Aminocalix[4]arene: The effect of pH on the dynamics of gate and portals on the hydrophobic cavity

Article abstract of DOI:10.1016/j.tetlet.2010.09.067

The pH of a solution shows a significant effect on the dynamics of the gate (formed by eight benzylic functions) and portal on the hydrophobic cavity of receptor. At pH 5.8 the gate closes and prohibits the entry of anionic guests. However, at pH 7.3 the gate opens and allows the entry of anionic guests into the hydrophobic cavity. It is the first time that anionic receptor efficiently recognizes anionic guests.

Full text of DOI:10.1016/j.tetlet.2010.09.067

Tetrahedron Letters 51 (2010) 6156–6160
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Aminocalix[4]arene: the effect of pH on the dynamics of gate and portals
on the hydrophobic cavity
Satish Balasaheb Nimse a, Keum-Soo Song b, Junghoon Kim b, Hyung-Sup Kim b, Van-Thuan Nguyen a,
Woon-Young Eoum b, Chan-Yong Jung b, Van-Thao Ta a, Taisun Kim a,
⇑
a Institute for Applied Chemistry and Department of Chemistry, Hallym University, Chuncheon 200-702, Republic of Korea
b Biometrix Technology, Inc., 202 BioVenture Plaza, Chuncheon 200-161, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
The pH of a solution shows a significant effect on the dynamics of the gate (formed by eight benzylic
functions) and portal on the hydrophobic cavity of receptor. At pH 5.8 the gate closes and prohibits
the entry of anionic guests. However, at pH 7.3 the gate opens and allows the entry of anionic guests into
the hydrophobic cavity. It is the first time that anionic receptor efficiently recognizes anionic guests.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 26 August 2010
Accepted 17 September 2010
Available online 7 October 2010
Keywords:
Calix[4]arene
Molecular recognition
Receptor
Host–guest systems
Gate and portals
Hydrophobic cavity
1. Introduction
In this Letter, we describe the dynamics of water-soluble amin-
ocalix[4]arene based receptor 1 at different pH. The gate of the
Water plays a pivotal role in many aspects of living organisms,1
such as for correct behavior of enzymes,2,3 enzyme–substrate
interactions,4 membranes, and DNA structure.5 In particular,
enzymes are complex machines whose functions are strictly con-
nected to their biological environment in which water molecules,
as a solvent, have a major role.6 The gates and portals on the cavity
of the receptor can be opened or closed for the entry of substrate
with specific stimulus.7 Solvent effects also play a crucial role in
controlling anion binding strength and selectivity.8 Anions are
ubiquitous throughout biological systems. They carry genetic
information (DNA is a polyanion) and the majority of enzyme–sub-
strates and co-factors is anionic.9
The design of selective receptors for anions requires that the
geometry and basicity of the anion and the nature of the solvent
medium be taken into account. Complementarity between the
receptor and anion is clearly crucial in determining selectivities.10
Water-soluble cyclophanes were the first examples with polar sol-
ubilizing groups.10 Receptors based on clefts,11 porphyrins,12 and
calixarenes/resocinarenes13,14 have followed. Examples of good
binding selectivity based on the pH effect for anions by the anionic
receptor in water remain elusive.15–17
receptor 1 closes at pH 5.8 and discriminates the anionic guests,
such as sulfonic acid and carboxylic acid derivatives. Whereas, at
pH 7.3 the gate of receptor 1 opens and allows the access of guests
to the cavity and shows strong binding.
Receptor 1 can be obtained independently from 2 or 3 through a
common intermediate 4, as shown in Scheme 1. Precursors 2 and 3
were prepared according to the previously reported method.18,19
As summarized in Scheme 1, precursor 2 was converted into the
corresponding octa-substituted derivative of aminecalix[4]arene
4 by reacting with methyl 4-(bromomethyl)benzoate following
route 1. In the other route, previously reported intermediate 3
was also reacted with the methyl 4-(bromomethyl)benzoate under
similar conditions to obtain intermediate 4. Interestingly, follow-
ing either route to obtain 4 the yield is in the range of 86.3–
88.5%. It is important to notice that being a single step reaction
route one is the high yielding and convenient path to synthesize
intermediate 4 or different derivatives.
Reaction of compound 4 with ethyl 2-bromoacetate allowed to
isolate intermediate 5 in 89.8% yield. Saponification of 5 with KOH
in ethanol and water mixture (2:1) resulted in receptor 1 as a
dodecapotassium salt in 95.4% yield (for the detail synthesis of
the compounds, please check the Supplementary data). The nega-
tively charged carboxylate functions found on the wide and narrow
rim of receptor 1 enabled its solubility in water at pH/pD = 5.8, 7.3
⇑
Corresponding author. Tel.: +82 33 258 6098; fax: +82 33 256 3421.
E-mail address: tskim@hallym.ac.kr (T. Kim).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.09.067

S. B. Nimse et al. / Tetrahedron Letters 51 (2010) 6156–6160
6157
Scheme 1. Synthesis of receptor 1. Reagents and conditions: (i) Br–C6H4–COOCH3, C6H5N, CH3CN; (ii) Br–C6H4–COOCH3, C6H5N, CH3CN; (iii) Br–CH2COOCH2CH3, K2CO3,
CH3CN, Ar, reflux, 8 h; (iv) aq KOH, EtOH/water (2:1).
at levels up to 50–100 mM. Compound 1 loses its solubility below
pH 5.4 in water.
Small changes in NMR spectra with change in pH through the
partial protonation dodeca acid functions may be related to the
dynamics of the arms (eight benzylic functions) on the wide rim
of aminecalix[4]arene nucleus to close or open the gate on the
hydrophobic cavity. To investigate the dynamics of arms on the
cavity, the molecular recognition experiments were conducted at
pD = 5.8 and 7.3 using the anionic, cationic, and neutral guests as
shown in Figure 2.
A 1 mM sample of 1 in D2O provides the NMR spectrum that has
sharp signals and shows the characteristics and symmetry ex-
pected for a time-averaged C4v conformation. The 1H NMR spec-
trum of compound 1 showed a typical AB pattern for methylene
bridge protons represented by two pairs of doublets at d = 3.13
and 4.54 ppm for the axial and equatorial protons, respectively.
This indicates that receptor 1 existed in a symmetrical cone confor-
mation at both pH/pD = 5.8 and 7.3. As can be seen in Figure 1 the
benzylic aromatic protons at d = 7.19 and 7.82 ppm shift slightly
upfield, while the calix[4]arene aromatic protons at d = 6.42 shift
downfield at pD = 5.8 as compared to that at pD = 7.3. The major
change in ortho protons indicates that at pD = 5.8 benzoate groups
(arms) come close to each other. In the upfield region, change is
seen only for the benzylic –CH2 protons, the peak at d = 4.44 ppm
shifts upfield at pD = 5.8 as compared to that of pD = 7.3 also con-
firms that the benzoate groups (arms) come close to each other,
associating the phenomenon of flexing of arms.
The benzylic aromatic protons on the wide rim of compound 1
show correlations with the aceto-acetate proton on the lower rim
at either pH, hence, the NOESY spectra of compound 1 did not
show any clue for the flexing of the arms. To find a clue, we
performed the NOESY experiment on compound 5 in CDCl3. Com-
pound 5 in its NOESY spectra shows the characteristic of opened
arms. The –CH3 in the benzoate function on the wide rim shows
a correlation with the –CH3 in the ethylacetate function on the nar-
row rim. This behavior of lipophilic compound 5 in organic solvent
gives a clue for the open arm conformation of hydrophilic com-
pound 1 in aqueous solution (See the Supplementary data). It
was of particular interest then to investigate the effect of pH/pD
on the flexing of arms. The phenomenon of flexing of arms was
not clear from the NOESY experiments, thus probing the effect by
the molecular recognition of anions by anionic receptor 1 was
inevitable.
Receptor 1 at pH 5.8 recognizes guests 11, 15–16, whereas at
pH 7.3 it recognizes guests 6, 7, 10–12, and 15–18. The 1:1 bind-
ing stoichiometry between receptor 1 and guest 6 is confirmed
by Job’s plot (see the Supplementary data). As shown in Figure
3A, the 1H NMR titrations revealed that the protons of the methyl
group para to the carboxylate moiety in guest 6 showed a max-
imum complexation-induced upfield shift (CIUS) of d = 1.01 ppm
upon complexation with receptor 1 at pH 7.3 and do not show
any change at pH 5.8. Similarly, the protons on the para position
to the functional groups in guests 7–18 showed maximum CIUS
upon recognition by receptor 1 at pH 5.8 or 7.3. The binding of
guests in this mode positions the guest’s functional group (anio-
nic, cationic, or neutral) in the proximity of the mouth of recep-
tor 1. It is noticeable that at pH 5.8 receptor 1 failed to recognize
guest 6, while shows strong recognition at pH 7.3. Receptor 1 at
pH 5.8 showed similar patterns of recognitions toward anionic
guests. These results indicate that at pH 5.8 the carboxylate func-
tions on the benzylic groups form a gate on top of the hydropho-
bic cavity of receptor 1, thus showing strong electrostatic
repulsion toward anionic guests. Whereas, the strong recognition
for anionic guests by receptor 1 at pH 7.3 indicates that the car-
boxylate functions on the benzylic groups on the top of the
hydrophobic cavity of receptor 1 are far enough from each other
making a room for the entry of anionic guests into the hydropho-
bic cavity. The recognition pattern of anionic guests by receptor 1
at pH 5.8 and 7.3 explains the flexing of arms on the hydrophobic
cavity.

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S. B. Nimse et al. / Tetrahedron Letters 51 (2010) 6156–6160
Figure 1. Partial 1H NMR spectra of receptor 1 at pD = 5.8 and 7.3.
Figure 2. Guests 6–18.
The association constants (Log K) obtained by 1H NMR titrations
functions closes the gate on the hydrophobic cavity and rejects
the entry of anionic guest molecules. The Log K of 3.4, 3.9, and
4.4 for complexes between receptor 1 and guests 11, 17, and 18
at pD = 5.8 indicates that, though now the gate is closed, the small
portals between the arms allow the entry of hydrophobic function
of ammonium guests as the ammonium function simultaneously
interacts with the carboxylate functions to loosen the grip of the
gate.
As shown in Figure 3A, receptor 1 does not interact with guest 6
at pD = 5.8, while it binds guest molecules at pD = 7.3 with the
Log K of 4.3. Guest 7 also follows the trend but with a reduced
Log K of 2.1. Receptor 1 not only shows a similar behavior toward
of 1 mM of guests with increasing concentration of receptor 1 from
0.008 to 2.5 mM at pD = 5.8 and 7.3 are presented in Table 1. It is
interesting to notice that at pD = 5.8 receptor 1 forms a complex
with guests 11, 16–18 and does not interact with guests 6–10
and 12–14. Whereas, at pD = 7.3 receptor 1 forms complexes with
all guests except 8, 9, 13, and 14.
Recently, we have reported that the cavity of aminoca-
lix[4]arene receptors has a preference for neutral and cationic
19b
guests, whereas, it refused to recognize the anionic guests.
Receptor 1 follows the same trend at pD = 5.8 indicating that
the flexing of arms having negatively charged carboxylate

S. B. Nimse et al. / Tetrahedron Letters 51 (2010) 6156–6160
6159
Figure 3. Partial 1H NMR spectra of complexes between receptor 1 and molecule of guests 6, 17, and 18 at pD = 5.8 and 7.3. (A) (a) guest 6, (b) receptor 1 at pD = 5.8, (c)
receptor 1 and guest 6 (2:1) at pD = 5.8, (d) receptor 1 at pD = 7.3, (e) complex between receptor 1 and guest 6 at pD = 7.3; (B) (a) guest 17, (b) receptor 1 at pD = 5.8, (c)
complex between receptor 1 and guest 17 (1:1) at pD = 5.8, (d) receptor 1 at pD = 7.3, (e) complex between receptor 1 and guest 17 (1:1) at pD = 7.3; (C) (a) guest 18, (b)
receptor 1 at pD = 5.8, (c) complex between receptor 1 and guest 18 (1:1) at pD = 5.8, (d) receptor 1 at pD = 7.3, (e) complex between receptor 1 and guest 18 (1:1.5) at
pD = 7.3. ( ) and (N) indicates upfield shift of methyl and methylene protons, respectively, after complexation, (d) indicates free guest.
guests 10 and 12 but also shows a preference for sulfonate deriva-
tives. The preference for sulfonates can be assigned to the tripodal
symmetry of sulfonate function instead of dipodal in carboxylate
and its electron withdrawing effect. The tripodal symmetry gives
extra room for negative charges of guest molecules on the cavity
of receptor 1 reducing the electrostatic repulsion. The electron
withdrawing effect prevails and increases the
p–p stacking inter-
actions between the guest and receptor. The recognition of anionic

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S. B. Nimse et al. / Tetrahedron Letters 51 (2010) 6156–6160
Table 1
Log K values of complex formation of guests 6–18 by receptor 1 in D2O at pD = 5.8 and 7.3; 25 °Ca
Guests
6.0
7.0
8.0
9.0
10
11
12
13
14
15
16
17
18
pD/pH = 5.8
pD/pH = 7.3
Ns
4.3
Ns
2.1
Ns
Ns
Ns
Ns
Ns
2.4
3.4
5.1
Ns
2.7
Ns
Ns
Ns
Ns
2.9
3.1
2.4
3.5
3.9
4.8
4.4
4.9
Ns indicates no change in chemical shift of protons in guests upon addition of receptor.
a
The guest concentration was kept constant (1 Â 10À3 M) while the receptor concentration was varied from 8 Â 10À4 to 2.5 Â 10À3 M and the chemical shifts of the protons
of guest were recorded at each concentration. The obtained 1H NMR data were analyzed by the nonlinear least square regression analysis, which allowed the calculation of
association constant (Log K).
guests only at pD = 7.3 pinpoints that receptor 1 opens the gate by
straitening the arms, while at pD = 5.8 it flexes the arms to close
the gate. Noticeably, the guests which do not bind with receptor
1 at either pD are 8, 9, 13, and 14. Guests 13 and 14 were barred
from the cavity because of the steric effect, as their size is unsuit-
able to fit into the cavity of receptor. As that of guest 6 and 7,
guests 8 and 9 should interact with receptor 1 but receptor 1 failed
to recognize them. The –CH3 in ethyl can interact with cavity, but
by doing so it tilts the rest of the guest 109° putting the negative
charges around the carboxylate functions on the opened arms of
receptor 1 giving strong electrostatic repulsion at pD = 7.3.
Guests 15 and 16 being neutral showed strong binding with
receptor 1 at both pD with a preference at pD = 7.3. This is also
an indication of the opened arms at pD = 7.3, whereas at pD = 5.8
due to closed arms the steric hindrance prevails and leads to the
reduced Log K.
Technology Foundation (KOTEF) through the Human Resource
Training Project for Regional Innovation.
Supplementary data
Supplementary data (experimental procedure and 1H, 13C NMR
spectra) associated with this article can be found, in the online ver-
sion, at doi:10.1016/j.tetlet.2010.09.067.
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We are working on the development of aminocalix[4]arene
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Acknowledgments
This research was financially supported by the Ministry of
Education, Science and Technology (MEST) and Korea Industrial