A bambusuril macrocycle that binds anions in water with high affinity and selectivity

Article abstract of DOI:10.1002/anie.201409895

Synthetic receptors that function in water are important for the qualitative and quantitative detection of anions, which may act as pollutants in the environment or play important roles in biological processes. Neutral receptors are particularly appealing

Full text of DOI:10.1002/anie.201409895

Angewandte
Chemie
DOI: 10.1002/anie.201409895
Anion Receptors
A Bambusuril Macrocycle that Binds Anions in Water with High
Affinity and Selectivity**
Mirza Arfan Yawer, Vaclav Havel, and Vladimir Sindelar*
Abstract: Synthetic receptors that function in water are
important for the qualitative and quantitative detection of
anions, which may act as pollutants in the environment or play
important roles in biological processes. Neutral receptors are
particularly appealing because they are often more selective
than positively charged receptors; however, their affinity
towards anions in pure water is only in range of 1–
103 LmolÀ1. The anion-templated synthesis of a water-soluble
bambusuril derivative is shown to be an outstanding receptor
for various inorganic anions in pure water, with association
constants of up to 107 LmolÀ1. Furthermore, the macrocycle
discriminates between anions with unprecedented selectivity
(up to 500000-fold). We anticipate that the combination of
remarkable affinity and selectivity of this macrocycle will
enable the efficient detection and isolation of diverse anions in
aqueous solutions, which is not possible with current supra-
molecular systems.
ions are common.[3] The disadvantage of these receptors is
that they rely on non-directional electrostatic binding, which
results in poor anion selectivity. Moreover, positively charged
receptors are always accompanied by counteranions that
compete with the anions of interest for the binding site. There
is therefore interest in neutral receptors, which lack these two
disadvantages. The supramolecular interactions that stabilize
the complexes of neutral receptors and anions are often
suppressed by the strong solvating effect of water. This is why
no association constant in excess of 104 LmolÀ1 between an
inorganic anion and a neutral receptor in 100% water has
previously been reported.[4] Another drawback of neutral
synthetic receptors is their low solubility in water, which
significantly limits their applications.
The bambusurils are a family of neutral macrocyclic
compounds that can act as anion receptors.[5] We have
previously shown that bambusuril macrocycles containing
six repeating constitutional units can bind anions with
affinities of up to 3 ꢀ 109 LmolÀ1 in chloroform. However,
binding studies in polar solvents were complicated by the low
solubility of the bambusurils, particularly in the absence of
anions. Herein, we describe the synthesis of a water-soluble
bambusuril 5 and demonstrate its ability to behave as an
outstanding receptor for inorganic anions in pure water.
The solubility of bambusurils can be tuned by varying the
substituents on the nitrogen atoms of the macrocyclic portals.
We therefore investigated the water solubility of the ben-
zoate-substituted bambusuril 5 (Scheme 1). It was synthesized
by transforming 4-methoxycarbonylbenzylamine 1 into dia-
lkylurea 2 and then dialkylglycoluril 3. The macrocycle was
prepared by coupling glycoluril 3 with paraformaldehyde in
toluene under p-toluenesulfonic acid catalysis. Tetrabutylam-
monium bromide was added to the macrocyclization mixture
so that the bromide anions would serve as templates and drive
the reaction towards the desired six-membered ring. In the
absence of template, the only identifiable macrocyclic prod-
uct was a four-membered ring. The six-membered macrocycle
4 was isolated as a 1:1 complex with bromide, which was
subsequently treated with LiCl. The resulting macrocycle–
chloride complex was then treated with KOH in methanol/
dichloromethane mixture to convert their ester groups into
potassium carboxylates. The anionic form of the macrocycle
was neutralized by treating with CF3COOH in water and
washed with methanol and acetonitrile. The water-soluble
macrocycle 5 free of anion was obtained from 1 in an overall
yield of 15%. Anion impurity can be easily monitored by
1H NMR spectroscopy as the macrocycle dissolved in
[D6]DMSO forms very strong complexes with all tested
anions in this solvent arising to a new set of signals next to
those of anion-free macrocycle. The products of all five
A
nions play key roles in biochemical processes and some of
them are known to be essential for human health.[1] Receptors
for both anions and cations have been investigated since the
late 1960s.[2] However, the majority of these efforts have
focused on the supramolecular chemistry of cations. The
major difficulty in developing supramolecular anion receptors
stems from their high free energy of solvation compared to
cations of similar size, which means that anion receptors must
compete more efficiently with the solvent. Water is the most
strongly competitive solvent and can negate most of the
stabilizing intermolecular forces that enable the binding of
anions in host–guest systems. Synthetic receptors with high
affinities for anions in nonpolar solvents are therefore usually
nonfunctional in aqueous environments. To increase their
binding efficiency in water, many anion receptors are
cationic; multiply charged species such as polyammonium
[*] Dr. M. A. Yawer, V. Havel, Prof. V. Sindelar
Department of Chemistry and RECETOX, Masaryk University
Kamenice 5, 625 00 Brno (Czech Republic)
E-mail: sindelar@chemi.muni.cz
[**] This work was supported by the Czech Science Foundation (13-
15576S), the Czech Ministry of Education (projects LM2011028,
LO1214, and LH11012). V.H. acknowledges the Brno Ph.D. Talent
Scholarship program sponsored by Brno City Municipality. We thank
Prof. M. Wimmerovꢀ for the access to the ITC instrument in her
laboratory.
Supporting information for this article (including detailed exper-
imental conditions and procedures, syntheses and compound
characterizations, 1H and 13C NMR spectroscopic analyses, mass
spectrometry data, titration data and analyses, and ITC data) is
available on the WWW under http://dx.doi.org/10.1002/anie.
201409895.
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Scheme 1. Synthesis of the water-soluble bambusuril macrocycle 5. TBA=tetra-n-butylammonium, PTSA=p-toluenesulfonic acid.
synthetic steps were isolated in pure form as precipitates with
no need for any chromatographic purification. This synthesis
enabled the gram-scale preparation of the macrocycle.
isotherms very well when plotted against anion concentration,
yielding the calculated equilibrium association constants (Ka)
shown in Table 1.
The supramolecular properties of 5 were studied using
1H NMR spectroscopy in D2O at 308C. The solution of the
macrocycle at the typical concentration of 1 mm in 20 mm
K2DPO4 was prepared resulting in a pD of 7.1. These
conditions ensured that the macrocycle was soluble at
milimolar level for NMR experiments. The inclusion of an
anion in the macrocycle cavity was accompanied by changes
in the chemical shifts of the methine and methylene protons
of the macrocycle (Figure 1A,B). Rate of chemical exchange
between free and bound macrocycle depends on the overall
stability of resulting complexes. Anions with lower affinity
Table 1: Equilibrium association constants Ka[a] for 1:1 complexes of the
bambusuril macrocycle 5 with various anions determined by 1H NMR
spectroscopy (D2O, 20 mmK2DPO4, pD 7.1, 303.15 K).
Anion
Ka [LmolÀ1
]
FÀ
1.1ꢁ102
9.1ꢁ102
1.1ꢁ103
6.5ꢁ103
3.0ꢁ104
1.4ꢁ105
4.8ꢁ105
2.2ꢁ106
4.3ꢁ106
1.0ꢁ107
5.5ꢁ107
ClÀ
CNÀÀ[b]
IO4
À
ReO4
BrÀ
(FÀ, ClÀ, CNÀ, IO4À, ReO4 ) interact with the macrocycle in
À
À
fast exchange regime on the NMR timescale. In these cases
the chemical shifts of the host signals fitted 1:1 binding
NOÀ3
PF6À
BF4
IÀ
À
ClO4
[a] Standard deviations calculated from two independent measurements
are lower than 10%. [b] CNÀ is largely protonated under experimental
conditions.
On the other hand, slow exchange regime and quantita-
tive complex formation at the submilimolar concentrations
were recorded for strongly bound anions (BrÀ, NO3À, PF6
,
À
BF4À, IÀ, ClO4À; Supporting Information, Figures S20 and
S21). Therefore, the Ka values for the strongly bound anions
were determined by competition experiments using another
anion, usually ClÀ (see the Supporting Information for
a detailed description of this method). The formation of
equimolar host–guest complexes was further confirmed by
the observation of signals corresponding to the expected m/z
values of these complexes in MALDI-TOF mass spectra (see
the Supporting Information).
The most intriguing property of 5 is its high affinity
towards all tested inorganic anions in pure water. Anion
receptors are usually potent in organic solvents but exhibit
significantly reduced binding abilities in the presence of
water. For example, the affinity of Jeong’s foldamer for BrÀ
decreases from 1350 to 19 LmolÀ1 on going from a 4:1 (v/v)
DMSO/MeOH mixture to water.[6] The most stable previously
known host–guest system involving a neutral receptor and an
inorganic anion in pure water is that formed by Schmidtchen’s
zwitterionic host and IÀ, which has an association constant of
6480 LmolÀ1.[3] Bambusuril 5 is able to capture IÀ three orders
of magnitude more strongly than this, with an association
constant of 1.0 ꢀ 107 LmolÀ1 (Table 1).
Figure 1. A) NMR titration of 5 with ClÀ as its sodium salt (D2O,
20 mmK2DPO4, pD 7.1, 500 MHz, 303 K). B) Plot of the chemical shift
of the methine proton (a) of 5 (1 mm) in the presence of increasing
ClÀ concentrations (0–4 mm). The curve shows the best fit of the
experimental data to a 1:1 binding model.
2
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Particularly interesting is the unprecedentedly strong
À
binding of 5 to the weakly coordinating anions BF4À, PF6
,
and ClO4À: the association constants for the formation of 1:1
complexes of these ions with 5 in water were all above 2.2 ꢀ
106 LmolÀ1. The highest binding affinity among all tested
anions was achieved for ClO4À (5.5 ꢀ 107 LmolÀ1). Moreover,
À
5 is selective for ClO4 in preference to anions that occur
naturally in the environment, such as ClÀ (Ka(ClO4 )/Ka-
À
(ClÀ) = 60440). ClO4 is considered to be toxic because it
À
interferes with iodine uptake into the thyroid gland. The high
À
affinity and selectivity of 5 for ClO4 may be useful in
detecting and remediating this pollutant.
The high binding affinities observed in this work are
presumably due to the isolation of the bound anion from
water molecules by its inclusion in the receptor’s hydrophobic
pocket, as discussed by Kubik, You, Jeong, and Flood.[7] In the
previously reported cases, the pocket was formed by the
inclusion of one anion between two rather flat-shaped
receptor molecules or by the receptor folding around the
anion. The latter principle is typical for natural systems, where
anion binding often occurs in a cavity formed by a folded
peptide chain. Host 5 consists of glycoluril units connected by
a row of methylene bridges along the equator of the macro-
cycle (Figure 2A). The alternating glycoluril units form
a deep electropositive cavity in which one anion is included
and isolated from water molecules. This binding mode is
shown in Figure 2B–D and derived from the structures of
several bambusuril-based complexes that have been deter-
mined previously by crystal diffractometry.[5c] We believe that
Figure 2. A) Chemical formula of 5. B) Pictorial representation of the
5·ClÀ complex with hydrogen atoms omitted for clarity; C) side and
D) top views of the 5·ClÀ complex with benzyl substituents omitted for
clarity. Weak hydrogen bonding interactions C H···ClÀ are showed as
dashed lines.
À
highest enthalpic penalty for desolvation, resulting in the
lowest enthalpy of host–guest association compared to lager
halides.[11] Entropy of binding is negative for all of the halides.
The negative entropy of binding is surprising particularly for
fluoride. This strongly coordinating anion is known to be
solvated by well-ordered water molecules. Release of these
water molecules from the solvation shell prior to anion
inclusion was expected to result in a large gain in entropy. The
reason for the observed negative entropy of binding is not
À
also in water the complex is stabilized by multiple weak C
H···AÀ hydrogen bonding interactions between the methine
hydrogen atoms of the macrocycle and the anion. The internal
volume of the host cavity can
adapt to the size of the anion
by tilting the glycoluril uni-
ts.As a result, the receptor can
accommodate anions of vari-
ous shapes and radii, ranging
À
from FÀ (1.33 ꢁ)[8] to PF6
(2.76 ꢁ).[9] Desolvation plays
an important role during the
complexation process.[10] FÀ is
solvated much more strongly
than ions such as IÀ and
ClO4À, and the increase in
binding affinity from FÀ to
À
ClO4 is therefore attributed
to the decreasing solvation
energy in this anion series.
This is consistent with ther-
modynamic parameters for
the binding of halides
(Figure 3) that were obtained
using isothermal titration cal-
orimetry (ITC; Table 2).Th-
ese measurements show that
Figure 3. Raw heats of binding obtained by ITC when A) BrÀ and B) IÀ as its tetramethylammonium (TMA)
salts where mixed with 5 (H2O, pH 7.1, 303.15 K). Binding isotherms fitted to the raw data using 1:1
the inclusion of anions in the
bambusuril 5 cavity is driven
by enthalpy. Fluoride pays the binding model as indicated.
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Table 2: Equilibrium association constants Ka[a] and standard free energy
DG, enthalpy DH, and entropy changes TDS for the formation of 1:1
complexes between the bambusuril macrocycle 5 and BrÀ and IÀ anions
as determined by isothermal titration calorimetry (H2O, pH 7.1,
303.15 K).
In conclusion, the bambusuril macrocycle 5 binds to
inorganic anions with affinities and selectivities that are
unprecedented for a neutral receptor in water. Its association
constants range from 110 LmolÀ1 for the most strongly
solvated anion, FÀ, to 5.5 ꢀ 107 LmolÀ1 for ClO4À. The
macrocycle was prepared on a gram scale and exhibits good
solubility in neutral and basic water. There is considerable
interest in the development of effective anion receptors that
function in water; we have demonstrated that the bambusuril
5 is such a receptor and as such may find diverse applications
in biology as well as medicinal and analytical chemistry.
Anion[a]
Ka [LmolÀ1
]
DH
TDS
DG
[kJmolÀ1
]
[kJmolÀ1
]
[kJmolÀ1
]
FÀ
13
À18.5
À45.4
À68.0
À84.1
À12.0
À30.3
À41.8
À47.5
À6.5
ClÀ
BrÀ
IÀ
4.0ꢁ102
3.3ꢁ104
2.0ꢁ106
À15.1
À26.2
À36.6
[a] Standard deviations calculated from two independent measurements
are lower than 10% for ClÀ, BrÀ, and IÀ, and lower than 50% for FÀ.
Received: October 9, 2014
Published online: && &&, &&&&
fully understood at the present time. One possible explan-
ation relies on two entropy contributions that overcompen-
sate anion desolvation effect. The first contribution is
represented by configurational entropy resulting from the
loss of mobility of the receptor and anion after the complex
formation. The second, negative entropy contribution is
connected with H2PO4À. This anion is present in solutions
under titration conditions and it is expected to undergo
Keywords: anion recognition · host–guest systems ·
macrocycles · self-assembly · supramolecular chemistry
.
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2013, 15, 480.
[6] J. Suk, K. S. Jeong, J. Am. Chem. Soc. 2008, 130, 11868.
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Soc. 2013, 135, 14908; b) Y. Hua, Y. Liu, C. H. Chen, A. H.
Flood, J. Am. Chem. Soc. 2013, 135, 14401; c) H. Juwarker, K. S.
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40, 2648; Angew. Chem. 2001, 113, 2722.
À
complexation with the macrocycle. During titration, H2PO4
positioned in the host cavity is replaced by anion of interest,
for example, fluoride. Thus positive entropy of fluoride
desolvation upon its complexation is compensated by the
solvation of released H2PO4À anion. The association constants
determined by NMR and ITC are in a good agreement given
the different principles of the two techniques and slightly
different experimental conditions (differences in pH and
buffer concentration).
Finally, we decided to test possible influence of counter-
cations, ionic strength, and pH on the anion interaction with 5.
We did not find any countercation effects on the macrocycle
binding affinities. The association constant for the formation
of the complex between 5 and ClÀ in the form of its sodium
salt was identical (within experimental error) to that for the
TMACl and CsCl (Supporting Information, Figure S7 and
S8). We also determined Ka of the 5·ClO4À complex by three
independent competition experiments differing with the
concentration of competitive ClÀ anion (0.2, 0.4, and 0.8m
NaCl; Supporting Information, Figure S14). Ka values differ-
ing by less than 15% were obtained (Supporting Information,
Table S1), which demonstrates a minor influence of ionic
strength on the stability of complex inside this concentration
window. Recently it was reported that strength of binding
À
between an octa-acid host and ClO4 in water significantly
increases with increasing content of NaCl.[12] We did not
observe similar trend, presumably because attractive forces
between 5 and ClO4À predominate over the salting out effect
of NaCl. All of the NMR experiments presented in this study
were performed at pD 7.1. To investigate the influence of pD
on the binding we performed the titration of 5 with ClÀ in
20 mm Na3PO4 buffer of pD 11.3. The titration yielded the Ka
of 1.4 ꢀ 103 LmolÀ1 which is about 45% higher compared to
that one determined at pD 7.1. Thus, we did not observed
significant influence of pD on the binding ability of 5. The
insolubility of the macrocycle upon the protonation of its
carboxylate groups precluded the binding study at low pD.
[8] J. W. Steed, J. L. Atwood, Supramolecular Chemistry, Wiley,
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[10] F. P. Schmidtchen, Chem. Soc. Rev. 2010, 39, 3916.
[11] These conclusions are consistent with recently published results:
F. Sommer, S. Kubik, Org. Biomol. Chem. 2014, DOI: 10.1039/
C4OB01497A.
[12] R. S. Carnegie, C. L. D. Gibb, B. C. Gibb, Angew. Chem. Int. Ed.
2014, 53, 11498; Angew. Chem. 2014, 126, 11682.
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Chemie
Communications
Anion Receptors
One ring binds them all: A bambusuril
macrocycle is able to selectively capture
a large number of different inorganic
anions in water. The high stability of the
resulting inclusion complexes is due to
M. A. Yawer, V. Havel,
V. Sindelar*
&&&& — &&&&
À
A Bambusuril Macrocycle that Binds
Anions in Water with High Affinity and
Selectivity
multiple C H···anion hydrogen bonding
interaction in hydrophobic cavity of the
macrocycle.
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These are not the final page numbers!