Chiral recognition in cucurbituril cavities

Article abstract of DOI:10.1021/ja063323p

For the first time, achiral cucurbiturils (CBs) were endowed with significant enantiomeric and distereomeric discrimination by incorporating a strong chiral binder. Calorimetric, nuclear magnetic, light-scattering, and mass spectral studies revealed that (S)-2-methylbutylamine (as a strong binder) can be discriminated by two enantiomeric supramolecular hosts, composed of CB[6] and (R)- or (S)-2-methylpiperazine, with an unprecedented 95% enantioselectivity in aqueous NaCl solution. This is the highest enantioselectivity ever reported for a supramolecular system derived from an achiral host. Similarly, CB[7], with a larger cavity, exhibited diastereoselectivities up to 8 times higher for diastereomeric dipeptides, as demonstrated for L-Phe-L-Leu-NH₃ ⁺ versus L-Phe-D-Leu-NH₃⁺.

Full text of DOI:10.1021/ja063323p

Published on Web 10/27/2006
Chiral Recognition in Cucurbituril Cavities
Mikhail V. Rekharsky,† Hatsuo Yamamura,‡ Chika Inoue,‡ Masao Kawai,‡
Issey Osaka,# Ryuichi Arakawa,# Kouhei Shiba, Akihiro Sato,| Young Ho Ko,§
Narayanan Selvapalam,§ Kimoon Kim,*,§ and Yoshihisa Inoue*,†
Contribution from the Entropy Control Project, ICORP, JST, 4-6-3 Kamishinden, Toyonaka
560-0085, Japan, Graduate School of Technology, Nagoya Institute of Technology, Gokiso-cho,
Showa-ku, Nagoya 466-8555, Japan, Department of Applied Chemistry, Kansai UniVersity,
Yamatecho, Suita 564-0073, Japan, Scientific Instrumentation DiVision, Sysmex Company, 4-4-4
Takatsuka-dai, Nishi-ku, Kobe 651-2271, Japan, JASCO International Company, 1-11-10
Myojin-cho, Hachioji 192-0046, Japan, and National CreatiVe Research InitiatiVe Center for
Smart Supramolecules and Department of Chemistry, DiVision of Molecular and Life Sciences,
Pohang UniVersity of Science and Technology, San 31 Hyojadong, Pohang 790-784, Korea
Received May 12, 2006; E-mail: inoue@chem.eng.osaka-u.ac.jp
Abstract: For the first time, achiral cucurbiturils (CBs) were endowed with significant enantiomeric and
distereomeric discrimination by incorporating a strong chiral binder. Calorimetric, nuclear magnetic, light-
scattering, and mass spectral studies revealed that (S)-2-methylbutylamine (as a strong binder) can be
discriminated by two enantiomeric supramolecular hosts, composed of CB[6] and (R)- or (S)-2-
methylpiperazine, with an unprecedented 95% enantioselectivity in aqueous NaCl solution. This is the highest
enantioselectivity ever reported for a supramolecular system derived from an achiral host. Similarly, CB[7],
with a larger cavity, exhibited diastereoselectivities up to 8 times higher for diastereomeric dipeptides, as
+
+
demonstrated for L-Phe-L-Leu-NH3 versus L-Phe-D-Leu-NH3
.
Chart 1. Chemical Structure of the Cucurbit[n]uril Macrocycle
Introduction
The pumpkin-shaped macrocycle cucurbit[6]uril (CB[6])1 was
first synthesized in 1905,2 and its chemical structure (Chart 1)
was established in 1981.3 Nevertheless, fundamental and ap-
plication studies on CB[6] have been rather limited until
recently, mostly due to its low solubility in both water and
common organic solvents. The use of CB[6], however, under-
went an explosion in various fields of science after the discovery
of the fact that it becomes readily soluble in aqueous solutions
that contain alkali or other metal cations, as well as organic
ammonium ions.4 It was subsequently found that metal cations
(M+) coordinate to both portals of the barrel-shaped CB[6] to
give a dicationic complex, [CB[6]‚2M]2+. We also reported the
syntheses of smaller and larger CB homologues, cucurbit[n]-
uril (CB[n], n ) 5-8).5 Since then, a wide variety of
supramolecular architectures and devices based on CB macro-
cycles have been designed by us as well as by other groups,6,7
as can be seen in a recent review.8
Challenges associated with the chirality induction in an achiral
host via noncovalent interactions with chiral ligands/auxiliaries
were previously delineated by Aoyama et al.9a and discussed
in further detail in our recent account.9b We now demonstrate,
for the first time, that the intrinsically achiral CB[n] macrocycles,
† Entropy Control Project, ICORP, JST.
‡ Nagoya Institute of Technology.
# Kansai University.
Sysmex Co.
(6) Lee, J. W.; Samal, S.; Selvapalam, N.; Kim, H.-J.; Kim, K. Acc. Chem.
Res. 2003, 36, 621.
| JASCO International Co.
§ Pohang University of Science and Technology.
(1) For reviews, see: (a) Mock, W. L. In ComprehensiVe Supramolecular
Chemistry; Vo¨gtle, Ed.; Pergamon: Oxford, 1996; Vol. 2, p 447. (b) Cintas,
P. J. Inclusion Phenom. 1994, 17, 205. (c) Kim, K. Chem. Soc. ReV. 2002,
31, 96-107.
(2) Behrend, R.; Meyer, E.; Rusche, F. Justus Liebigs Ann. Chem. 1905, 339,
1.
(3) Freeman, W. A.; Mock, W. L.; Shin, N.-Y. J. Am. Chem. Soc. 1981, 103,
7367.
(4) (a) Jeon, Y.-M.; Kim, J.; Whang, D.; Kim, K. J. Am. Chem. Soc. 1996,
118, 9790. (b) Whang, D.; Heo, J.; Park, J. H.; Kim, K. Angew. Chem.,
Int. Ed. 1998, 37, 78.
(5) Kim, J.; Jung, I.-S.; Kim, S.-Y.; Lee, E.; Kang, J.-K.; Sakamoto, S.;
Yamaguchi, K.; Kim, K. J. Am. Chem. Soc. 2000, 122, 540.
(7) Recent examples: (a) Ko, Y. H.; Kim, K.; Kang, J.-K.; Chun, H.; Lee, J.
W.; Sakamoto, S.; Yamaguchi, K.; Fettinger, J. C.; Kim, K. J. Am. Chem.
Soc. 2004, 126, 1932. (b) Tuncel, D.; Steinke, J. H. G. Macromolecules
2004, 37, 288. (c) Jeon, W. S.; Kim, E.; Ko, Y. H.; Hwang, I.; Lee, J. W.;
Kim, S. Y.; Kim, H. J.; Kim, K. Angew. Chem., Int. Ed. 2005, 44, 87. (d)
Ooya, T.; Inoue, D.; Choi, H. S.; Kobayashi, Y.; Loethen, S.; Thompson,
D. H.; Ko, Y. H.; Kim, K.; Yui, N. Org. Lett. 2006, 8, 3159. (e) Sindelar,
V.; Silvi, S.; Kaifer, A. E. Chem. Commun. 2006, 2185. (f) Sobransingh,
D.; Kaifer, A. E. Org. Lett. 2006, 8, 3247.
(8) Lagona, J.; Mukhopadhyay, P.; Chakrabarti, S.; Isaacs, L. Angew. Chem.,
Int. Ed. 2005, 44, 4844.
(9) (a) Kikuchi, Y.; Kobayashi, K.; Aoyama Y. J. Am. Chem. Soc. 1992, 114,
1351. (b) Borovkov, V. V.; Hembury, G. A.; Inoue, Y. Acc. Chem. Res.
2004, 37, 449.
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J. AM. CHEM. SOC. 2006, 128, 14871-14880
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A R T I C L E S
Rekharsky et al.
Figure 1. Formation of 1:2 complexes of CB[6] (a) with (R,R)- or (S,S)-trans-1,2-diaminocyclohexane (DC) and (b) with (R)- or (S)-2-methylpiper-
azine (MP).
particularly CB[6] and CB[7], can serve as effective chiral
discriminators.
to achieve enantiodifferentiation by accommodating two dif-
ferent chiral guests inside a self-assembled achiral capsule;
however, the chiral recognition ability was not very high, with
a maximum difference in affinity of only 1.2 times. Strictly
speaking, the molecular recognition event observed in Rebek’s
supramolecular system11 as well as in the system described here
(Figures 1 and 2) is not an enantioselection in a classical
definition of this term, but rather a special kind of diastereo-
selectivity that arises from supramolecular assembling of chiral
and achiral building blocks. More precisely, such a molecular
recognition event may be called “assembled enantioselection”
or “assembled enantiorecognition.” Here, we report our results
on the assembled enantiorecognition upon guest-exchange
reactions with enantiopure CB[6]-based supramolecular com-
plexes.
Aqueous solutions of 1:2 host-guest complexes (R,R;R,R)-
and (S,S;S,S)-[CB[6]‚2DC]4+, formed in situ, were subjected
to microcalorimetric (ITC) titration with aqueous (R)-leucinea-
mide (Leu-NH2) to reveal the formation of two 1:1:1 complexes,
i.e., (R,R;R)- and (S,S;R)-[CB[6]‚DC‚Leu-NH2]3+. The thermo-
dynamic parameters for these diastereomeric complexes were
exactly the same, meaning no chiral recognition (Table 1). One
of the most likely explanations for this result is the relatively
large distance between the stereogenic centers of the two chiral
guests in the termolecular complexes. Enantiomeric (R)- and
(S)-sec-butylamines (BA) also gave practically the same
affinities upon complexation with (S,S;S,S)-[CB[6]‚2DC]4+
(Table 1).
Experimental Section
Microcalorimetric experiments were performed using an isothermal
titration calorimeter (VP-ITC, MicroCal, Northampton, MA). Each
experiment consisted of 25-35 consecutive injections (5-10 µL) of a
guest solution into the microcalorimetric reaction cell (1.4 mL) charged
with a solution of CB[6] or CB[7]. The heat of reaction was corrected
for the heat of dilution of the guest solution, determined in separate
experiments. All solutions were degassed prior to the titration experi-
ment according to the method provided by MicroCal. Computer
simulations (curve fitting) were performed using the ORIGIN 7.0
software, adapted for ITC data analysis. The “Single Set of Identical
Sites” model was applied in all cases.
1D and 2D NMR spectra, including ROESY, COSY, and HOHAHA,
were obtained at 600 MHz in D2O at 25 °C on a Bruker Avance 600
instrument. HOHAHA experiments were performed by using the
MLEV-17 pulse sequence with a mixing time of 120 ms, while ROESY
spectra were recorded with a mixing time of 200 ms.
A Zetasizer Nano ZS instrument (Malvern Instruments, Worcester-
shire, UK) was used for light-scattering measurements at 25 °C and at
a 173° angle. The Rayleigh equation was applied to estimate the
molecular weight of nanoparticles in the solution.10
CB[6] and CB[7] were synthesized and purified by the method
reported previously.3-5 All other reagents, of the highest purity available,
were obtained from Aldrich and Wako and were used without further
purification.
Results and Discussion
Interestingly, in the case of (R)- and (S)-BA guests, the
differences between ∆H° and ∆S° values obtained for the
diastereomeric (S,S;R)- and (S,S;S)-[CB[6]‚DC‚BA]3+ com-
plexes are clearly outside the experimental errors, despite their
having practically the same complex stabilities (Table 1), for
which the frequently observed enthalpy-entropy compensation
effect12,13 is responsible. The different ∆H° and ∆S° values
Enantiodifferentiation in the Cucurbit[6]uril Cavity. Dis-
solving CB[6] in an aqueous solution of an enantiopure organic
amine, such as (R)- or (S)-2-methylpiperazine (MP) or (R,R)-
or (S,S)-trans-1,2-diaminocyclohexane (DC), leads to formation
of the respective enantiopure complex, i.e., (R;R)- or (S;S)-[CB-
[6]‚2MP]4+ or (R,R;R,R)- or (S,S;S,S)-[CB[6]‚2DC]4+ (Figure
1). An appropriate chiral guest, added to this solution, can
replace one of the originally bound enantiopure amines (MP or
DC) at the CB portals, as illustrated in Figure 2. This idea is
reminiscent of that proposed by Rebek et al.,11 who attempted
(11) Scarso, A.; Shivanyak, A.; Hayashida, O.; Rebek, J. J. Am. Chem. Soc.
2003, 125, 6239.
(12) Williams, D. H.; Stephens, E.; O’Brian, D. P.; Zhou, M. Angew. Chem.,
Int. Ed. 2004, 43, 6596.
(13) Grunwald, E. Thermodynamics of Molecular Species; Wiley-Interscience:
New York, 1996.
(10) Bloomfield, V. A. Biopolymers 2000, 54, 168-172.
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Chiral Recognition in Cucurbituril Cavities
A R T I C L E S
Figure 2. Replacement of (a) trans-1,2-diaminocyclohexane (DC) or (b) 2-methylpiperazine (MP) auxiliary by another chiral guest, such as sec-butylamine,
leucineamide, or 2-methylbutylamine (G).
Table 1. Stability Constant (K) and Standard Enthalpy (∆H°) and Entropy Changes (T∆S°) for Complexation of Cucurbit[6]uril (CB[6]) with
(S)-2-Methylbutylamine ((S)-MB), (R)- and (S)-sec-Butylamine ((R/S)-BA), and Related Guest Molecules in Aqueous Solutions of 0.025 M
(R,R)- or (S,S)-1,2-Diaminocyclohexane ((R,R/S,S)-DC, soln 1), 0.025 M (R)- and (S)-2-Methylpiperazine ((R/S)-MP, soln 2), and 0.1 M
NaCl (soln 3) at 298.15 K
1
1
1
complexation reaction
K/M-
∆H°
/kJ mol-
T∆S°
/kJ mol-
[CB[6]‚2((R,R)-DC)]4+ + (R)-Leu-NH2+ f
315 ( 10
320 ( 10
1030 ( 50
1070 ( 50
15 000 ( 3000
800 ( 100
∼20
-19.5 ( 0.2
-19.3 ( 0.2
-19.8 ( 0.3
-18.6 ( 0.3
-5.0 ( 0.5
13.4 ( 0.8
∼ -3
-5.2 ( 0.3
-5.0 ( 0.3
-2.6 ( 0.3
-1.3 ( 0.3
18.8 ( 0.6
30.0 ( 1.0
∼4
[CB[6]‚(R,R)-DC‚(R)-Leu-NH2]3+ + (R,R)-DC2+ (soln 1)
[CB[6]‚2((S,S)-DC)]4+ + (R)-Leu-NH2+ f
[CB[6]‚(S,S)-DC‚(R)-Leu-NH2]3+ + (S,S)-DC2+ (soln 1)
[CB[6]‚2((S,S)-DC)]4+ + (R)-BA+
f
[CB[6]‚(S,S)-DC‚(R)-BA]3+ + (S,S)-DC2+ (soln 1)
[CB[6]‚2((S,S)-DC)]4+ + (S)-BA+
[CB[6]‚(S,S)-DC‚(S)-BA]3+ + (S,S)-DC2+ (soln 1)
[CB[6]‚2((R)-MP)]4+ + (S)-MB+
[CB[6]‚(R)-MP‚(S)-MB]3+ + (R)-MP2+ (soln 2)
[CB[6]‚2((S)-MP)]4+ + (S)-MB+
[CB[6]‚(S)-MP‚(S)-MB]3+ + (S)-MP2+ (soln 2)
[CB[6]‚2Na]2+ + (S)-MP2+
[CB[6]‚(S)-MP‚Na]3+ + Na+ (soln 3)
[CB[6]‚(S)-MP‚Na]3+ + (S)-MP2+
[CB[6]‚2((S)-MP)]4+ + Na+ (soln 3)
f
f
f
f
f
∼20
∼ -17
∼ -10
[CB[6]‚2Na]2+ + (S)-MB+
f
10 500 ( 500
-17.4 ( 0.2
5.6 ( 0.2
[CB[6]‚(S)-MB‚Na]3+ + Na+ (soln 3)
observed for (R)- and (S)-BA imply that the supramolecular
enantiomeric complex of CB[6] can effectively recognize the
guest’s chirality when the right combination of chiral portal
auxiliaries and replacing guest is chosen.
of such criteria, we selected chiral 2-methylbutylamine (MB)
as the most promising guest in the following ITC experiments.
MB has a crucial advantage over BA, since the stereogenic
center is located R to the ammonium group in BA, but at the â
position in MB. Consequently, the stereogenic center of MB
should be forced to penetrate deeply into the chiral nanospace
produced by the chiral auxiliary (MP) of the CB-MP complex.
Indeed, the choice of chiral MB as guest was very successful.
As can be seen from the thermodynamic parameters shown in
Table 1, the diastereomeric (S;R)- and (S;S)-[CB[6]‚MP‚MB]3+
ternary complexes exhibited an unprecedented 19-fold difference
in stability, or an enantioselectivity of 95%, which is the best
“assembled enantioselectivity” reported for a supramolecular
system incorporating an achiral host. To further elucidate the
To optimize the stereospecific interactions in the CB[6]
ternary complex, we changed the chiral auxiliary from (R,R)-
and (S,S)-DC to (R)- and (S)-2-methylpiperazine (MP). The key
idea is that the MP’s hydrophobic methyl substituent is most
likely included inside the CB cavity, so as to form a chiral
nanospace for further inclusion of another guest. The guest’s
structure is also important, and hence we searched for a chiral
amine that possesses a stereogenic center at a position remote
from the amino group yet that is small enough to be accom-
modated in the same CB cavity together with MP. On the basis
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Rekharsky et al.
Figure 3. NMR spectra of supramolecular complexes composed of (R)/(S)-2-methylpiperazine, CB[6], and/or (S)-2-methylbutylamine in D2O: (a) 5 mM
(R)-2-methylpiperazine‚2HCl, (b) 0.5 mM CB[6] + 5 mM (R)-2-methylpiperazine‚2HCl, (c) 0.5 mM CB[6] + 0.5 mM (S)-2-methylbutylamine‚HCl + 5
mM (R)-2-methylpiperazine‚2HCl, and (d) 0.5 mM CB[6] + 0.5 mM (S)-2-methylbutylamine‚HCl + 5 mM (S)-2-methylpiperazine‚2HCl.
mechanism and the factors that lead to such a remarkable
difference in complex stability, we performed additional ITC
and NMR examinations.
nition upon formation of ternary [CB[6]‚MP‚MB]3+ complexes.
Formation of the (S;S)-[CB[6]‚MP‚MB]3+ complex is exclu-
sively entropy-driven and associated with a large unfavorable
∆H° of 13.4 kJ mol-1. Such high endothermicity is unusual
for a CB complex, and may be attributed to severe dislocation/
displacement of the remaining (S)-MP in the CB cavity upon
replacement of one of the initially bound two (S)-MP auxiliaries.
In contrast, in the [CB[6]‚2((R)-MP)]4+ case, the remaining (R)-
MP does not suffer such a large dislocation, probably as a result
of smaller steric hindrance within the CB cavity, to afford the
moderately exothermic ∆H° of -5.0 kJ mol-1 and the highly
positive T∆S° of 18.8 kJ mol-1, which are jointly responsible
for the contrasting behavior of the diastereomeric host-guest
combination and the 19-fold difference in K.
The thermodynamic parameters for the complexation of chiral
auxiliary (S)-MP with CB[6] in aqueous 0.1 M NaCl solution
indicate that Na+ and MP2+ are very comparable guests,
competing for the CB[6] portals, since the replacement of Na+
by MP2+ is associated with small free energy changes, as judged
from the K values of ca. 20 M-1 for the successive complexation
(Table 1). The favorable enthalpy changes observed for both
steps strongly indicate that the MP2+ molecules occupy the
optimal position at the CB portals, as is the case with Na+, and
therefore removal or severe displacement/dislocation of MP2+
from the optimal position leads to a large enthalpy loss.
The data presented in Table 1 allow us to elucidate the
molecular events leading to the large assembled enantiorecog-
Additional support for severe unfavorable dislocation/
displacement of the remaining (S)-MP upon formation of the
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Chiral Recognition in Cucurbituril Cavities
A R T I C L E S
of (S)-MB into the cavity, the small sodium cation coordinated
at the opposite portal should not be significantly disturbed.
Accordingly, we observed a large negative enthalpy of com-
plexation, indicating optimization of host-guest van der Waals
contacts inside the cavity and positive entropy arising from
desolvation.
NMR studies provided us with strong evidence in support of
the above discussion. The signals for MP’s ring protons (Figure
3) are shifted downfield upon 1:2 host-guest complexation with
CB[6], indicating partial insertion of the guest into the cavity.
Replacement of one of the two (R)- or (S)-MP molecules by a
stronger guest, i.e., (S)-MB, leads to the opposite upfield shifts
(Figure 3), indicating displacement from the optimal position/
location of the remaining (R)- or (S)-MP upon insertion of (S)-
MB. This effect (upfield shift) is more pronounced for (S;S)-
[CB[6]‚MP‚MB], in nice agreement with the highly endothermic
∆H° and the lower stability, suggesting larger dislocation/
displacement of chiral MP auxiliary from its original optimal
position/location. The MP’s methyl protons also exhibit similar
upfield/downfield shifts upon complexation, but the observed
shifts are smaller (Supporting Information).
Interestingly, supramolecular interactions in a solution con-
taining (S)-MB and (S;S)-[CB[6]‚2MP]4+ or (R;R)-[CB[6]‚
2MP]4+ appear to be more complex than those of ternary (S;S)-
[CB[6]‚MP‚MB]3+ or (S;R)-[CB[6]‚MP‚MB]3+ species only.
These ternary complexes are the predominant species only at
very low concentrations of CB[6] in the range of 0.1-0.3 mM.
The “Single Set of Identical Sites” model was successful in
simulating the experimental data obtained at a 0.12 mM
concentration of (S;S)-[CB[6]‚2MP]4+ or (R;R)-[CB[6]‚2MP]4+
(Figure 4). Previously,14,15 we discussed in more detail the
gradual simplification of a reaction mixture upon dilution,
eventually leading to the simplest possible species. In the present
case, these simplest species are ternary (S;S)-[CB[6]‚MP‚MB]3+
or (S;R)-[CB[6]‚MP‚MB]3+ complexes. The thermodynamic
parameters of the ternary complex formation are presented in
Table 1; their formation is predominantly (for (S;R)-[CB[6]‚
MP‚MB]3+) or even exclusively (for (S;S)-[CB[6]‚MP‚MB]3+
)
entropy-driven.
Higher CB[6] concentrations lead to spontaneous supramo-
lecular assembling, as indicated by the results of ITC experi-
ments (Figure 4). To reveal the molecular structure of supramo-
lecular assemblies in the solution, we apply light-scattering and
ESI-MS techniques. We estimated that the average molecular
weight of supramolecular aggregates in 1.2 mM CB[6] solution
(the same concentration used for ITC experiments; Table 1) is
2.2 kDa, which corresponds to a dimer composed of [MP‚CB-
[6]‚MP‚CB[6]‚MP]. We succeeded in detecting the cation [MP‚
CB[6]‚MP‚CB[6]‚MP]3+ using ESI-MS techniques (Supporting
Information). Certainly, the molecular weight of 2.2 kDa
obtained from light-scattering experiments represents an average
size of the nanoparticles, and in the real solution there must be
a distribution among monomer, dimer, trimer, and probably
some higher molecular aggregates. It is reasonable to expect
that, at higher CB[6] concentrations, the distribution is shifted
to the higher molecular weight side. Indeed, the average
Figure 4. ITC titration curves upon addition of (S)-MB to solutions of
CB[6] of various concentrations; experiments performed with (a) 0.025 M
(R)-MP and (b) 0.025 M (S)-MP.
(S;S)-[CB[6]‚MP‚MB]3+ complex can be derived from the
thermodynamic parameters for the complexation of (S)-MB with
CB[6] in 0.05 M NaCl (Table 1 and Supporting Information).
Indeed, it is expected that, after insertion of the aliphatic chain
(14) Rekharsky, M. V.; Inoue, Y. J. Am. Chem. Soc. 2000, 122, 10949-10955.
(15) Rekharsky, M. V.; Inoue, Y. J. Am. Chem. Soc. 2000, 122, 4418-4435.
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Figure 5. Results of light-scattering experiments: (a) intensity of scattered light and (b) estimated molecular weight of nanoparticles in solutions 1-3
(indicated in the horizontal scale) of varying MB concentrations, which contain (R)-MP (25 mM), CB[6] (4 mM), and (S)-MB at 0, 0.7, and 4 mM (red
circles); (R)-MP (25 mM), CB[6] (1.2 mM), and (S)-MB at 0, 0.24, and 1.2 mM (black squares); and (S)-MP (25 mM), CB[6] (1.2 mM), and (S)-MB at 0,
0.24, and 1.2 mM (green triangles).
molecular weight of supramolecular aggregates in the presence
of 4 mM CB[6] in the solution was determined as 3.2 kDa
(Figure 5).
to destroy higher molecular aggregates. The results of the NMR
study are also consistent with the dominant formation of the
monomeric ternary complex in the solution where (S)-MB is
equal to CB[6] in concentration. Indeed, the observation of two
doublets of equal intensity for each of Hx and Hy protons of
CB[6] (Supporting Information) unequivocally reveals that the
two portals of CB[6] are not identical to each other, and one
portal is occupied by (S)-MB but another by (R)- or (S)-MP.
Since the supramolecular complex formed from (R)-MP, (S)-
MB, and CB[6] is diastereomeric to that from (S)-MP, (S)-MB,
and CB[6], they may exhibit separate distinguishing patterns if
the NMR spectra are measured in the presence of both (R)-MP
and (S)-MP. However, in reality, it appears that exchange of
(R)- and (S)-MP molecules between supramolecular species and
bulk solution is very fast on the NMR time scale, and we
Addition of (S)-MB into the solution leads to gradual
reduction of the size of the nanoparticles, and the solution of
(S)-MB and CB[6] in equal concentrations contains predomi-
nantly the monomeric species, i.e., (S;S)-[CB[6]‚MP‚MB]3+ or
(S;R)-[CB[6]‚MP‚MB]3+ (Figure 5). It should be emphasized
that the molecular weight of the supramolecular aggregates in
the solution of (R)-MP and CB[6] decreases faster than that in
the solution of (S)-MP and CB[6] of the same concentration
(Figure 5). This is consistent with the higher stability of (S;R)-
[CB[6]‚MP‚MB]3+ than that of (S;S)-[CB[6]‚MP‚MB]3+ ob-
tained in the ITC experiments (Table 1). Thus, a smaller
concentration of (S)-MB is necessary to shift the equilibrium
to the ternary monomeric complex, and consequently, it is easy
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Figure 6. Hierarchical supramolecular assembling in the three-component “Commander-Sergeants-Soldiers” system, composed of (S)-2-methylbutylamine
(A), CB[6] (B), and (R/S)-2-methylpiperazine (C).
Table 2. Stability Constant (K) and Standard Enthalpy (∆H°) and Entropy Changes (T∆S°) for Complexation of Cucurbit[7]uril (CB[7]) with
Various Phenylalanine-Containing Dipeptidesa in Aqueous 0.1 M NaCl Solutions at 298.15 K
K
LL/KDL or
LL/KLD
1
1
1
complexation reaction
K/M-
K
∆H°
/kJ mol-
T∆S°
/kJ mol-
[CB[7]‚2Na]2+ + Phe-Ala+-
f
(7.9 ( 0.3) × 105
(1.3 ( 0.1) × 106
(2.9 ( 0.2) × 104
(5.0 ( 0.2) × 104
(5.3 ( 0.2) × 106
(1.3 ( 0.1) × 106
(1.4 ( 0.2) × 107
(1.7 ( 0.1) × 106
0.61
0.58
4.1
-30.6 ( 0.3
-32.5 ( 0.3
-24.8 ( 0.3
-27.1 ( 0.3
-36.6 ( 0.4
-29.0 ( 0.3
-36.4 ( 0.4
-24.2 ( 0.3
3.1 ( 0.4
2.4 ( 0.4
0.7 ( 0.4
[CB[7]‚Phe-Ala‚Na]+(+ -) + Na+
[CB[7]‚2Na]2+ + D-Phe-Ala+
f
[CB[7]‚D-Phe-Ala‚Na]+(+ -) + Na+
[CB[7]‚2Na]2+ + Phe-Pro+ -
[CB[7]‚Phe-Pro‚Na]+(+ -) + Na+
f
[CB[7]‚2Na]2+ + Phe-D-Pro+ -
f
-0.3 ( 0.4
1.8 ( 0.5
5.9 ( 0.4
4.4 ( 0.5
11.4 ( 0.4
[CB[7]‚Phe-D-Pro‚Na]+(+ -) + Na+
[CB[7]‚2Na]2+ + NH3+-Phe-Phe-CONH2 f
[CB[7]‚NH3-Phe-Phe-CONH2‚Na]2+ + Na+
[CB[7]‚2Na]2+ + NH3+-D-Phe-Phe-CONH2 f
[CB[7]‚NH3-D-Phe-Phe-CONH2‚Na]2+ + Na+
[CB[7]‚2Na]2+ + NH3+-Phe-Leu-CONH2 f
[CB[7]‚NH3-Phe-Leu-CONH2‚Na]2+ + Na+
[CB[7]‚2Na]2+ + NH3+-Phe-D-Leu-CONH2 f
[CB[7]‚NH3-Phe-D-Leu-CONH2‚Na]2+ + Na+
8.2
a
L-Amino acid residue used unless stated otherwise.
observed only averaged signals for all species existing in the
solution (Supporting Information).
(S)-MB controls the size/weight of oligomeric nanoparticles
composed of enantiopure (R)-MP and CB[6] or enantiopure (S)-
MP and CB[6]. Since nanoparticles consisting of (R)-MP/(S)-
MB/CB[6] vs (S)-MP/(S)-MB/CB[6] are diastereomeric species,
they possess distinctly different properties, as demonstrated in
our ITC and light-scattering experiments. The properties of our
three-component system are ruled by the new “Commander-
Sergeants-Soldiers” principle. First, the strong binder, 2-me-
thylbutylamine (MB), functioning as the Commander, replaces
one 2-methylpiperazines (MP) under the control of CB[6] in
the original oligomeric species (see the above discussion).
Second, upon gradual increase of the MB concentration (too
By combining the results of light-scattering, ITC, NMR, and
ESI-MS experiments, we can conclude that the molecular events
occurring in the CB[6]-MP-MB system involve gradual
reduction of the size/weight of oligomeric nanoparticles existing
in the initial solution of CB[6] and (R)- or (S)-MP upon
subsequent addition of (S)-MB, as illustrated in Figure 6.
Addition of (S)-MB is considered as an interesting extension
of the “Soldiers and Sergeants” principle, which has amply been
observed in several supramolecular systems.16-22 However, in
our three-component system, only one enantiopure molecule
9
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A R T I C L E S
Rekharsky et al.
many Commanders), the size/weight of oligomeric nanoparticles
decreases, ultimately leading to monomeric ternary 1:1:1 CB-
[6]-MP-MB complexes.
Diastereorecognition in the Cucurbit[7]uril Cavity. Above,
we demonstrated that the right choice of two different chiral
guests, which can be simultaneously accommodated in the same
CB[6] cavity, forming a 1:1:1 complex, leads to an unprec-
edented assembled enantioselectivity. Indeed, the stability of
(S;R)-[CB[6]‚MP‚MB]3+ is 19 times higher than that of the
diastereomeric (S;S)-[CB[6]‚MP‚MB]3+. In this case, the chiral
centers of the two guests are not connected by a covalent bond.
However, it is reasonable to assume that an appropriate selection
of a guest having two chiral centers connected covalently may
also reveal high chiral recognition in the achiral CB cavity if
the guest size, the distance between chiral centers, and the
functional groups are carefully considered. Consequently, CB
complexation reactions gave us a rare opportunity to compare
diastereorecognition in a pair of two separate enantiomers, i.e.,
MP + MB, with the pair(s) of “classical” diastereomers, in
which two asymmetric carbons are located in the same molecule.
A large variety of dipeptides, which are readily available, are
certainly among the best targets. CB[7] forms complexes with
larger guest molecules that are not included in CB[6].6,23 For
example, CB[7] forms a 1:1 complex with a variety of positively
charged aromatic compounds, including naphthalene, viologen,
stilbene, and ferrocene derivatives.5,23a-h Recently, we also
found that zwitterionic phenylalanine interacts strongly with CB-
[7] in an aqueous 0.1 M NaCl solution (K ) 2.5 × 104 M-1
)
and even more strongly in pure water (K ) 1.8 × 106 M-1).23h
This is an interesting finding, since it was believed that only
positively charged and/or neutral compounds form strong
complexes with CB[7].6
Various phenylalanine-containing dipeptide diastereomers
were subjected to the ITC experiments employing CB[7] as host
molecule. We selected two diastereomeric pairs of zwitterionic
dipeptides, i.e., (1) L-Phe-L-Ala versus D-Phe-L-Ala and (2)
L-Phe-L-Pro versus L-Phe-D-Pro, and two diastereomeric pairs
of positively charged dipeptide amides, i.e., (1) NH3+-L-Phe-
+
L-Phe-CONH2 versus NH3+-D-Phe-L-Phe-CONH2 and (2) NH3
-
L-Phe-L-Leu-CONH2 versus NH3+-L-Phe-D-Leu-CONH2. The
choice of a solution appropriate for the ITC experiment is also
(16) Green, M. M.; Reidy, M. P.; Johnson, R. J.; Darling, G.; O’Leary, D. J.;
Wilson, G. J. Am. Chem. Soc. 1989, 111, 6452.
(17) Prins, L. J.; Timmerman, T.; Reinhoudt, D. N. J. Am. Chem. Soc. 2001,
123, 10153.
(18) Ikeda, A.; Udzu, H.; Zhong, Z.; Shinkai, S.; Sakamoto, S.; Yamaguchi, K.
J. Am. Chem. Soc. 2001, 123, 3872.
(19) Brunsveld, L.; Lohmejer, B. G. G.; Vekemans, J. A. J. M.; Meijer, E. W.
Chem. Commun. 2000, 2305.
(20) Brunsveld, L.; Schenning, A. P. H. G.; Broeren, M. A. C.; Janssen, H. M.;
Vekemans, J. A. J. M.; Meijer, E. W. Chem. Lett. 2000, 292.
(21) Hiraoka, S.; Fujita, M. J. Am. Chem. Soc. 1999, 121, 10239.
(22) Palmans, A. R. A.; Vekemans, J. A. J. M.; Havinga, E. E.; Meijer, E. W.
Angew. Chem., Int. Ed. 1997, 36, 2648.
Figure 7. 1H NMR spectra of (a) L-Phe-D-Leu-NH2 (5 mM) and (b) L-Phe-
L-Leu-NH2 (5 mM) in the absence and in the presence of 1 equiv CB[7] in
0.1 M NaCl in D2O.
(23) (a) Wagner, B. D.; Stojanovic, N.; Day, A. I.; Blanch, R. J. J. Phys. Chem.
B 2003, 107, 10741. (b) Choi, S. W.; Park, S. H.; Ziganshina, A.; Ko, Y.
H.; Lee, J. W.; Kim, K. Chem. Commun. 2003, 2176. (c) Kim, H.-J.; Jeon,
W. S.; Ko, Y. H.; Kim, K. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5007.
(d) Ong, W.; Gomez-Kaifer, M.; Kaifer, A. E. Org. Lett. 2002, 4, 1791.
(e) Moon, K.; Kaifer, A. E. Org. Lett. 2004, 6, 185. (f) Ong, W.; Kaifer,
A. E. J. Org. Chem. 2004, 69, 1383. (g) Ong, W.; Kaifer, A. E.
Organometallics 2003, 22, 4181. (h) Jeon, W. S.; Moon, K.; Park, S. H.;
Chun, H.; Ko, Y. H.; Lee, J. Y.; Lee, E. S.; Samal, S.; Selvapalam, N.;
Rekharsky, M. V.; Sindelar, V.; Sobransingh, D.; Inoue, Y.; Kaifer, A. E.;
Kim, K. J. Am. Chem. Soc. 2005, 127, 12984-12989. (i) Marquez, C.;
Nau, W. M. Angew. Chem., Int. Ed. 2001, 40, 4387. (j) Blanch, R. J.;
Sleeman, A. J.; White, T. J.; Arnold, A. P.; Day, A. I. Nano Lett. 2002, 2,
147. (k) Marquez, C.; Pischel, U.; Nau, W. M. Org. Lett. 2003, 5, 3911.
(l) Jeon, Y. J.; Kim, S.-Y.; Ko, Y. H.; Sakamoto, S.; Yamaguchi K.; Kim
K. Org. Biomol. Chem. 2005, 3, 2122.
an important issue. It is widely recognized that addition of
hydrophobic residues and/or positive charges to a guest leads,
in general, to a large enhancement of the affinity toward CB[7]
and other CB homologues.1a,6,24 Thus, we may expect much
higher affinities for the selected dipeptides compared to zwit-
terionic phenylalanine. Taking into account the instrumental
(24) Mock, W. L.; Shih, N.-Y. J. Org. Chem. 1986, 51, 4440.
9
14878 J. AM. CHEM. SOC. VOL. 128, NO. 46, 2006

Chiral Recognition in Cucurbituril Cavities
A R T I C L E S
Figure 8. Molecular events leading to diastereodifferentiation upon interaction of Phe-containing dipeptides with CB[7].
upper limit of ITC in affinity determination of about 107-108
M-1, we used aqueous 0.1 M NaCl solution rather than pure
water. The results of our ITC experiments are given in
Table 2.
CONH2 and NH3+-L-Phe-L-Leu-CONH2 with CB[7] are larger
than those for the corresponding D,L- or L,D-diastereomers
(Table 2).
A higher degree of diastereomeric recognition is expected to
occur with positively charged dipeptides than with zwitterionic
guests. Indeed, the major origin of the recognition of zwitterionic
guests is the long-reaching electrostatic repulsion of the guest’s
carboxylate against the host’s carbonyl oxygens. In contrast,
with dipeptide amides, the origin of recognition is the short-
range van der Waals interactions. In good agreement with the
Phe-Ala is the simplest diastereomeric dipeptide possessing
a Phe residue at the N-terminus. The D,L-isomer reveals 1.65
times higher affinity for CB[7] than the L,L-isomer. This affinity
enhancement is entirely enthalpic in origin. For a higher degree
of diastereomeric recognition, we further examined Phe-Pro.
The idea was to exploit the rigidity of the proline ring, with the
expectation that one of the diastereomers would fit nicely into
the CB[7] cavity, while the other causes large steric hindrance,
leading to higher discrimination. However, in reality the
affinities of the two Phe-Pro diastereomers are 25-30 times
lower than those of the corresponding Phe-Ala, and the affinity
enhancement for L-Phe-D-Pro versus L-Phe-L-Pro was merely
1.72 times, which is about the same as the value observed for
Phe-Ala diastereomers.
above discussion, both peptides reveal higher diastereomeric
+
selectivities of 4.1 for NH3+-L-Phe-L-Phe-CONH2 versus NH3
-
D-Phe-L-Phe-CONH2 and of 8.2 for NH3+-L-Phe-L-Leu-CONH2
versus NH3+-L-Phe-D-Leu-CONH2.
To gain a better understanding of the molecular origin of the
high diastereomeric recognition, we subjected the diastereomeric
complex pair of [CB[7]‚NH3+-L-Phe-L-Leu-CONH2] and [CB-
[7]‚NH3+-L-Phe-D-Leu-NH2] to NMR study. As shown in Figure
7, the signals for the aromatic protons of Phe are shifted upfield
(strongly indicating the insertion of the aromatic ring into the
cavity), while those for the protons in the Leu side group are
shifted downfield, suggesting that this group is located outside
the cavity. The ROESY spectra (Supporting Information) clearly
reveal that the Leu residue is residing outside the cavity, since
the NOE signals are seen between the aliphatic protons of Leu
residue and the CB[7] protons located outside the cavity.
Unusually, the large downfield shift of the γ-CH proton of the
Leu residue in the [CB[7]‚NH3+-L-Phe-L-Leu-CONH2] complex
and the large upfield shift of the â-CH proton of Phe in the
[CB[7]‚NH3+-L-Phe-D-Leu-CONH2] complex underscore the
differences in the structures of these two complexes. Missing
NOE signals between one of â-CH protons of Leu and the outer
surface protons of CB[7] in the [CB[7]‚NH3+-L-Phe-L-Leu-
CONH2] complex (Supporting Information) suggest the deeper
penetration of Phe compared with that in the diastereomeric
[CB[7]‚NH3+-L-Phe-D-Leu-NH2]. Indeed, a deeper insertion of
the Phe residue reduces the effective length of the peptide chain
outside the cavity, which is necessary to place the â-CH proton
in the nearest proximity of the outside host protons. As amply
observed in host-guest chemistry, deeper insertion leads to
improvement of the direct van der Waals contacts and is
therefore associated with large favorable (negative) enthalpy
changes (Table 2).
Insertion of a zwitterionic guest into a CB[7] cavity is
illustrated by three successive events: (a) inclusion of the
hydrophobic moiety of the guest in the CB cavity, (b)
disengagement of the ion-pairing interactions in the zwitterionic
guest to liberate the ammonium ion (and also to expel the
negatively charged carboxylate from the CB portal), and (c)
coordination of the guest’s ammonium ion by the carbonyl
oxygens at the CB portal. From this picture, one may expect
that zwitterionic and positively charged dipeptides display
different chiral preferences toward the CB[7] cavity, simply
because there is no second molecular event that disengages the
ion-pairing interactions in the latter case. The lower affinities
observed for the homochiral zwitterionic dipeptides than for their
heterochiral analogues indicate that it is sterically more difficult
in the former case to expel the negatively charged carboxylate
from the CB portal upon complexation, with the remaining
electrostatic repulsion between the carboxylate and portal
oxygens resulting in the lowering of affinity. In contrast, in the
positively charged dipeptide case, there is no electrostatic host-
guest repulsion, and thus all dipeptide residues can participate
in the formation of a strong complex. Consequently, upon
complexation, the conformation of the homochiral dipeptide,
which is unfavorable for optimal disengagement of the ion-
pairing interactions, would be most likely favorable for optimiz-
ing the short-range van der Waals interactions inside the cavity,
eventually leading to the enhanced affinity. This simple
structural rationale is well compatible with the experimental
data. Indeed, the complex stabilities of NH3+-L-Phe-L-Phe-
Figure 8 is a schematic illustration of molecular events leading
to diastereodifferentiation upon interaction of Phe-containing
dipeptides with CB[7], as elucidated from the NMR and ITC
data. Insertion of the Phe residue into the cavity leads to
9
J. AM. CHEM. SOC. VOL. 128, NO. 46, 2006 14879

A R T I C L E S
Rekharsky et al.
conformational restriction (fixation) of the guest, allowing a
pronounced stereoselective interaction of the second amino acid
residue at the outer surface of CB[7].
of diastereomeric (S;R)- and (S;S)-[CB[6]‚MP‚MB]3+ ternary
complexes, is the highest chiral recognition reported for an
achiral host.
(2) An interesting expansion of the “Soldiers and Sergeants”
principle was realized in the three-component system, where
only one enantiopure molecule, (S)-MB, controls the degree of
chiral supramolecular assembling of (R)-MP or (S)-MP with
CB[6], which is strictly ruled by the novel hierarchical “Com-
mander-Sergeants-Soldiers” principle.
(3) A diastereomeric pair of two separate enantiomers can
exhibit similar or even higher selectivity toward an achiral host
as compared with the pair of “classical” diastereomers in similar
complexation reactions, as demonstrated by comparison of a
MP + MB diastereomeric pair versus diastereomeric dipeptides.
These results further indicate that an inherently chiral host is
not necessarily required for chiral recognition, but a combination
of achiral host (CB) and chiral inductor can also serve as a
supramolecular chiral host for such a purpose, where the chiral
recognition ability may be tuned by choosing the chiral inductor.
The “Commander-Sergeants-Soldiers” system will not be
restricted to the combination of CB, strong binders, and weaker
binders, but the basic concept could potentially be expanded to
other supramolecular systems.
The observed affinity enhancement is clearly enthalpy-driven
for all the diastereomeric recognition processes shown in Table
2, and thus we conclude that the optimization of van der Waals
interactions is the major factor that determines the magnitude
of diastereomeric recognition. It should be emphasized that the
host’s chirality (chiral â-CD or achiral CB[7]) plays practically
no role in the recognition of Phe-containing dipeptides, since
nearly the same magnitude of diastereomeric recognition was
reported for inclusion of Phe-containing dipeptides with chiral
â-CD.25 Consequently, we may predict that any hydrophobic
cavity of appropriate size and shape can serve as an effective
selector to recognize, distinguish, or separate homochiral from
heterochiral dipeptides (oligopeptides) with hydrophobic resi-
dues.
Interestingly, the magnitude of recognition of all the diaster-
eomeric dipeptides listed in Table 2 by CB[7] is actually low
as compared with that of (S)-MB + (R)-MP vs (S)-MB + (S)-
MP by CB[6], underscoring the high potential of the supramo-
lecular approach for the design of hosts for enantiorecognition.
It should also be noted that both types of recognition reactions
(dipeptides by CB[7] and MB + MP by CB[6]) reveal
similarities in their thermodynamics. Indeed, in both cases,
increasing affinity is exclusively enthalpy-driven, thus empha-
sizing the importance of optimization of intracavity host-guest
interactions.
Acknowledgment. We gratefully acknowledge the support
of this work by the ICORP (Y.I. and K.K.), Creative Research
Initiatives, and BK 21 programs (K.K.).
Supporting Information Available: Figures showing interac-
tion of (S)-MB cation with CB[6] in aqueous 0.1 M NaCl;
methyl proton signals of MP in NMR spectra of D2O solutions;
ESI-MS spectra of an aqueous solution of CB[6], (R)/(S)-MP,
and (S)-MB; NMR spectra of D2O solutions of mixtures of CB-
[6], (R)-MP‚2HCl, (S)-MB‚2HCl, and (S)-MB‚HCl; NMR
spectrum of a mixture of diastereomeric complexes; and ROESY
spectra of complexes of CB[7] with Phe-D-Leu-NH2 and Phe-
L-Leu-NH2. This material is available free of charge via the
Internet at http://pubs.acs.org.
Conclusions
The main findings of this first comprehensive study on the
chiral recognition of guests by achiral hosts (CBs) using
calorimetric, light-scattering, nuclear magnetic, and mass spec-
tral techniques can be summarized as follows:
(1) The assembled enantiorecognition of MB of up to 95%
ee (or 19 times difference in affinity), achieved upon formation
(25) Rekharsky, M. V.; Yamamura, H.; Kawai, M.; Inoue, Y. J. Org. Chem.
2003, 68, 5228.
JA063323P
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14880 J. AM. CHEM. SOC. VOL. 128, NO. 46, 2006