A polyoxapolyaza macrobicyclic receptor for the recognition of zwitterions

Article abstract of DOI:10.1039/c2ob25725d

A polyoxapolyaza heteroditopic macrobicyclic compound (btpN ₄O₃) was synthesized. The acid-base behaviour of the compound as well as its binding ability for zwitterionic amino acids were studied by potentiometry at 298.2 ± 0.1 K in H₂O-MeOH (50:50 v/v) and at I = 0.10 ± 0.01 M in NMe₄TsO. The H _nbtpN₄O₃ⁿ⁺ showed preference for amino acids containing tetrahedral anionic groups.

Full text of DOI:10.1039/c2ob25725d

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COMMUNICATION
A polyoxapolyaza macrobicyclic receptor for the recognition of zwitterions†
Pedro Mateus,a Rita Delgado,*a Paula Brandãob and Vítor Félixc
Received 13th April 2012, Accepted 19th June 2012
DOI: 10.1039/c2ob25725d
Another possible approach for binding zwitterionic species in
more competitive aqueous solutions consists in developing
heteroditopic macrobicyclic compounds in which the anion
binding sites are positively charged ammonium groups and the
cation binding sites are ether groups. Macrobicyclic compounds
offer the possibility to encapsulate substrates within their well
defined cavities and allow the placement of several binding units
in strategic points of the molecule for multiple and cooperative
binding.6 It was shown that tris(2-aminoethyl)amine (tren)
capped macrobicyclic polyammonium receptors strongly bind
anions in aqueous media by combining electrostatic and hydro-
gen bonding interactions,6c,d and they were particularly useful in
the binding of carboxylates.4e In addition, it was recently demon-
strated that a new family of benzene capped polyammonium
cryptands are also useful for the binding of anions in aqueous
solutions.7 On the other hand, triethanolamine capped and
benzene capped macrobicyclic receptors have been shown to
bind ammonium,8 amino alcohols and amino acids.9 Thus it was
intended to join these two types of compounds to produce
“hybrid” polyoxapolyaza cryptands which were believed to be
suitable for the recognition of the amino acids depicted in
Scheme 1.
Mixed polyoxapolyaza macrobicyclic receptors have been first
prepared by Bharadwaj et al.10 and only recently their ability to
bind anions has been evaluated.11 However the mixed anion–
cation encapsulation has never been attempted in this class of
molecules, perhaps due to their relatively small cavities. Herein a
new mixed polyoxapolyaza macrobicyclic compound is
described, with a larger cavity than the ones previously reported
by Bharadwaj et al., and its ability to bind zwitterionic amino
acids in a highly competitive aqueous solution is evaluated.
The synthesis of heteroditopic macrobicyclic compounds
requires two critical steps: formation of a tripodal intermediate
and macrobicyclization through a [1 + 1] “tripod–tripod
A polyoxapolyaza heteroditopic macrobicyclic compound
(btpN4O3) was synthesized. The acid–base behaviour of the
compound as well as its binding ability for zwitterionic
amino acids were studied by potentiometry at 298.2 0.1 K
in H2O–MeOH (50 : 50 v/v) and at I = 0.10
NMe4TsO. The HnbtpN4O3 showed preference for amino
acids containing tetrahedral anionic groups.
0.01 M in
n+
Amino acids are very important anion containing substrates, not
only for their use as building blocks of proteins but also for their
role in the metabolism, neurotransmission and biosynthesis.1
Thus, the design of artificial receptors for the selective recog-
nition and sensing of amino acids in aqueous solutions may have
important applications. Consequently, over the last thirty years
supramolecular chemists have strived to design receptors selec-
tive for amino acids.2 However, this task has proven extremely
challenging due to the intrinsic characteristics of amino acids.
The zwitterionic nature of the amino acids at physiological pH
requires both the existence of a cation and an anion binding site
in the receptor, which from a synthetic point of view is not a
trivial task. With this in mind, much work in the area has
focused on the recognition of derivatized amino acids in which
either the carboxylate or the ammonium group is protected.3
Moreover, strong binding of zwitterions in aqueous solutions is
hampered by the high energetic cost of desolvation of the double
ion. Indeed, few receptors were conceived to work in aqueous
solutions.4
Very recently two heteroditopic hemicryptophanes were
shown to be able to encapsulate zwitterionic amino acids in up
to 20% water in MeCN medium through a combination of
hydrogen bonding, cation–π and anion–π interactions.5
aInstituto de Tecnologia Química e Biológica, Universidade Nova de
Lisboa, Av. da República, 2780-157 Oeiras, Portugal.
E-mail: delgado@itqb.unl.pt; Tel: (+351) 214469737
bDepartamento de Química, CICECO, Universidade de Aveiro,
3810-193 Aveiro, Portugal
cDepartamento de Química, CICECO, and Secção Autónoma de
Ciências da Saúde, Universidade de Aveiro, 3810-193 Aveiro, Portugal
†Electronic supplementary information (ESI) available: Experimental
details; NMR (1H, 13C, COSY, NOESY, and HMQC), ESI-mass spectra
of btpN4O3; table with the protonation constants of btpN4O3 and of the
amino acids; table with overall and stepwise association constants
(log βH L A ). Crystal data of btpN4O3 together with the refinement
h
l
a
details are also given. CCDC 877715. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c2ob25725d
Scheme 1 Target zwitterionic substrates.
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coupling” strategy.12 The synthetic strategy requires a bifunc-
tional building block in which one of the functionalities is pro-
tected while the other is available to react and produce tripodal
compounds that are the precursors of the desired heteroditopic
macrobicyclic receptors. We have previously reported the use of
this strategy and 4-(diethoxymethyl)benzaldehyde as the bifunc-
tional building block to prepare a polyaza macrobicyclic com-
pound analogous to btpN4O3 in 79% yield.13 btpN4O3, being a
mixed polyoxapolyaza macrobicyclic compound, required the
use of (4-diethoxymethylphenyl)methanol as the bifunctional
building block and a slightly different synthetic procedure, as
outlined in Scheme 2.
The protonation constants of btpN4O3 were determined by
potentiometry in H2O–MeOH (50 : 50 v/v) solution at 298.2 K
and ionic strength 0.10 M in NMe4TsO. The results are collected
in Table S2 in the ESI† and the corresponding species distri-
bution diagram represented in Fig. 2.
The (4-diethoxymethylphenyl)methanol (2) has an acetal-
protected aldehyde group and an OH group available to react
with 1,3,5-tris(bromomethyl)-2,4,6-triethylbenzene (1) in a Wil-
liamson ether synthesis procedure. After deprotection of the
aldehyde groups the trialdehyde tripodal intermediate (3) was
obtained and reacted with tren in a Schiff-base condensation
cyclization step.
Surprisingly, with the same reaction conditions used to
prepare the polyaza analogue (reagents at 5.6 mM in MeCN
at r.t.) only polymeric materials were obtained in the attempted
synthesis of btpN4O3. Since the only difference in the synthetic
strategy is the use of a 2,4,6-triethylbenzene derived tripodal
intermediate in the synthesis of btpN4O3 instead of a tren
derived one in the synthesis of the polyaza analogue, it seems
that the cyclization reaction requires a certain degree of flexi-
bility of the tripodal intermediate. Indeed it has been pointed out
that the rigidity of the 2,4,6-triethylbenzene scaffold requires the
use of suitably preorganized spacers, namely meta-substituted
aromatic units, in the synthesis of macrobicyclic compounds.7b,14
This is why there are fewer 2,4,6-triethylbenzene derived macro-
bicyclic compounds reported in the literature when compared
with the tren derived ones. In order to circumvent this problem
the cyclization was tried in refluxing MeCN and in ten times
more diluted conditions. With this method btpN4O3 was
obtained after sodium borohydride reduction in 40% yield.
The single crystal X-ray diffraction determination of btpN4O3
revealed that one acetonitrile molecule is hosted in the macrobi-
cyclic cavity as shown in Fig. 1. The three independent N–H⋯N
hydrogen bonds between the acetonitrile and secondary amine
binding groups of btpN4O3 are established with N⋯N distances
of 3.27(2), 3.30(2) and 3.40(2) Å and the N–H⋯N correspond-
ing angles of 167.4(14)°, 162.8(15)° and 160.1(14)°, respect-
ively (for crystal data and refinement details, see Table S1 of
ESI†).
Fig. 1 X-ray single crystal structure of btpN4O3 showing an MeCN
solvent molecule encapsulated in the macrobicyclic binding pocket.
Hydrogen bonds are shown as dashed lines and the C–H hydrogen
atoms of the receptor, apart from the N–H binding sites, have been
omitted for clarity.
Fig. 2 Species distribution diagram of the protonation of btpN4O3 in
H2O–MeOH (50 : 50 v/v) solution at 298.2 K and 0.10 M NMe4TsO.
CbtpN O = 1.0 × 10−3 M.
4
3
Scheme 2 Synthetic procedure of bptN4O3.
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H2amp > H2aep ≈ Htau > Hbala ≈ Hgly > Hgaba (Fig. 3). Inter-
estingly, the receptor has lower affinity for amino acids contain-
ing a carboxylate group than for amino acids with a tetrahedral
anionic group. This suggests that the latter are preferred due to
the 3-fold symmetry of their anionic group which should be
complementary to the tren subunit of the cryptand17 and possibly
due to the formation of a higher number of hydrogen bonds.
In conclusion, a polyoxapolyaza heteroditopic macrobicyclic
compound was synthesized through a [1 + 1] “tripod–tripod
coupling” strategy to be used as a receptor for the recognition of
amino acids. The binding studies showed that the protonated
receptor binds the zwitterionic amino acid substrates through a
combination of hydrogen bonding, electrostatic interactions and
possibly cation–π interactions with association constants in the
range of 1.63–3.21 log units, the highest values in comparison
with reported ones determined under identical experimental con-
ditions (H2O–MeOH solutions). The receptor showed a prefer-
ence for amino acids containing a tetrahedral anionic group due
to the 3-fold symmetry of their anionic moiety, complementary
to the tren subunit of the cryptand.
Fig. 3 Plot of Keff versus pH for the associations formed between the
indicated amino acids and HnbtpN4O3n+, in H2O–MeOH (50 : 50 v/v)
solution at 298.2 K and 0.10 M NMe4TsO.
The acid–base behaviour of btpN4O3 is straightforward: three
protonation constants were found in the working pH region
(3.0–10.0), corresponding to the successive protonations of the
secondary amines. The tertiary amine is very acidic due to the
nearby presence of three positive charges, hence its protonation
constant cannot be obtained in the working pH region. As shown
in Fig. 2, the fully protonated form of the receptor,
H3btpN4O33+, exists as the main species below pH ≈ 6.2. This
receptor species exhibits a compartment with three protonated
amines to interact with the anionic group of the amino acids
through electrostatic interactions and hydrogen bonding. The
other compartment has three ether oxygen atoms available to
accept hydrogen bonds from the ammonium group of the sub-
strates and an aromatic unit which may contribute to the esta-
blishment of cation–π interactions.
Acknowledgements
The NMR spectrometers are part of the National NMR Network
and were purchased in the framework of the National Program
for Scientific Re-equipment, contract REDE/1517/RMN/2005,
with funds from POCI 2010 (FEDER) and Fundação para a
Ciência e a Tecnologia (FCT). M. C. Almeida from the Elemen-
tal Analysis and Mass Spectrometry Service at the ITQB is
acknowledged for providing elemental analysis and ESI-MS
data. Pedro Mateus thanks FCT for the grant (SFRH/BD/36159/
2007).
The association constants of the protonated forms of btpN4O3
with several amino acids were determined by potentiometry in
H2O–MeOH (50 : 50 v/v) solution at 298.2 K and 0.10 M
NMe4TsO. The values obtained are collected in Table S3 of the
ESI.†
Notes and references
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