Synthesis of a water-soluble pillar[6]arene dodecaamine and its selective binding of acidic amino acids in water

Article abstract of DOI:10.1016/j.tetlet.2017.10.025

A water-soluble pillar[6]arene dodecaamine has been synthesized. ¹H NMR and fluorescence studies indicate that pillar[6]arene dodecaamine could selectively and strongly bind acidic amino acids, i.e. glutamic acid and aspartic acid in water. And

Full text of DOI:10.1016/j.tetlet.2017.10.025

Tetrahedron Letters xxx (2017) xxx–xxx
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of a water-soluble pillar[6]arene dodecaamine and its selective
binding of acidic amino acids in water
Qunpeng Duan a, , Wenjie Zhao , Kui Lu
⇑
b
a,b,
⇑
a
School of Material and Chemical Engineering, Henan University of Engineering, Zhengzhou 451191, PR China
School of Chemistry and Chemical Engineering, Henan University of Technology, Zhengzhou 450001, PR China
b
a r t i c l e i n f o
a b s t r a c t
1
Article history:
A water-soluble pillar[6]arene dodecaamine has been synthesized. H NMR and fluorescence studies indi-
cate that pillar[6]arene dodecaamine could selectively and strongly bind acidic amino acids, i.e. glutamic
acid and aspartic acid in water. And the complexation behavior of pillar[6]arene dodecaamine towards
Received 18 August 2017
Revised 30 September 2017
Accepted 10 October 2017
Available online xxxx
acidic tripeptide glutathione and short chain length (C
investigated.
3
to C
8
) dicarboxylic acids in water is also
Ó 2017 Published by Elsevier Ltd.
Keywords:
Supramolecular chemistry
Pillararenes
Recognition
Amino acids
Host–guest complex
The recognition of biologically relevant substrates with syn-
thetic receptors is a topic of great interest in supramolecular chem-
istry.1 In this respect, numerous efforts have been devoted to the
design and synthesis of macrocyclic receptors with specific proper-
ties and functions revealing their affinity and selectivity towards
acid and aspartic acid in specific regions of the brain are closely
1
2
related to Parkinson’s disease. Therefore, the selective recogni-
tion of acidic amino acids has a wide range of potential applica-
tions in the field of biomedicine. At present there were many
reports on selective recognition of basic amino acids by anionic
2
3
8a,13
biologically relevant molecules. Along with the crown ethers,
macrocycles.
However, to the best of our knowledge, the
cyclodextrins, calixarenes5 and cucurbibituril,6 the pillararenes
4
pillararene-based selective acidic amino acids recognition in
water has not been reported yet. Herein, we report the facile
synthesis of a new water-soluble pillar[6]arene dodecaamine 1
are one of the most important categories of macrocyclic receptors.
7
Pillararenes, as a new class of synthetic macrocycles, have
received much attention owing to its intriguing and versatile
host–guest binding properties and many potential applications in
chemistry, biology and materials science.8 Pillararene-based
molecular recognition in organic media has been mostly investi-
gated due to the inherent poor solubility of pillararene analogues
and its selective binding of acidic amino acids, i.e.,
acid (Glu) and -aspartic acid (Asp) in water. Furthermore, its
molecular recognition with acidic tripeptide glutathione (GSH)
L-glutamic
L
and short chain length (C
investigated.
3 8
to C ) dicarboxylic acids were also
8c,j,9
10
in aqueous medium. To combat that, anionic,
cationic and
The synthetic approach depicted in Scheme 1 outlines the
preparation of pillar[6]arene dodecaamine. Compound 4 was first
prepared from commercially available 1,4-bis(2-hydroxyethoxy)
benzene according to previously reported procedure.1
Treatment of 4 and metaformaldehyde with boron trifluoride
diethyl etherate in chloroform at room temperature gave 3 in
25% yield. Pillar[6]arene derivative 3 was then reacted with
sodium azide in N,N-dimethylformamide (DMF) at 100 °C to
produce 2 in 89% yield. Palladium-catalyzed hydrogenation of 2
in methanol at 50 °C afforded 1 as a colourless solid in 99% yield.
1
1
neutral water-soluble pillararenes have been synthesized and
shown to act as scaffolds to various cationic, anionic and biologi-
cally significant guests.
0a
Amino acids are the basic structural building blocks of proteins
and other biomolecules. The acidic amino acids, glutamic acid and
aspartic acid, play critical roles in the central nervous system as
excitatory neurotransmitters. Concentration changes of glutamic
1
4
⇑
Corresponding authors at: School of Material and Chemical Engineering, Henan
University of Engineering, Zhengzhou 451191, PR China.
The structure of pillar[6]arene dodecaamine 1 was confirmed
1
13
by H NMR, C NMR and high-resolution mass spectrometry
(HRMS).
E-mail addresses: qpduan@haue.edu.cn (Q. Duan), luckyluke@haue.edu.cn (K.
Lu).
https://doi.org/10.1016/j.tetlet.2017.10.025
0
040-4039/Ó 2017 Published by Elsevier Ltd.
Please cite this article in press as: Duan Q., et al. Tetrahedron Lett. (2017), https://doi.org/10.1016/j.tetlet.2017.10.025

2
Q. Duan et al. / Tetrahedron Letters xxx (2017) xxx–xxx
Fig. 2. Partial 2D NOESY NMR spectrum of Gluꢀ1 (400 MHz, D
time = 300 ms), [1] = 1.00 mM. [Glu] = 1.00 mM.
2
O, 298 K, mixing
Scheme 1. Synthesis of water-soluble neutral pillar[6]arene dodecaamine 1 and
structural illustration of Glu, Asp, GSH and short chain length (C
acids.
3 8
to C ) dicarboxylic
(Fig. S8) and similar upfield shifts along with peak broadening
were observed for the Asp protons (H1–2) compared to free Asp,
indicating
Furthermore, NOE correlations were observed between aromatic
proton H of 1 and methylene protons H1-2 of Asp, confirming that
these protons were located in the cavity of 1 to form an interpen-
etrated geometry (Fig. S12). Next, the host–guest complexation of
a strong threaded host–guest complex formation.
The host–guest complexation between the host 1 and native
-amino acids was then investigated by H NMR spectroscopy.
1
L
-
a
a
As shown in Fig. 1b, the 1:1 mixture of 1 and Glu in D
pD = 6.0) had substantial upfield shifts and broadening effects
for the Glu protons (H1–3) compared to free Glu (
2
O
(
D
d = À0.14 to
1
with other 17 naturally occurring amino acids was also per-
1
À0.28 ppm) (Fig. 1c), indicating a strong threaded host–guest com-
plex formation. And the presence of only one set of peaks for the
solution of 1 and Glu (Fig. 1b) suggests the host–guest complex for-
mation is a fast exchange process on the NMR time scale. The
formed in D
S10). The corresponding chemical shift changes (
2
O (pD 6.0) by H NMR spectroscopy (Figs. S9 and
d) of host 1 pro-
D
tons in the presence of amino acids guests were listed in Table S1.
As shown in Table S1, there were observed no noticeable chemical
1
downfield shift of aromatic proton H
a
of 1 (Fig. 1b) compared to
shift changes in their H NMR spectra when 1.0 equiv. of other 17
free 1 ( d = +0.10 ppm) (Fig. 1a), caused by deshielding gives an
D
naturally occurring amino acids were added (Table S1), respec-
additional support for interpenetrated complex formation. The
host–guest interaction in water can be further confirmed by 2D
NOESY, which has a maximal observation limit at a spatial proxim-
tively, which indicated that among 20 native
acidic amino acids, Glu and Asp, could effectively bind to the pillar
6]arene dodecaamine to form deep inclusion complexes.
According to the pK values of the branched carboxyls of the two
acidic amino acids (Glu: 4.25; Asp: 3.65) and the pI (isoelectric
point) values of the two acidic amino acids (Glu: 3.22; Asp:
L-a-amino acids, only
[
1
1
5
ity of 5 Å. From the 2D NOESY spectrum of a solution of 1 and Glu
Figs. 2 and S11), intermolecular correlations were observed
between aromatic proton H of 1 and methylene protons H1–3 of
a
(
a
Glu, which also confirmed the interpenetrated geometry.
Subsequently another acidic amino acid Asp was studied for
host–guest complex formation with 1. A 1:1 mixture of 1 and
2
.77), it can be concluded that both the branched carboxyls and
the -carboxyls of the two acidic amino acids should be in the
deprotonated form at pH 6.0. Conversely, the -amino groups of
the two acidic amino acids should be in the protonated form
a
a
Asp in D
2
O (pD = 6.0) was examined by 1H NMR spectroscopy
+
(
3
NH ) at pH 6.0. Therefore, we deduce that the interaction mecha-
nism of 1 with the two acidic amino acids is that the acidic amino
acids with two carboxylate anions could bind positively charged 1
bearing ammonium groups in aqueous solutions at pH 6.0, where
the cooperative electrostatic attraction forces between two car-
boxylate anions of the two acidic amino acids and two cationic
portals of the host 1 play a dominant role in the present host–guest
complexation. In addition to electrostatic interactions, it seems
also reasonable to assume that other noncovalent interactions,
1
6
interactions between the cavity-included NH+
3
such as cation–
p
groups of the two acidic amino acids and aromatic rings of 1, as
.
. . 17
well as N–H
p
interactions between ammonium hydrogen
-plane of 1, may con-
atoms of the two acidic amino acids and
p
tribute to the stabilization of these inclusion complexes. We also
1
recorded the H NMR spectroscopy of Glu in the absence and in
the presence of 1 in water at pH 4.0 (Fig. S14). Differing from the
case at pH 6.0, no obvious chemical shift changes of Glu protons
were found except a slightly downfield shift of H
pared to free Glu at pH 4.0, just caused by electrostatic attraction
force between negatively charged -carboxylate anion and
1
(Fig. S14b) com-
Fig. 1. 1H NMR spectra (400 MHz, D
.00 mM Glu, and (c) 1.00 mM Glu at pD 6.0.
O, 293 K) of (a) 1.00 mM 1, (b) 1.00 mM 1 +
2
a
1
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Q. Duan et al. / Tetrahedron Letters xxx (2017) xxx–xxx
3
positively charged 1. The difference can be explained by the fact
that the branched carboxylic group of Glu is in the protonated form
at pH 4.0, which demonstrated that the above-mentioned cooper-
ative electrostatic attraction forces are the decisive driving forces
in these inclusion complexes.
Investigations of the host–guest complexation between 1 and
the two acidic amino acids employing Job plots18 (Figs. S15 and
S17) using UV–vis absorption data indicated the formation of 1:1
complexes. The quenching of fluorescence intensity (Figs. S16
and S18) was found to be significant enough that we could quanti-
tatively measure (Table 1) the binding behaviors of Glu and Asp
with host 1. By using the Benesi-Hildebrand equation,1 the associ-
9
6
À1
ation constants (K
a
) were calculated to be (1.00 ± 0.01) Â 10 M
6
À1
and (0.98 ± 0.03) Â 10 M
Figs. S16 and S18).
To extend the scope of the host properties and to elucidate the
for Glu and Asp, respectively
(
affinity and selectivity of 1, the binding of 1 to an acidic tripeptide
containing the Glu residue (GSH) and aliphatic dicarboxylic acids
of different lengths (DA-n, n: carbon number, n = 3–8) was further
Fig. 3. 1H NMR spectra (400 MHz, D
.00 mM GSH, and (c) 1.00 mM GSH at pD 6.0.
O, 293 K) of (a) 1.00 mM 1, (b) 1.00 mM 1 +
2
1
1
examined. Fig. 3 shows the H NMR spectra of the 1:1 mixture of 1
2
and GSH in D O (pD = 6.0), from which the noticeable upfield
acids in water. This water-soluble pillar[6]arene exhibits strong
chemical shifts of GSH proton resonances H1–6 were observed upon
addition of 1 due to the shielding effect of the electron-rich cavities
of 1 for GSH, indicating the inclusion of GSH into 1 cavity. Besides,
binding affinities towards acidic amino acids, i.e., Glu and Asp
6
À1
(
with the K
a
values in the order of magnitude of 10 M in water)
-amino acids. The cooperative electrostatic interac-
against other
a
a
NOE correlations were observed between aromatic proton H of 1
tions between two carboxylate anions of the two acidic amino
acids and two cationic portals of the host 1 are the decisive driving
forces in these electrostatic inclusion complexes. Additionally,
and methylene protons H1–6 of GSH, confirming that GSH threaded
through the cavity of 1 (Fig. S13). The stoichiometry for the com-
plex of 1 with GSH was confirmed to be 1:1 in solution by Job plots
cation–
p
and N–HÁ Á Á
p interactions between cationic ammonium
(
a
Fig. S19). Additionally, The K value of GSH with 1 was calculated
6
À1
sites of both the two acidic amino acids and aromatic rings of 1
may also play an important role in the host–guest binding. Further
studies of the complexation of 1 with tripeptide GSH and short
to be (1.20 ± 0.50) Â 10 M
by using curve-fitting analysis
(Fig. S21). Similar to acidic amino acids, DA-n (n = 3–8) also form
1
deep inclusion complexes with 1, which can be confirmed by
H
chain length (C
able to form inclusion complex with both GSH and short chain
length (C to C ) dicarboxylic acids. Compared with Asp and Glu,
short chain length (C to C ) dicarboxylic acids guests have a lower
affinity (with the K values in the order of magnitude from 10 to
0 M ) with the 1 host. These selective recognitions of 1 toward
3 8
to C ) dicarboxylic acids show that 1 can also be
NMR spectra of DA-n (n = 3–8) in D O (pD = 6.0) recorded in the
2
absence and in the presence of 1.0 equiv. of the host (Figs. S24–
S29). But their binding abilities are considerably different.
Compared with Asp and Glu, DA-4 and DA-5 (both without
the ammonium moiety) have a dramatically lower affinity with
the host 1, respectively (Table 1). This may be attributed to the
3
8
3
8
3
a
4
À1
1
acidic amino acids may provide reference in design amino acid
molecular machines as well as biological applications.
additional cation–
acidic amino acids and 1. Additionally, the K
n = 3–8) with 1 which were calculated by using curve-fitting
analysis (Fig. S23) increase with increasing chain length of the
DA-n (Table 1). This hike in K values might be explained by the
p
and N–HÁ Á Á
p
interactions between the two
a
values of DA-n
(
Acknowledgments
a
increase in the number of methylene groups resulting in greater
hydrophobic effect.
This work was supported by the National Natural Science Foun-
dation of China (Nos. 201402040, 21572046 and 21675040).
In conclusion, we have successfully synthesized a new water-
soluble pillar[6]arene dodecaamine 1, and investigated the inclu-
sion complexation between 1 and 20 naturally occurring amino
A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at https://doi.org/10.1016/j.tetlet.2017.10.025.
Table 1
Association constants (K
a
a
) for 1:1 inclusion complexa-
tion of the guests with host 1 at 298 K.
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Please cite this article in press as: Duan Q., et al. Tetrahedron Lett. (2017), https://doi.org/10.1016/j.tetlet.2017.10.025