Oxacalix[2]arene[2]triazine based ion-pair transporters

Article abstract of DOI:10.1039/c5ob02291f

Heteracalixaromatics are a new generation of macrocyclic hosts showing a unique structure and versatile recognition properties towards various guests. Amazingly, the application of heteracalixaromatics as membrane transporters or ion channels has never been explored. We reported herein the elaborated design of a series of oxacalix[2]arene[2]triazine-based derivatives 1-10 and their ion transport properties. Among these compounds, 3, 8-10 can mediate the transport of chloride across the lipid bilayer of EYPC with activity (EC₅₀) ranging from 0.43 to 8.23 μM. These compounds serve as ion carriers during the transport process, and the transport activity is both anion- and cation-dependent, suggesting a Cl^-/M⁺ ion-pair transport model. Structure-activity studies indicate that hydrogen bonding, electron deficiency of the triazine rings, lipophilicity and macrocyclic frameworks are essential for ion transport.

Full text of DOI:10.1039/c5ob02291f

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Oxacalix[2]arene[2]triazine based ion-pair
transporters†
Cite this: DOI: 10.1039/c5ob02291f
Xu-Dong Wang, Sen Li, Yu-Fei Ao, Qi-Qiang Wang, Zhi-Tang Huang and
De-Xian Wang*
Heteracalixaromatics are a new generation of macrocyclic hosts showing a unique structure and versatile
recognition properties towards various guests. Amazingly, the application of heteracalixaromatics as
membrane transporters or ion channels has never been explored. We reported herein the elaborated
design of a series of oxacalix[2]arene[2]triazine-based derivatives 1–10 and their ion transport properties.
Among these compounds, 3, 8–10 can mediate the transport of chloride across the lipid bilayer of EYPC
with activity (EC50) ranging from 0.43 to 8.23 μM. These compounds serve as ion carriers during the trans-
port process, and the transport activity is both anion- and cation-dependent, suggesting a Cl−/M+ ion-
pair transport model. Structure–activity studies indicate that hydrogen bonding, electron deficiency of the
triazine rings, lipophilicity and macrocyclic frameworks are essential for ion transport.
Received 7th November 2015,
Accepted 24th November 2015
DOI: 10.1039/c5ob02291f
www.rsc.org/obc
transmembrane channel models on the basis of pillararenes, a
Introduction
new type of para-bridged macrocycle emerged in 2008.8
Ion transport across membranes catalyzed by proteins that
Heteracalixaromatics, or heteroatom bridged calix(het)arenes
serve as ion channels or carriers is one of the most important are a new type of macrocyclic molecule.9 In the traditional calix-
processes in living cells.1 The desire to understand the func- arene the cavity itself shows almost no recognition ability. The
tion and mechanism of proteins has inspired the development introduction of heteroatoms as bridging linkages and hetero-
of synthetic ion channel models.2 Owing to their unique struc- aromatic moieties into heteracalixaromatics endowed the fine-
ture and facile functionalization, calixarenes and the related tuned cavities and versatile recognition abilities. For example,
resorcinarenes have a rich history as transmembrane ion chan- these hosts can recognize various guest species, from metal
nels or transporters.3 Early work demonstrated the use of ions and clusters,10 neutral molecules11 to anions.12 As the
resorcinarenes and calixarenes as cation transporters or chan- representative examples of heteracalixaromatics, oxacalix[2]-
nels.3a,b Later on, several groups including Davis, Izzo and arene[2]triazines, 1,3-alternate shape-persistent macrocyclic
Tecilla reported the transport of anions through the phospho- molecules bearing electron-deficient triazine rings, have been
lipid bilayer membrane with 1,3-alternate or paco calix[4]arene shown as unique anion acceptors based on anion–π inter-
derivatives.2a,4 Very recently, Matile reported ditopic ion trans- actions.12 Despite their versatile recognition properties, the
port systems based on anion–π interactions and halogen application of heteracalixaromatics as transmembrane models
bonds by taking cone calix[4]arene as a molecular platform.5 has never been explored. Herein we report the use of a series of
Besides the largely explored classical calixarenes, other calix- oxacalix[2]arene[2]triazine derivatives (Fig. 1) as transmembrane
aromatics such as calixpyrroles, in which pyrrole instead of transporters. Based on the fluorescence assays, we present that
benzene units are incorporated into the macrocyclic frame- compounds 3, 8–10 can mediate the ion pair transport across
work, have been intensively studied as anion transporters by the lipid bilayers of EYPC. Hydrogen bonding, electron
the Gale and Sessler groups.6 In addition to the meta-bridged deficiency of the triazine rings, lipophilicity and macrocyclic
macrocycles, Hou and coworkers7 designed novel and versatile frameworks are considered to be essential factors in order to
mediate the effective ion transport.
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular
Results and discussion
Recognition and Function, Institute of Chemistry, CAS, Beijing, 100190, China.
E-mail: dxwang@iccas.ac.cn
†Electronic supplementary information (ESI) available. CCDC 1432620. For ESI
Ion transport activity measurements
Oxacalix[2]arene[2]triazine derivatives 1–10 (Fig. 1) were syn-
thesized according to our reported procedures or as described
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c5ob02291f
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Fig. 1 Structures of tetraoxacalix[2]arene[2]triazine derivatives 1–10,
and the control compounds 11 and 12.
in the Experimental part (ESI†).11b,13 To evaluate the anion
transport activities of these compounds, we chose the fluo-
rescence assays with the halide selective dye lucigenin as the
probe.2a Briefly, the egg yolk phosphatidylcholine (EYPC) lipid
film was rehydrated with an aqueous solution containing
10 mM HEPES, 1 mM lucigenin and 100 mM sodium nitrate.
After freeze–thaw cycles, extrusions through a membrane with
100 nm pores and size exclusion chromatography to remove the
external lucigenin, EYPC large unilamellar vesicles (LUVs) con-
taining lucigenin were prepared (for experimental details, see
the ESI†). A NaCl solution (100 mM) was added to the extra-
vesicular buffer to create a chloride gradient. Upon addition of
compounds 1–10, the fluorescence emission of intravesicular
lucigenin was measured over time.
Fig. 2 Evaluation of the ion transport activity of transporters 1–10 by
lucigenin⊂EYPC fluorescence assays. (A) All compounds and blank DMF,
with the concentration for each compound is 5 μM except for 9 is
7.5 μM, (B) 3, (C) 8, (D) 9 and (E) 10 in varied concentrations.
Effect of substituents on the ion transport activity
when OH groups in 3 are replaced with perfluorophenoxy (4)
or 3,5-dimethoxyl-1-triazinoxyl (5) the activity is negligible,
suggesting that the OH groups are probably essential to
mediate the ion transport. However, this speculation is argued
by the little activity observed for 6 and 7, which still contain
OH groups but the p-tolyl groups are replaced with –Cl and
–OMe respectively. Considering the p-tolyl groups are more
lipophilic than –Cl and –OCH3, it is envisioned that ion trans-
port occurs when both hydrogen bonding and lipophilicity
take effects. To verify this hypothesis, compound 8 with N,N-
dipropyl groups attached on the triazine rings was investi-
gated. 8 is more lipophilic than 3 as suggested by a longer
retention time (t = 20.57 vs. 19.69 min) on an ODS-HPLC
column (Table 1). Accordingly the obtained effective concen-
tration show that 8 is more active than 3 (EC50 = 2.56 vs.
4.31 μM).
The ion transport activity, as illustrated in Fig. 2, closely depends
on the structure of these compounds. For example, compounds
1 and 2, which have been previously reported as good anion-π
receptors,12a,b,14 show little ion transport activity. The lack of
activity for 1 probably owes to its poor lipophilicity and thus it
can’t be inserted into the lipid membrane, as indicated by pre-
cipitation that occurred when 1 was injected into the buffer solu-
tion. Compound 2, with enhanced lipophilicity by introduction
of the p-tolyl groups and no precipitation observed, still can’t
mediate the ion transport. In contrast, compound 3 with
hydroxyl groups attached on the lower rim of benzene rings
shows significant ion transport activity. Hill analysis of the dose–
response curve gives an effective concentration EC50 = 4.31 μM
and the Hill coefficient n = 2.9 (Table 1 and ESI†). According to
our previous results,13c with OH groups on the lower rim, 3 can
form cooperative hydrogen bonding and anion–π interactions
with chloride and lead to an enhanced binding affinity. This
outcome could contribute to the activity observed.
The influence of the electronic nature of the substituents
on ion transport activity was found to be significant. For
example, when electron donating groups such as p-OMe-
phenyl were used to replace the p-tolyl groups in 3, the result-
ing compound 9 shows decreased activity (EC50 = 8.23 μM).
However, with electron withdrawing groups such as p-CF3-
Encouraged by the transport activity observed for 3, the
influence of the macrocyclic host structure on transport
activity was further systematically evaluated. For example,
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Table 1 Summary of ion transport activity (EC50), Hill coefficient (n), OH chemical shift, triazine ring surface potential (E), Cl− association constant
(Ka) and retention time for 3, 8–10
EC50 a (μM)
n
1H NMRb (ppm)
E (kcal mol−1
)
Ka(1:1) (L mol−1
)
Retention timec (min)
3
8
9
10
4.31 0.60
2.65 0.62
8.23 0.68
0.43 0.19
2.9 1.1
1.9 0.7
2.8 0.7
1.5 0.7
10.90
10.36
10.86
11.00
20.1
15.8
18.3
27.8
2.4 × 104
3.6 × 103
1.2 × 104
3.6 × 104
19.7
20.6
16.9
20.4
a Effective concentration, the molar ratios of transporters to lipids are: 1 : 40 for 3, 1 : 60 for 8, 1 : 20 for 9, 1 : 400 for 10. b Measured in DMSO-d6.
c Determined by HPLC, ODS column, mobile phase is acetonitrile containing 0.1% formic acid.
phenyl, compound 10 gives increased activity with an EC50 DPPC at 25 °C suggests that the ion transport process is gov-
value as low as 0.43 μM, about one order of magnitude lower erned by a carrier mechanism (Fig. S9a†).
than 3. According to our previous report,12a the substituent
Then we performed a carboxyfluorescein (CF) release experi-
properties regulate the electron density of the triazine rings. ment to understand the transport process. CF is a self-
With electron withdrawing substituents the electron density of quenched dye at a high concentration and hence it is weakly
triazine decreases and accordingly anion–π interactions fluorescent when entrapped in the vesicles. Enhancement of
strengthen. As listed in Table 1 and Fig. S12,† the surface CF fluorescence could be detected if the transporters cause the
potential of the triazine rings increases from the electron membrane defects of LUVs and lead to the release of CF to the
donating to electron withdrawing substituents (from 9, 3 to diluted extravesicular buffer solution. As illustrated in Fig. S7,†
10). This clearly demonstrates the considerable substituent addition of transporters 3, 8–10 to the suspension of LUVs
effect on the π-deficiency of triazine rings. The association con- containing CF shows no fluorescence enhancement at all, indi-
stants between 9, 3, 10 and chloride (as tetrabutylammonium cating that the transporters do not induce the membrane
salt, in acetonitrile) are 1.2 × 104, 2.4 × 104 and 3.6 × 104 M−1 defects. To investigate whether the transport is through a Cl−/
−
respectively (Fig. S13–16†), following the order of the surface NO3 anti-port process, we prepared the vesicles containing
potentials of the triazine rings. As the hydrogen bonding lucigenin and sodium chloride. Sodium nitrate was added to
ability of the hydroxyl groups on the benzene rings is less the extravesicular buffer solution. In the control experiment,
affected by the substituents on the triazine rings (could be sodium sulfate was applied as the external salt solution. As
judged from the very similar OH chemical shifts, δ = 10.86, shown in Fig. S8,† in both cases the intensity of quenched luci-
10.90, 11.00 ppm for 9, 3, 10, respectively, Table 1), the genin is recovered with the release of chloride to the extra-
increased association constant hence mainly benefits from the vesicular buffer. Hence, the chloride efflux mediated by the
electron deficiency of triazine rings. These outcomes indicate transporters is independent of NO3− or SO42− contained in the
that the anion–π interaction probably makes one of the main external solution. As sulfate is an extremely hydrophilic anion
contributions to the efficient ion transport activity for 10. In and it is normally impossible to be transported from the
addition to the electron withdrawing nature, the hydrophobi- aqueous solution to the lipid bilayer membrane, the results
city of CF3 is also considerable (retention time = 20.37 min), thus indicate less possibility of a Cl−/NO3− anti-port exchange.
hence the contribution of lipophilicity to the high activity
could not be excluded.
The influence of the external cations (alkali metal) and
anions (Cl− and Br−) on the transport activity was further
Based on the above structure-dependent ion transport studied. As illustrated in Fig. S6,† the transport activity (EC50)
activity, we believe that hydrogen bonding, lipophilicity and of 3, 8–10 is both anion- and cation-dependent. For example,
anion–π interactions are all essential driving forces for mediat- the chloride transport is more active than bromide except for
ing effective ion transport. Although it’s hard to distinguish 9. For different cations, the activities of 3, 9 and 10 follow an
which contribution is dominant, these contributions should overall sequence of Li+ < Na+ < K+ < Rb+ ≈ Cs+. Noticeably, 8
take effect in a subtly balanced and cooperative way. Moreover, gives the almost reversed activity sequence. Although the exact
when control compounds 11 and 12 were used negligible ion reason is not very clear, one can speculate that it might be
transport activity was observed (Fig. S1†). This highlights the difficult for the larger cation to enter the cavity of 8 owing to
function of a cavity of this type of macrocyclic transporter.
the steric hindrance by the substituents. These outcomes indi-
cate that the ion transport is most probably through an X−/M+
ion-pair process (Fig. 3). We also investigated the ion transport
activity at pH 7.6 by taking compound 3 as a representative
example. In principle, a higher pH value would facilitate the
deprotonation of OH groups, and hence the enhancement of
binding with cations. If the ion transport is dominated
by cations, a decreased effective concentration should be
obtained at a higher pH value (pH = 7.6). However, as shown
Mechanism of the transport process
In order to investigate the transport mechanism, we first set
up an assay using vesicles prepared with dipalmitoyl phosphatidyl-
choline (DPPC). As DPPC has a gel to liquid crystalline
transition phase at 41 °C, transport assays at 45 °C and 25 °C
were performed respectively. The dramatically reduced trans-
port activity caused by the decrease of membrane fluidity of
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Fig. 3 Proposed schematic description of the ion-pair transport.
in Fig. S10,† the calculated EC50 values are almost the same at
pH 7.6 and pH 7.0, respectively (EC50 = 4.59 vs. 4.31 μM),
suggesting less possibility of cation-dominated ion-pair trans-
port. In combination with the aforementioned investigations,
we postulated the ion-pair transport entity as: the transporters
form a complex with the anion through cooperative anion–π
interactions and hydrogen bonding, the cation as the counter
ion forming a compact ion-pair with the complex (Fig. 3).
To verify our postulation and get deep insights into the ion
transport mechanism at the molecular level, we cultivated the
single crystals of the complexes of the compounds and halide
anions.‡ Fortunately, single crystals of the complex between 7
and bromide were obtained through slow evaporation of the
solvent. Although 7 shows no transport activity, its structural
similarity with the active transporters could provide detailed
information on the host–guest interactions. As revealed by the
crystal structure (Fig. 4A and B), 7 interacts with bromide
through cooperative anion–π interactions and hydrogen
bonding. Bromide locates over one of the triazine rings with
the distances to the plane and centroid of the triazine ring
being 3.266 Å (dBr1–plane) and 3.315 Å (dBr1–centroid) respectively,
forming typical non-covalent anion–π interactions. Meanwhile
the two hydroxyl groups interact with bromide through mul-
tiple hydrogen bonds. The tetraethylammonium cation locates
outside the cavity and shows a short contact with bromide and
oxygen atoms of one of the hydroxyl groups. Two such com-
plexes, with the aid of intermolecular C–H⋯Br hydrogen
bonds between OCH3 on the triazine rings and bromides, self-
assemble into a dimeric structure (2 + 2) (Fig. 4B). Based on
the crystal structure, we set up DFT modeling at the B3LYP/
6-31G* level to speculate the complex structure of 3 and NaCl.
DFT models give an energy favorable ion-pair complex
(Fig. 4D). The optimized model shows that 3 binds Cl−
through the aforementioned cooperative anion–π and hydro-
Fig. 4 Crystal structure of (A) the complex of
7 and tetraethyl-
ammonium bromide (Et4N+(7·Br−)), (B) dimeric structure, DFT optimized
structure of (C) side view and (D) top view of the complex of 3 and NaCl.
gen bonding interactions, meanwhile sodium as the counter
cation seats at the outer rim of the cavity and interacts with
one of oxygen atoms (dNa–O = 2.325 Å) through dipole-ion inter-
actions (Fig. 4C). It is worth addressing that the ion-pair
complex structure observed in both the crystal structure and
DFT calculation all support our hypothesized ion-pair trans-
port entity. Moreover, from the dimeric crystal structure and
the Hill coefficient (n) obtained around 2 from Hill analysis of
the dose–response curve, it is likely that the transporters
mediate the transport of ions through a 2 + 2 dimeric complex.
Conclusions
We have designed a series of oxacalix[2]arene[2]triazine deriva-
tives and studied their capabilities as ion transporters through
the double layer membrane of EYPC. The fluorescence assays
demonstrated that compounds 3, 8–10 can mediate the ion
pair transport and act as transmembrane carriers. Hydrogen
bonding, electron deficiency of the triazine rings, lipophilicity
and macrocyclic frameworks were revealed to be essential for
mediating ion transport. This work hence provides new dimen-
sions of heteracalixaromatics as a molecular model of ion
transporters or ion channels. Design of heteracalixaromatics-
based ion channels is underway in our laboratory.
Acknowledgements
‡Crystallographic data for 2([7·Br−]Et4
) (CHCl3) (C65H77Br2Cl3N14O24): Mr =
+
1704.58, monoclinic, space group P2(1)/n, a = 21.688(6), b = 17.152(4), c = 22.729(5)
Å, α = 90.00°, β = 115.043°, γ = 90.00°, V = 7660(3) Å3, T = 173(2) K, full-matrix
least-squares refinement on F2 converged to RF = 0.1495 [I > 2σ(I)], 0.1745 (all data)
We thank Prof. Stefan Matile and Dr Naomi Saikai from the
University of Geneva for their helpful discussions. This work
was financially supported by MOST (2013CB834504) and
NNSFC (21272239, 91427301, 21521002).
and Rw(F2)
CCDC 1432620.
= 0.3675 [I > 2σ(I)], 0.3798 (all data), goodness of fit 1.882.
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