C3v symmetric receptors show high selectivity and high affinity for phosphate

Article abstract of DOI:10.1021/ja021390n

The binding of phosphate to tripodal metalloreceptors 1 and 2 is reported. Receptors 1 and 2 are C_3v symmetric, designed to complement three sides of a tetrahedron. The receptors derive from tripodal ligands that are preorganized through binding to a central Cu(II) atom. These metalloreceptors demonstrate high selectivity and affinity for the molecular recognition of phosphate in aqueous media at neutral pH. The binding of phosphate and other anions to the cavities of receptors 1 and 2 was monitored by UV/vis titration techniques. Binding algorithms were used to determine the affinity of phosphate to 1 and 2 with association values (K_a) of 2.5 × 10⁴ and 1.5 × 10⁴ M^-1, respectively. Copyright

Full text of DOI:10.1021/ja021390n

Published on Web 03/18/2003
C3v Symmetric Receptors Show High Selectivity and High Affinity for
Phosphate
Suzanne L. Tobey, Benjamin D. Jones, and Eric V. Anslyn*
Department of Chemistry and Biochemistry, UniVersity of Texas, Austin, Texas 78712
Received November 30, 2002; E-mail: anslyn@ccwf.cc.utexas.edu
Table 1. Binding Affinities Determined for a Series of Anions
Using 1 and the Control Hosts 3 and 4
The design of synthetic receptors for the purpose of binding
anions in water with high selectivity and affinity is one of the goals
of current molecular recognition pursuits.1 Inorganic anions provide
a variety of interesting targets due to their differences in size, shape,
and charge. Examples of host/guest associations between synthetic
receptors and anionic guests such as acetate, nitrate, chloride,
fluoride, carbonate, sulfate, and phosphate have been reported.2
Many of the receptors for phosphate display moderate to high
affinity constants in organic media and low affinities in aqueous
media. Lehn,3 Bazzicalupi,4 and Beer5 have reported host/phosphate
affinities in water as high as 105 M-1 at low pH, but the selectivity
of these polyamine hosts for phosphate over other anions was either
not reported or was low. Recently, Kim reported a sensor ensemble
with high affinity and selectivity for phosphate in water.6 However,
the lack of a general strategy to create effective receptors for
phosphate in water provides the impetus for the work described
herein. We report a specific design principle to target tetrahedral
oxyanions.
receptor
aniona
stoichiometry anion:1
binding constantsb (M-1
)
2-
1
1
1
1
1
1
1
1
3
3
4
HPO4
1:1
1:1
1:1
1:1
1:1
2:1
2:1
n.d.
1:1
1:1
1:1
2.5 × 104 (( 6 × 102)
2.5 × 104 (( 6 × 102)
2.0 × 103 (( 7 × 102)
<900
2-
HAsO4
-
ReO4
AcO-
-
NO3
<20
-
HCO3
n.d.c
n.d.
n.d.
Cl-
SO4
HPO4
-2
2-
8.0 × 103 ((7 × 102)
9.0 × 103 ((7 × 102)
9.0 × 102 ((3 × 102)
2-
HAsO4
2-
HPO4
a All counterions to the anions are sodium. b Ka values were determined
from a curve fit analysis to UV/vis data collected as aliquots of analyte
solution were added to an aqueous solution of 1 at pH 7.4 at 25 °C. c n.d.
) not able to determine.
The cavities we designed are complementary to three faces of a
tetrahedron (eq 1). Accordingly, the design of receptors 1 and 2
features a C3V symmetric cavity with a single Cu(II) along the C3
axis for phosphate binding. Three additional functional groups
bearing positive charges are positioned to provide the additional
binding interactions commensurate with the general design strategy.
Receptor 1 is derived from a tris(2-ethylamino)amine unit with
appended benzylamine groups, similar to designs exploited by
Fabrizzi7 and others.8 Similarly, receptor 2 is derived from a tris-
[(2-pyridyl)methyl]amine subunit functionalized with appended
guanidinium groups, analogous to compounds studied by Canary,9
Karlin,10 and others.11 A stoichiometric amount of copper(II)
chloride preorganizes the ligands to yield the desired receptors. UV/
vis spectroscopy was followed as aliquots of copper(II) chloride
were added to solutions of the ligands (1.2 mM as chloride salts).
The data was used to generate a mole ratio plot, showing a 1:1
binding stoichiometry of ligand to Cu(II) for both apo-1 and apo-
2.
margins, due to the similarity in size, shape, and charge. Perrhenate,
also tetrahedral in shape, has a binding constant on the order of
103, likely due both to the larger size and reduced charge relative
to phosphate. Addition of sulfate to 1 gave so small a spectral
change that the scatter in the data was too large to determine a
reproducible binding constant. Anions of different geometries
demonstrated much lower affinities to 1, and in some cases the
binding stoichiometry was 2:1 guest:host (Cl- and HCO-3, see
Table 1). Therefore, the shape, size, and charge of 1 lead to a high
affinity and selectivity for phosphate relative to perrhenate, and
spherical or trigonal bipyramidal anions.
A 98:2 H2O/MeOH solution of 1 was prepared for titration
purposes and buffered at pH 7.4 (5 mM HEPES). The change in
the absorbance of 1 (0.71 mM) was monitored by UV/vis
spectroscopy as aliquots of a phosphate solution (12.94 mM) were
introduced. The resulting binding isotherm was fit12 with a curve
indicative of a 2.5 × 104 M-1 binding constant. This is one of the
largest binding constants of phosphate to a synthetic receptor in
water at neutral pH reported to date.
To decipher the roles of the various binding sites on 1, pH
titrations on 1 were performed, and hosts 3 and 4 were examined.
The pKas of the ammonium groups in 1 are above 8.0 and therefore
are predominately fully protonated at the pH that the studies were
performed (the protonation state of phosphate when bound is not
known). While the affinity of phosphate with 3 is comparable to 1
(three times smaller), it serves to demonstrate that the cavity of 1
appears to be slightly better suited for small tetrahedral oxyanions.
The Ka value of 900 M-1 for the binding of phosphate to 4 suggests
that a large portion of the binding of phosphate to 1 is due to the
metal center, a key feature in the host design. However, it also
Similar methods were used to determine the binding affinities
of several other anions to 1 in aqueous media (Table 1). The affinity
of arsenate with 1 is the same as phosphate, within our error
9
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J. AM. CHEM. SOC. 2003, 125, 4026-4027
10.1021/ja021390n CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Binding Affinities Determined for a Series of Anions
Using 2 and the Control Host 5
the cavities, which provides excellent shape, size, and charge
complimentarity to the anion. The high affinities reported for
phosphate to both 1 and 2 are attributed to the combined charge-
pairing interactions of the ammoniums/guanidiniums and the Cu(II)
center with the oxygens of the tetrahedral anion. The inherent flex-
ibility of 1 compared to that of 2 decreases its selectivity for
phosphate. In contrast, the rigidity of 2 leads to a decrease in affinity
for phosphate while increasing its selectivity. Such high selectivity
is not generally observed with receptor designs based on polyaza
macrocycles or clefts which compliment oxyanions in charge but
not in size or in shape.
receptor
aniona
stoichiometry anion:2
binding constantsb (M-1
)
2-
2
2
2
2
2
2
2
2
5
HPO4
1:1
1:1
1.5 × 104 (( 6 × 102)
2-
HAsO4
1.7 × 104 (( 6 × 102)
-
ReO4
<100
AcO-
<100
-
NO3
<100
-
HCO3
<100
Cl-
SO4
HPO4
<100
-2
<100
2-
1:1
4.0 × 103 (( 7 × 102)c
a All counterions to the anions are sodium. b Ka values were determined
from a curve fit analysis to UV/vis data collected as aliquots of analyte
solution were added to an aqueous solution of 1 at pH 7.4 at 25 °C.
c Determined using 15% MeOH/water solution due to low solubility.
Acknowledgment. We gratefully acknowledge the support of
the National Institutes of Health (GM57306) and the Welch
Foundation.
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indicates that the ammonium groups are contributing significantly
to the observed affinity constant for phosphate.
To further test the utility of C3V symmetric receptors, an analogue
to 1 was studied which was predicted to be even more comple-
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titrations of solutions containing receptor 2 (1.16 mM) with anions
were performed. Modulations in the UV/vis absorption of 2 were
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added at pH 7.4 (10 mM TRIS). The resulting binding isotherm
was fit with a 1:1 algorithm to yield an affinity of 1.5 × 104 M-1
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The titration with arsenate resulted in a similar binding affinity
(Table 2), much as in the case of 1. Although the Ka for phosphate
and 2 is slightly smaller than that of the 1:phosphate complex, this
is still one of the highest reported in the literature to date. Titrations
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system was used. Titration of a phosphate solution (29.3 mM) into
a solution of receptor 5 (1.5 mM), which lacks the guanidinium
groups, demonstrates a binding affinity of 4.0 × 103 M-1. This
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