, being comparable to that of DMAP,
and associates with hexoses to form helical complexes with
characteristic ICDs. These ICDs and the binding constants were
enhanced by the addition of trifluoroacetic acid of ca. 0.5 molar
equivalence versus the pyridine rings on 2, and further addition
of the acid suppressed the ICDs. The half-protonation on 2
would stabilize the helical conformation probably with the aid
of the preferable local cisoid conformation and the enforced
intramolecular pyridinium-pyridine π-stacking. This preference
for the cisoid conformation was supported by DFT calculations.
The DMAP-like moieties in 2 are potentially expected to serve
as catalysis centers, and that in combination with the recognition
abilities will allow realization of artificial enzymes of a new
type, a study now under way.
for two possible isomeric conjugate acids of a DMAP-like
molecule, 2,6-diethynyl-4-(N,N-dimethylamino)pyridine (12)
(Chart 2). One conjugate acid is 12-H+-ring protonated at 1-N
in the ring, and the other is 12-H+-amino protonated at the
dimethylamino nitrogen. The DFT calculations indicated that
12-H+-ring is much more stable than 12-H+-amino by 120 kJ
mol-1. Therefore, the protonation at the dialkylamino groups
in 2 is regarded as a negligible process in the following
computational studies. Furthermore, the calculation confirmed
that the dipole moment on 12-H+-ring is reversed as compared
to that on 12 (see above).
Acknowledgment. We gratefully acknowledge the Takeda
Science Foundation for financial support.
Supporting Information Available: The entire Experimental
Section, Figures S1-S8, 1H NMR spectra for compounds 2, 5,
7, 8, 9, and 11, and the results of computational analyses for
12, 13, and their conjugate acids shown in Charts 2 and 3. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Next, the stabilities for transoid and cisoid conformations in
2 were computationally assessed by using bis[4-(N,N-dimethyl-
amino)-6-ethynyl-2-pyridyl]ethyne (13) as a model dimeric
JA054134U
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16196 J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005