Social self-sorting: Alternating supramolecular oligomer consisting of isomers

Article abstract of DOI:10.1021/ja903988c

The isomers of cinnamoyl α-CDs (2-CiO-α-CD and 3-CiO-α-CD) organize by themselves to give different types of supramolecular complexes in aqueous solutions. 2-CiO-α-CD was found to form a double-threaded dimer, which was characterized by single crystal X-r

Full text of DOI:10.1021/ja903988c

Published on Web 08/06/2009
Social Self-Sorting: Alternating Supramolecular Oligomer
Consisting of Isomers
Naoki Tomimasu, Akira Kanaya, Yoshinori Takashima, Hiroyasu Yamaguchi, and
Akira Harada*
Department of Macromolecular Science, Graduate School of Science, Osaka UniVersity,
Toyonaka, Osaka 560-0043, Japan
Received May 21, 2009; E-mail: harada@chem.sci.osaka-u.ac.jp
Abstract: The isomers of cinnamoyl R-CDs (2-CiO-R-CD and 3-CiO-R-CD) organize by themselves to give
different types of supramolecular complexes in aqueous solutions. 2-CiO-R-CD was found to form a double-
threaded dimer, which was characterized by single crystal X-ray analysis. The molecular sizes of
supramolecular complexes consisting of CiO-R-CDs were estimated by pulsed field gradient spin-echo
NMR, which gives the diffusion coefficient (D). D of 2-CiO-R-CD was found to be 2.3 × 10-10 m2/s at
concentrations over 10 mM and D was saturated. It should be noted that the hydrodynamic radius derived
from D of 2-CiO-R-CD was found to be 9.6 Å, which closely matched the result from the single crystal
X-ray analysis. D of 3-CiO-R-CD, however, was smaller than that of 2-CiO-R-CD at concentrations over 32
mM, indicating that 3-CiO-R-CD formed a supramolecular oligomer. Two-dimensional (2D)-ROESY spectra
of the mixture of 2-CiO-R-CD and 3-CiO-R-CD did not show a correlation between the same species; rather,
correlation peaks between 2-CiO-R-CD and 3-CiO-R-CD were observed. D’s of a mixture of 2-CiO-R-CD
and 3-CiO-R-CD were found to be larger than those of 2-CiO-R-CD and 3-CiO-R-CD, respectively, indicating
that the mixture of 2-CiO-R-CD and 3-CiO-R-CD formed an alternative supramolecular oligomer and not a
self- or random- supramolecular complex. CDs recognize a guest molecule and the substitutional position
of a guest molecule on CD, suggesting that CDs have social self-sorting capability.
coordination bonds,8-17 solvophobic effects,18,19 dynamic co-
valent bonds.20-22 In recent years, supramolecular assemblies23-26
Introduction
Biological materials, such as microtubules1 and hemoglobin,2
are composed of two kinds of building blocks. These molecules
exhibit a high affinity for one another and, hence, are referred
to as social self-sorting.3 Notable self-sorting systems have been
constructed on the basis of the formation of hydrogen bonds,7
and supramolecular polymers27-29 from enantiomers were
designed by researchers. In many cases, the mixture of isomers
led to formation of homosupramolecular assemblies and su-
pramolecular polymers, indicating that in the process of narcis-
sistic self-sorting,30 molecules that show a high affinity for
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Figure 1. Structures of 2-CiO-R-CD and 3-CiO-R-CD.
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it extremely difficult to form an alternating supramolecular
polymer using a mixture of isomers. In our study, we have
prepared isomers of cinnamoyl R-CD. We observed the forma-
tion of homosupramolecular complexes by each isomer, and
the formation of an alternating supramolecular oligomer by the
mixture of isomers in such a way that represents a social self-
sorting system.
Results and Discussion
Preparation of Cinnamoyl-r-CDs (2-CiO-r-CD and 3-CiO-
r-CD). Cinnamoyl R-CDs (CiO-R-CD) on the secondary
hydroxyl group were prepared by the reaction of m-nitrophenyl
cinnamate with R-CD in a basic aqueous solution (pH ) 9).
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Figure 3. 600 MHz ROESY spectrum of 2-CiO-R-CD in D2O (24 mM) at 20 °C (mixing time ) 100 s).
The reaction involved an ester exchange by way of inclusion
of the ester in the CD cavity. The product was found to be a
mixture of 2-cinnamoyl R-CD (2-CiO-R-CD) and 3-cinnamoyl
R-CD (3-CiO-R-CD) (Figure 1). Both isomers were purified by
reversed phase HPLC.
Formation of Homosupramolecular Oligomer from 2-CiO-
1
r-CD or 3-CiO-r-CD. The H NMR spectra of 2-CiO-R-CD
showed that the peaks of 2-CiO-R-CD shifted downfield with
an increase in the concentration (Figure 2), suggesting the
intermolecular supramolecular complex in aqueous solutions.
The 2D ROESY spectrum of 2-CiO-R-CD showed correlation
peaks between C(3)-H in the inner protons of R-CD and
cinnamoyl protons in D2O, whereas the C(5)-H of the inner
protons of R-CD did not show correlation peaks with protons
of the cinnamoyl group (Figure 3). These results indicate that
the cinnamoyl group is included from the secondary hydroxyl
group side (wider rim). We obtained a single crystal of 2-CiO-
R-CD suitable for X-ray crystallographic analysis. The X-ray
crystallographic analysis showed formation of a double-threaded
dimer (Figure 4). This result is in an excellent agreement with
the 2D ROESY experiments.
Figure 4. Crystal structure of 2-CiO-R-CD. Carbon and oxygen of R-CD
are shown in gray and red, respectively. Carbon and oxygen of cinnamoyl
groups are shown in blue and light green, respectively.
was found to be 9.6 Å, which is in agreement with that
calculated from single crystal X-ray analysis. These results
indicate that 2-CiO-R-CD forms a double threaded dimer over
10 mM. It should be noted that the D’s of 3-CiO-R-CD showed
a concentration dependence, but D’s of 2-CiO-R-CD did not.
These results indicate that 3-CiO-R-CD forms a supramolecular
oligomer at concentrations greater than 32 mM in aqueous
solutions.
The correlation peaks of the 2D ROESY spectrum of 3-CiO-
R-CD were similar to that of 2-CiO-R-CD (Figure 5), suggesting
that the cinnamoyl part is included from the wider rim of the
R-CD.
Formation of Heterosupramolecular Oligomer from the
Mixture of 2-CiO-r-CD or 3-CiO-r-CD. Although 2-CiO-R-
CD is sparingly soluble in D2O, when 2-CiO-R-CD was mixed
with 3-CiO-R-CD in water, 2-CiO-R-CD was easily solubilized
and gave a clear solution. This result suggests that 2-CiO-R-
CD formed complexes with 3-CiO-R-CD to give soluble
supramolecules. The 2D ROESY spectrum of the mixture of
2-CiO-R-CD and 3-CiO-R-CD clearly showed that olefin and
aromatic protons of 3-CiO-R-CD correlated with inner protons
To compare the molecular size of supramolecular complexes
incorporating CiO-R-CDs, diffusion coefficients (D’s) of the
supramolecular complexes were determined by the pulse field
gradient spin-echo (PFG) NMR technique. Although the D of
2-CiO-R-CD shows an unvarying value over 10 mM and
reached 2.3 × 10-10 m2/s at 32 mM, the D of 3-CiO-R-CD
decreased monotonously with increasing concentration, then
reached 1.7 × 10-10 m2/s at 64 mM. The hydrodynamic radius
of 2-CiO-R-CD calculated by D’s at concentrations over 10 mM
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Figure 5. 600 MHz ROESY spectrum of 3-CiO-R-CD in D O (24 mM)
2
at 20 °C (mixing time ) 200 ms).
(C(3)-H) of 2-CiO-R-CD and vice versa (Figure 6). However,
cinnamoyl protons of 3-CiO-R-CD and 2-CiO-R-CD, respec-
tively, did not correlate with their own inner protons (C(3)-H).
It should be noted that although correlation peaks between
2-CiO-R-CD and 3-CiO-R-CD were observed, correlation peaks
between the same species were not. These results indicate that
a mixture of 2-CiO-R-CD and 3-CiO-R-CD forms an alternating
supramolecular complex and not a self-supramolecular complex.
D’s of a 1:1 mixture of 2-CiO-R-CD and 3-CiO-R-CD were
calculated from the proton intensity decay of 2-CiO-R-CD and
3-CiO-R-CD, respectively. Figure 7 shows the D’s of the
mixture of 2-CiO-R-CD and 3-CiO-R-CD calculated from the
2a proton intensity of 2-CiO-R-CD. Even though calculated by
a 2a proton, the D’s of the mixture showed much lower values
than those of individual isomers, especially those of 2-CiO-R-
CD. These results indicate that the mixture of 2-CiO-R-CD and
3-CiO-R-CD did not form individual self-supramolecular com-
plexes, but rather cooperatively formed heterosupramolecular
oligomers.
Figure 6. Two-dimensional ROESY NMR spectra of the mixture of 2-CiO-
R-CD and 3-CiO-R-CD in D2O (32 mM) at 20 °C. The spectrum shows
the partial area between inner protons of CiO-R-CDs and olefin protons of
the cinnamoyl group. The correlation peaks were observed in the area of
solid circles, whereas in the dash circles were not observed. Full spectrum
is shown in Figure S5 in Supporting Information.
The turbo ion spray TOF mass spectrum provides direct
evidence of the formation of a supramolecular oligomer. The
mass spectrum of the 1:1 mixture of 2-CiO-R-CD and 3-CiO-
R-CD showed oligomeric peaks assigned as sodium cation
adducts in aqueous solutions (Figure 8). The spectrum showed
that the peaks appeared at intervals of av. 1102.4 Da, which
corresponded to a CiO-R-CD monomer unit. These results
indicate that the mixture of 2-CiO-R-CD and 3-CiO-R-CD
formed a supramolecular oligomer in an aqueous solution.
Association Constants for Formation of Homo- and Hetero-
supramolecular Oligomers. Formations of supramolecular com-
plexes consisting of CiO-R-CDs are summarized in Figure 9.
The association constant (K) of 2-CiO-R-CD for dimerization
is found to be K2-CiO ) 4.1 ( 0.55 × 102 M-1, and that of 3-CiO-
R-CD is K3-CiO ) 1.0 ( 0.17 × 102 M-1. Single crystal X-ray
analysis of 2-CiO-R-CD indicated the formation of a double
Figure 7. Plots of diffusion coefficients (D’s) of 2-CiO-R-CD (red ]),
3-CiO-R-CD (black O) and the mixture of 2-CiO-R-CD and 3-CiO-R-CD
(red [). D’s of 2-CiO-R-CD, 3-CiO-R-CD, and the mixture of 2-CiO-R-
CD and 3-CiO-R-CD were calculated by using a 2a proton, a 3a proton,
and a 2a proton, respectively, in D2O at 30 °C.
threaded dimer. The complementary dimerization of 2-CiO-R-
CD led K2-CiO to be larger than K3-CiO. In contrast, KHetero between
2-CiO-R-CD and 3-CiO-R-CD was found to be KHetero-1 ) 2.1
( 0.33 × 103 M-1 (dimerization) and KHetero-2 )1.5 ( 0.23 ×
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nmax ) 8 at 32 mM and nmax ) 11 at 64 mM.43-49 On the basis
of these studies of the diffusion coefficients, the mass spec-
trum,50 and the association constant, KHetero, the mixture of
2-CiO-R-CD and 3-CiO-R-CD forms alternating supramolecular
oligomer.
Conclusion
2-CiO-R-CD and 3-CiO-R-CD organized by themselves to
give a double threaded dimer and a supramolecular oligomer
at high concentrations, respectively. The addition of 2-CiO-R-
CD to an aqueous solution of 3-CiO-R-CD triggered the
formation of heterosupramolecular complexes by a social self-
sorting system. It should be noted that CiO-R-CDs recognized
the difference in the substitution position on a glucopyranose
unit. Moreover, the mixture of 2-CiO-R-CD and 3-CiO-R-CD
formed alternating supramolecular oligomer over 32 mM and
not self- or random-supramolecular complexes. We conclude
that it is important not only that hydrophobic interactions but
also π-π stacking and hydrogen-bonding interactions function
cooperatively to create social-self-sorting system.
Figure 8. Turbo-ion spray TOF mass spectrum of the 1:1 mixture of 2-CiO-
R-CD (8.0 mM) and 3-CiO-R-CD (8.0 mM) in an aqueous solution (total
concentration;16 mM).
Acknowledgment. We thank Mr. S. Adachi and Dr. A. Hashid-
zume in the Graduate School of Science, Osaka University, for help
with the NMR experiments and for fruitful discussions. We are grateful
to M. Yamazaki (Rigaku Corporation, Japan) for measuring the single
X-ray analysis. This work has been supported by “Core Research for
Evolutional Science and Technology” program of Japan Science and
Technology Agency, Japan.
Supporting Information Available: Selected NMR data (1D
NMR and ROESY spectra), determination of the association
constant and the diffusion coefficient; movie file of the crystal
structure of 2-CiO-R-CD; CIF file. This material is available
free of charge via the Internet at http://pubs.acs.org.
JA903988C
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Figure 9. Illustration of the formation of supramolecular complexes using
CiO-R-CDs.
103 M-1 (trimerization), which are much higher than those of
individual homocomplexes. These results indicate that the
formation of heterosupramolecular complexes between 2-CiO-
R-CD and 3-CiO-R-CD have a priority over that of the
homosupramolecular complexes in diluted conditions. Previ-
ously, Meijer has reported that the maximum degree of
polymerzation, nmax ) (KC)1/2, in which K is the association
constant and C is the concentration. When we follow this
equation, the mixture of 2-CiO-R-CD and 3-CiO-R-CD formed
(45) Oosawa, F.; Kasai, M. J. Mol. Biol. 1962, 4, 10.
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(50) The species of supramolecular oligomer up to the 20 mer were
observed in the mass spectrum in Figure 7 because the supramolecular
oligomer has a distribution and the desolvation process of ionizations
leads to formation of larger supramolecular oligomeric species by an
increase in the concentration.
9
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