Synthesis of an ammonium ionophore and its application in a planar ion-selective electrode

Article abstract of DOI:10.1021/ac0257851

A modular technique was used to synthesize an ammonium-selective ionophore based on a cyclic depsipeptide structure. The ionophore was incorporated into a planar ion-selective electrode sensor format and the selectivity tested versus a range of metal cations in a commercial clinical diagnostic "point-of-care" instrument. Four sensor membrane formulations were tested, all of which consisted of plasticized PVC. Formulations differed as to the type of plasticizer used and whether an ionic additive was present. It was found that the membrane containing the polar plasticizer nitrophenyl octyl ether in the absence of ionic additive exhibited near-Nernstian behavior (slope, 60.1 mV/decade at 37 °C) and possessed high selectivity for ammonium ion over lithium and the divalent cations, calcium and magnesium (log K_NH4+j^POT= -7.3, -4.4, and -7.1 for lithium, calcium, and magnesium ions, respectively). The same membrane also exhibited sodium and potassium selectivity that was comparable to that reported for nonactin (log K_NH4+j^POT= -2.1 and -0.6 for sodium and potassium, respectively, compared to -2.4 and -0.9 in the case of nonactin). Membranes containing the less polar plasticizer, dioctyl phthalate, showed sub-Nernstian behavior (slope, <50 mV/decade at 37 °C). In all cases, the presence of the ionic additive potassium tetrakis(4-chlorophenyl)borate substantially reduced the selectivity observed. The flexible modular synthetic technique developed and reported here will allow the cyclic depsipeptide structure to be tuned for optimum selectivity.

Full text of DOI:10.1021/ac0257851

Anal. Chem. 2003, 75, 152-156
S yn t h e s is o f a n Am m o n iu m Io n o p h o re a n d It s
Ap p lic a t io n in a P la n a r Io n -S e le c t ive Ele c t ro d e
J ohn S. Be nc o, Hube rt A. Nie na be r, a nd W. Gra nt Mc Gim ps e y*
Department of Chemistry and Biochemistry, Worcester Polytechnic Institute, Worcester, Massachusetts 01609
date, the most common strategy involves the use of ion-selective
electrodes (ISEs) that employ the natural antibiotic nonactin as
an ionophore doped into plasticized poly(vinyl chloride) (PVC)
membranes.10 However, it is well known that nonactin-based
ammonium ISEs are limited in their utility because while non-
A modular technique was used to synthesize an am-
monium-selective ionophore based on a cyclic depsi-
peptide structure. The ionophore was incorporated into
a planar ion-selective electrode sensor format and the
selectivity tested versus a range of metal cations in a
commercial clinical diagnostic “point-of-care” instrument.
Four sensor membrane formulations were tested, all of
which consisted of plasticized P VC. Formulations differed
as to the type of plasticizer used and whether an ionic
additive was present. It was found that the membrane
containing the polar plasticizer nitrophenyl octyl ether in
the absence of ionic additive exhibited near-Nernstian
behavior (slope, 60.1 mV/ decade at 37 °C) and possessed
actin has reasonable ammonium ion/ sodium ion selectivity (log
pot
K
∼ -2.4), it has poor selectivity over potassium ions
+
+
NH4 ,Na
pot
(log
K
NH4 ,K
∼ -0.9).1 Consequently, significant effort is now
+
+
being made to develop new synthetic ionophores with higher
ammonium ion selectivity.11-14
In early work, Lehn et al. synthesized macrotricyclic cryptands
that exhibited a substantial enhancement in NH4+/ K+ selectivity,
as determined by NMR studies.11 Greater selectivity was attributed
to the tetrahedral geometry provided by the ionophore and its
ability to donate four hydrogen bonds to stabilize the cation. This
result pointed out the particular importance of hydrogen bonding
and symmetry considerations in the design of ammonium ion
recognition sites. Another consideration especially important in
sensor applications is the reversibility of ion binding as indicated
by the magnitude of the ion/ ionophore complex dissociation
constant. In the systems studied by Lehn, the dissociation constant
was several orders of magnitude smaller than for nonactin, which
leads to the conclusion that such systems cannot be used for rapid
reversible detection of ammonium ion in an ISE format. In effect,
the cryptands described were ammonium ion sinks.
high selectivity for ammonium ion over lithium and the
POT
NH4
divalent cations, calcium and magnesium (log
K
)
+
j
-7 .3 , -4 .4 , and -7 .1 for lithium, calcium, and magne-
sium ions, respectively). The same membrane also ex-
hibited sodium and potassium selectivity that was com-
POT
NH4
parable to that reported for nonactin (log
K
) -2 .1
+
j
and -0 .6 for sodium and potassium, respectively, com-
pared to -2 .4 and -0 .9 in the case of nonactin). Mem-
branes containing the less polar plasticizer, dioctyl
phthalate, showed sub-Nernstian behavior (slope, <5 0
mV/ decade at 3 7 °C). In all cases, the presence of the
ionic additive potassium tetrakis(4 -chlorophenyl)borate
substantially reduced the selectivity observed. The flexible
modular synthetic technique developed and reported here
will allow the cyclic depsipeptide structure to be tuned
for optimum selectivity.
Chin and co-workers synthesized 1,3,5-tri(3,5-dimethylpyrazol-
1-ylmethyl)-2,4,6-triethylbenzene in which the three pyrazole
groups provide hydrogen-bonding sites.12 An ISE incorporating
this molecule showed improvement in ammonium ion selectivity
pot
NH4
over potassium ion as compared to nonactin (log
K
+ ) -2.6),
Metabolites such as urea and creatinine are important disease
indicators1 and for this reason, considerable effort has been
expended in developing diagnostic tools for their detection. Since
the ammonium cation is a byproduct of various enzymatic
reactions involving these substrates, much work has focused on
fabricating sensors for selective ammonium ion detection.2-9 To
again illustrating the importance of hydrogen bonding and
symmetry. However, the limit of detection for this ionophore is 2
orders of magnitude higher than for nonactin, and therefore, it is
not sufficiently sensitive for some applications. Kim et al. inves-
tigated the use of thiazole-containing benzocrown ethers as
(7) Yasuda, K.; Miyagi, H.; Hamada, Y.; Takata, Y. Analyst 1 9 8 4 , 109, 61-64.
(8) Shin, J. H.; Yoon, S. Y.; Yoon, I. J.; Choi, S. H.,; Lee, S. D.; Nam, H.; Cha,
G. S. Sens. Actuators, B 1 9 9 8 , 50, 19-26.
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(10) Buhlmann, P.; Pretsch, E.; Bakker, E. Chem. Rev. 1 9 9 8 , 98, 1593-1687.
(11) Graf, E.; Kintzinger, J. P.; Lehn, J. M.; LeMoigne, J. J. Am. Chem. Soc. 1 9 8 2 ,
104, 1672-1678.
* Corresponding author. E-mail: wgm@wpi.edu.
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Eds.; Elsevier/ North-Holland Biomedical Press: Amsterdam, 1979.
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Chapter 9.
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Angew. Chem., Int. Ed. 1 9 9 9 , 38, 2756-2759.
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G. S.; Nam, H. Anal. Chem. 2 0 0 0 , 72, 4683-4688.
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152 Analytical Chemistry, Vol. 75, No. 1, January 1, 2003
10.1021/ac0257851 CCC: $25.00 © 2003 American Chemical Society
Published on Web 11/22/2002

1
ammonium ionophores and reported potassium selectivity com-
capillary melting point apparatus and are not corrected. H and
parable to nonactin and enhanced selectivity for ammonium over
13C NMR spectra were recorded with a Bruker Avance 400 in
CDCl3 unless otherwise noted. All solvents and reagents were
analytical reagent grade and used as supplied from Aldrich
Chemical Co. PVC, nitrophenyl octyl ether (NPOE), dioctyl
phthalate (DOP), potassium tetrakis(4-chlorophenyl)borate
pot
sodium ion (log
K
+ ) -3.9).13 Similarly, others have used 19-
NH4
crown-6 structures with decalino blocking groups to control
selectivity, reporting increased selectivity for ammonium over both
smaller and larger cations.14
Our approach to the design and synthesis of ammonium
ionophores has, as with Lehn and others, focused on the
incorporation of hydrogen bond donors in tetrahedrally symmetric
complexation sites. Given the structural complexity of some of
the synthetic ionophores reported, we have used a molecular motif
that both lends itself to straightforward synthesis and allows
structural modifications to be incorporated without substantial
changes in synthetic strategy. Our experience to date, as well as
that of others,15-19 has shown that ionophores based on cyclic
peptide and depsipeptide structures, i.e., those that are similar to
natural ionophores, can be readily synthesized in high yield by
either solution- or solid-phase methods.
(KtpClPB), and
Fluka AG (Buch, Switzerland). Amino acids
were purchased from Calbiochem-Novabiochem Corp. Buffers
were prepared with deionized water (18 MΩ‚cm).
D
-hydroxyisovaleric acid were purchased form
- and -valine-N-fmoc
L
D
Molecular Modeling Calculations. Molecular modeling was
performed on an SGI 320 running Windows NT. Calculations were
carried out using the Molecular Operating Environment version
2000.02 computing package (Chemical Computing Group Inc.,
Montreal, PQ, Canada). Structures were minimized first using the
AMBER94 potential control under a solvent dielectric of 5.
PEF95SAC was used to calculate partial charges. Minimized
structures were then subjected to a 30-ps molecular dynamics
simulation employing the NVT statistical ensemble. The structures
were heated to 400 K, equilibrated at 310 K, and cooled to 290 K
in the dynamics thermal cycle at a rate of 10 K/ ps. The lowest
energy structures obtained from these dynamics calculations were
then minimized again. Using the minimized structures, docking
energies of the ammonium and potassium cations were calculated
by employing the default parameters supplied with the program.
Synthesis of Ionophore (Scheme 1 ). A detailed description
of the synthesis and purification of the cyclodepsipeptide IV is
provided in the Supporting Information. Briefly, two building
blocks, I and II, were formed in solution from the corresponding
In the work reported here, we have taken valinomycin as our
inspiration for the design of a new ammonium ion-specific
ionophore. Valinomycin is a naturally occurring antibiotic having
high selectivity for potassium ions. It has a cyclic depsipeptide
structure consisting of alternating amide and ester units (6 of each,
12 in total) and has been synthesized on a solid-phase support.19
Valinomycin preorganizes through hydrogen bonding of its amide
carbonyl groups to form a pocket that presents its six ester
carbonyl groups as sources of electrostatic stabilization for
potassium ions.20 Thus, the pocket provides an octahedral-type
complexation site with a size that is a close match to the estimated
ionic radius of potassium (1.33 Å). We report here the synthesis
of an ammonium ion-specific ionophore, IV, which has some of
the same structural elements as valinomycin. IV is a cyclic
depsipeptide consisting of alternating amide and ester groups
(three of each, six in all) which is in effect, half of the valinomycin
structure. IV does not fold onto itself and therefore it provides a
complexation site that is approximately the same size as valino-
mycin, a necessary feature because the ammonium ionic radius
(1.43 Å) is comparable to that of potassium.11 An important
difference though, is that IV is not capable of providing an
octahedral binding site. However, it has hydrogen bond donors
arranged tetrahedrally (necessary for ammonium complexation),
and it is this distinction that we expected to allow the ionophore
to discriminate efficiently between potassium and ammonium ions.
Below we describe the synthesis of IV, the incorporation of
this ionophore into a planar ISE sensor format, testing of the
potentiometric response of the electrode in a commercial clinical
diagnostic “point-of-care” instrument, and the results of selectivity
studies for ammonium versus other metal cations.
L-lactic acid,
D-valine-N-fmoc residues (I) and D-hydroxyisovaleric
acid, -valine-N-fmoc residues (II). These were coupled sequen-
L
tially as I-II-I onto a Wang resin, then cleaved to give III, and
subsequently cyclized to give the title compound, IV.
ISE Membrane and Electrode P reparation. Four mem-
brane cocktails were prepared to test IV. The specific formulations
are as follows: M1, 69/ 30/ 1 wt % of NPOE/ PVC/ IV; M2, same
as M1 with 50 mol % of KtpClPB to IV; M3, 69/ 30/ 1 wt % of DOP/
PVC/ IV; M4, same as M3 with 50 mol % KtpClPB to IV.
Membrane cocktails were prepared as 10 wt % solutions in THF.
The base electrodes were constructed in a thick-film planar
format21 using a polymeric internal electrolyte layer.22,23 A single
wafer composed of 100 individual electrode elements was used
for the sensor construction. The polymer, methacrylamido-
propylmethylammonium chloride (MAPTAC), for the internal
electrolyte was prepared as a 10 wt % solution in EtOH, spun on
to the planar wafer at 750 rpm for 30 s, and allowed to dry for 1
h before membrane deposition. Internal electrolyte thickness was
∼3.5 µm. The wafer was then quartered giving 4 wafers of 25
sensors each. Membrane cocktails were deposited (0.9 mL) onto
the wafers and allowed to cure for 24 h before use, giving a
membrane thickness of ∼105 µm. The planar wafers were
singulated by hand, giving 25 sensors for each formulation.
ISE Testing. The sensors were housed in the proprietary flow-
through cell used with the Bayer Diagnostics Rapidpoint 400
critical care system. This system uses a saturated Ag/ AgCl
reference cell. Two flow cells were constructed, which contained
EXPERIMENTAL SECTION
Reagents. Mass spectra were performed by Synpep Corp.
(Dublin, CA). Melting points are measured in a Mel-Temp
(15) Kubik, S.; Goddard, R. J. Org. Chem. 1 9 9 9 , 64, 9475-9486.
(16) Ghadiri, M. R.; Granja, J. R.; Milligan, R. A.; McRee, D. E.; Khazanovich, N.
Nature 1 9 9 3 , 366, 324-327.
(17) Granja, J. R.; Ghadiri, M. R. J. Am. Chem. Soc. 1 9 9 4 , 116, 10785-10786.
(18) Hartgerink, J. D.; Granja, J. R.; Milligan, R. A.; Ghadiri, M. R. J. Am. Chem.
Soc. 1 9 9 6 , 118, 43-50.
(19) Isin, B. F.; Merrifield, R. B.; Tosteson, D. C. J. Am. Chem. Soc. 1 9 6 9 , 91,
(21) Benco, J. S.; Foos, J. U.S. Patent 5554272, 1996.
(22) Chan, A. D. C. U.S. Patent 5804049, 1998.
(23) Chan, A. D. C. U.S. Patent 5911862, 1999.
2691-2695.
(20) Pullman, A. Chem. Rev. 1 9 9 1 , 91, 793-812.
Analytical Chemistry, Vol. 75, No. 1, January 1, 2003 153

S c h e m e 1
3 sensors of each formulation for a total of 12 tested sensors. Each
cell was tested individually on the Rapidpoint system that
maintains a 37 °C temperature for the cell. The sensors were
tested using solutions containing NH4Cl (0.5-100 mM), 100 mM
Tris buffer (pH 7.2), and 0.05 g/ L Brij 700. Selectivity testing was
based on the separate solution method (SSM),24,25 where i ) j )
0.1 M.
RESULTS AND DISCUSSION
Ionophore Modeling and Synthesis. As we have noted, IV
possesses some structural similarities to valinomycin in that it
provides an appropriately sized binding pocket for complexation
of ammonium or potassium cations (ionic radii: 1.43 and 1.33 Å,
respectively).11 However, instead of providing an octahedral
complexation geometry like valinomycin, IV is only able to
stabilize ions with tetrahedral binding requirements such as
ammonium ion. It is on this basis that we predicted enhanced
ammonium ion/ potassium ion selectivity for IV over that of
valinomycin. This is supported by Figure 1, which shows the
results of modeling the binding of ammonium and potassium ions
with IV. These minimized structures (obtained as described
above) show the ammonium cation centrally located within the
pocket and able to hydrogen bond with at least five of the carbonyl
groups in IV. In contrast, the potassium cation adopts a position
that is shifted to one side, and while not shown in the figure, it
lies well above the plane of the disklike structure of IV and
therefore provides an unfavorable binding site for potassium.
Modeling of IV also indicates that it may offer enhanced
ammonium/ potassium selectivity over nonactin, the ammonium
ionophore commonly used in ISE applications. Minimized struc-
tures of nonactin with ammonium and potassium ions are shown
in Figure 2. The crown ether backbone of nonactin is quite flexible
+
Fig u re 1 . Minimized structures of IV complexed with NH4 (left)
and with K+ (right).
+
Fig u re 2 . Minimized structures of nonactin complexed with NH4
(left) and with K+ (right).
and allows for the formation of wrapping-type complexes with both
ions. In such complexes, the ions are enveloped by the nonactin
structure and have multiple binding opportunities with the ethereal
or carbonyl oxygen atoms present. It is important to note that
formation of this envelope is essential for binding potassium ions
because it is only in this conformation that an octahedral binding
geometry is provided. On the other hand, the cyclodepsipeptide
IV possesses a more rigid backbone structure that cannot easily
form a wrapping-type structure. As a result, an octahedral binding
site is not provided and potassium ion binding will not be
favorable. However, since a tetrahedral complexation geometry
is available, ammonium ion binding can occur.
(24) Bakker, E.; Buhlmann, P.; Pretsch, E. Chem. Rev. 1 9 9 7 , 97, 3083-3132.
(25) Buck, R. P.; Lindner, E. Pure Appl. Chem. 1 9 9 4 , 66, 2527-2536.
154 Analytical Chemistry, Vol. 75, No. 1, January 1, 2003

To estimate the efficiency of ammonium binding in IV
compared with that in nonactin, docking energies were obtained
for the ion/ ionophore complexes in each case. In the case of IV,
the difference in docking energies between ammonium ion and
potassium ion was calculated to be 6 kcal/ mol more negative than
that calculated for nonactin. While these calculations give relative
values only, they indicate qualitatively that IV should be at least
comparable to nonactin in terms of its NH4+/ K+ selectivity.
Scheme 1 shows the strategy used for the synthesis of IV. The
same solution- and solid-phase techniques reported previously for
the synthesis of valinomycin19 have been used here with the
exception that an Fmoc protection strategy was employed. This
strategy allows the synthesis to be carried out on a commercially
available solid-phase support (Wang resin). Also, cleavage can be
carried out under mild conditions. Thus, block components I and
II were synthesized in solution; I was loaded on a Wang resin
coupled with II and then again with I to yield III, which was
subsequently cleaved from the resin and cyclized to form IV.
Although IV was synthesized from the same component groups
Fig u re 3 . Potentiometric responses of planar ISEs to NH4+ (10-4
10-1 M) for membranes 1-4 based on IV.
-
Ta b le 1 . S e le c t ivit y o f IV ve rs u s Ot h e r Ca t io n s
POT
NH4
selectivity coefficients, log K
+
j
membranea
Li+
Na+
K+
Ca2+
Mg2+
slopeb
found in valinomycin (
-valine), it is clear that a variety of hydroxylated amino acid
L-lactic acid, D-hydroxisovaleric acid, L- and
D
M1
M2
M3
-7.3
-1.9
-5.0
-3.2
-4.44
-2.1
-1.5
-1.9
-1.5
-2.41
-0.6
-1.0
-0.6
-0.4
-0.91
-4.4
-1.3
-3.9
-3.4
-2.54
-7.1
-1.5
-7.3
-3.9
-4.24
60.1
55.8
45.8
39.4
59.3c
derivatives and natural amino acids can be used in order to
produce an ionophore with the appropriate binding site size and
symmetry. (We note that the stereochemistry of the individual
groups in valinomycin has been preserved in the design of IV.)
This approach, then, represents a flexible strategy that will allow
future systematic investigations of the effects of structure on the
efficiency and selectivity of ammonium ion complexation.
Sensor Fabrication and Testing. With the advent of micro-
fabrication techniques, planar-type electrodes have become an
attractive alternative to traditional Phillips body ISEs due to the
ability to construct and test many sensors at once. In addition,
planar-type electrodes make use of polymeric solid internal
contacts, which allow for the construction of an all-solid-state ISE.
The all-solid-state format has advantages over traditional Phillips
body ISEs such as ease of construction, cost-effectiveness, and
ability for miniaturization. Systems such as these have been shown
to be quite stable and have been shown to give potentiometric
selectivities that are comparable to traditional ISEs.26 In particular,
ammonium sensors have been constructed using the all-solid
contact concept and have been shown to possess selectivities that
are typical of nonactin-based Phillips body ISEs.27
These advances have led to a substantial increase in the use
of the planar format and have prompted manufacturers to offer
clinical diagnostic instrumentation that utilizes planar ISEs.28,29
Following this trend, IV was incorporated into a planar ISE
structure employing a polymeric solid contact material21-23 and
tested in a commercially available point-of-care clinical diagnostic
system. Four membrane formulations were tested in order to
determine which environment would yield the best potentiometric
properties of sensors constructed with IV. Each sensor membrane
consisted of plasticized PVC. Formulations differed as to the type
M4
nonactind
a
M1, 69/ 30/ 1 wt % of NPOE/ PVC/ IV; M2, same as M1 with 50
mol % of KtpClPB to IV; M3, 69/ 30/ 1 wt % of DOP/ PVC/ IV; M4, same
as M3 with 50 mol % KtpClPB to IV. b Determined between10-3 and
10-1 M cation, at 37 °C. c At 25 °C. d Data for nonactin taken from
references indicated.
of plasticizer used and whether an ionic additive was present.
NPOE and DOP were used as plasticizers since they have been
used in other ammonium-sensitive electrodes and yielded good
results.10,13 We also investigated the effect of a lipophilic ionic
additive, i.e., KtpClPB, in combination with the two plasticizers.
Figure 3 shows the potentiometric responses of the four
membrane formulations to increasing concentrations of aqueous
NH4Cl. Two effects can be observed in the data, one that we
attribute to the plasticizer and a second due to the ionic additive.
It is clear that membranes containing the plasticizer NPOE, both
in the presence and absence of KtpClPB, consistently produced
sensors with the highest slopes, i.e., the closest to the ideal
Nernstian condition (see Table 1). We attribute this effect to the
higher polarity that NPOE imparts to the membrane compared
to DOP. It is known that depsipeptide structures such as
valinomycin (and likely IV) experience intramolecular hydrogen-
bonding (H-bonding) interactions. In the case of IV, these
interactions would be expected to interfere with the complexation
of ammonium ions (since this complexation also requires H-
bonding). In a polar environment such as that provided by NPOE,
the intramolecular H-bonding will probably be decreased, thus
allowing for more efficient ammonium ion complexation.30,31 Such
would not be the case in lower polarity environments such as those
produced by the presence of DOP. In fact, our NMR dilution
(26) Cadogan, A.; Gao, Z,; Lewenstam, A.; Ivaska, A. Anal. Chem. 1 9 9 2 , 64,
2496-2501.
(27) Tinkilie, N.; Cubuk, O.; Isildak, I. Anal. Chim. Acta 2 0 0 2 , 452, 29-34.
(28) Zaloga, G. P.; Hill, T. R.; Strickland, R. A.; Kennedy, D.; Visser, M.; Ford,
K.; Whitley, J.; Holt, G.; Booker, C. Crit. Care Med. 1 9 8 9 , 17, 920-925.
(29) D, Orazio, P. A.; Maley, T. C.; McCaffrey, R. R.; Chan, A. D. C.; Orvedahl,
D.; Foos, J.; Degnan, S.; Benco, J.; Boden, M.; Murphy, C.; Edelman, P. G.;
Ludi, H. Clin. Chem. 1 9 9 7 , 43, 1804.
(30) Gallo, E. A.; Gellman, S. H. J. Am. Soc. 1 9 9 3 , 115, 9774-9788.
(31) Dobler, M. Ionophores and their Structures; Wiley-Interscience: New York,
1981.
Analytical Chemistry, Vol. 75, No. 1, January 1, 2003 155

studies support this suggestion. In this study, NMR spectra of IV
were obtained over a concentration range of 2-20 mM in nonpolar
(CDCl3) and polar (MeCN-d3) solvents. This concentration range
brackets the concentration of IV in the membranes tested. In the
nonpolar environment, spectra of IV at each concentration
exhibited broad and structureless NH resonances symptomatic
of intramolecular H-bonding, while in the polar medium, NH
resonances were sharp, suggesting the disruption of H-bonding.
In addition to this apparent polarity effect, it was observed that
a 50 mol % loading of the ionic additive, KtpClPB (in combination
with either the NPOE or DOP), results in significant deviations
from Nernstian behavior and substantially reduces the selectivity,
particularly over the divalent cations (see selectivity data in Table
1). Although it has been shown previously that modest numbers
of anionic sites within liquid polymeric membranes can improve
the potentiometric properties of ISEs, a low molar ratio of
ionophore to ionic sites can substantially degrade selectivity and
decrease the slope.24,32,33 This behavior is confirmed in the present
study. It is likely that the deleterious effect on selectivity of the
ionic additive, which is present in the membrane in a 1:2 ratio
relative to IV, is due to nonspecific ion exchange, likely through
the formation of ion pairs.34,35
Table 1 shows the results of selectivity studies that were
carried out using the separate solution method24,25 on four
membrane preparations. It is clear from these data that the
optimum potentiometric characteristics are obtained using the
NPOE plasticizer in the absence of added ionic sites. Membranes
produced with this formulation gave near-Nernstian responses
(slope, 60.1 mV/ decade). Taking membrane M1 as the best
example, this formulation exhibited excellent selectivity for
ammonium ion over the divalent cations calcium and magnesium
nonactin with respect to the divalent cations as well as lithium
ion. The high ammonium/ lithium selectivity can be attributed to
a size fit effect. As noted, the pocket of IV is designed to
accommodate the larger cations and this, coupled to the fact that
IV cannot form wrapping-type complexes, precludes the possibility
of favorable binding to lithium ion whose ionic radius (0.68 Å) is
much smaller than that of ammonium ion.
High selectivity of IV for ammonium ion over the divalent
cations is likely due to two effects, a size fit effect for both calcium
and magnesium ions and, in the case of calcium ion, a low
coordination geometry. The size fit effect is straightforward since
calcium and magnesium ions have radii of 0.99 and 0.82 Å,
respectively, and thus are too small to be effectively complexed.
The second effect specific to calcium ion can be attributed to the
fact that this ion has been shown to favor ligands with six
coordinating groups such as ETH 1001.10 Compound IV cannot
present all six carbonyl groups at one face due to dipole-dipole
repulsion and therefore would be not be able to stabilize the
calcium ion.
The data presented show that new ammonium ionophores
based upon cyclodepsipeptide motifs are an attractive alternative
to others presented in the literature. While the potentiometric data
are comparable to nonactin, modifications to the ionophore
backbone are expected to enhance its selectivity. In particular, it
has been shown that the addition of bulky substituents to other
ionophores has improved selectivities through steric effects.14 This
approach also has the added advantage of increasing the lipo-
philicity of the compounds, thereby making them more compatible
with nonpolar membrane environments. Due to the facile nature
of depsipeptide synthesis, the introduction of bulky groups such
as phenyl or long alkyl chains will allow for additional tuning of
the ionophore’s potentiometric properties.
as well as lithium and good selectivity over sodium and potassium
POT
NH4
(log
K
) -2.1 and -0.6, respectively) that is comparable to
+
j
POT
CONCLUSIONS
nonactin (log
K
-2.4 and -0.9 for sodium and potassium,
+
j
NH4
We have reported here the modular synthesis of a new
respectively).1,4 This selectivity dropped considerably for most ions
with the addition of ionic additive and is reflected in the non-
Nernstian slopes as previously noted. Membranes that were
formulated using DOP as the plasticizer exhibited substantial sub-
Nernstian behavior, making comparisons of selectivity less
meaningful. As indicated above, this effect is likely induced by
the low polarity of the polymeric solvent inducing intramolecular
hydrogen bonding and thus reducing the ability of the ionophore
to complex the cations. However, again, potassium selectivity was
comparable to that of nonactin.
ammonium-selective ionophore based upon a cyclic depsipeptide
motif. This approach yielded an ionophore that, when incorporated
into an ISE format, provides selectivity for ammonium ion over
potassium and sodium that is comparable to nonactin. We believe
that the flexible modular approach used will enable us to tune
the structure of similar molecules to achieve higher selectivity
and sensitivity characteristics.
SUPPORTING INFORMATION AVAILABLE
Detailed description of the synthesis and purification of the
cyclodepsipeptide IV. This material is available free of charge via
The ionic selectivity pattern for M1, NH4+ > K+ > Na+ . Ca2+
,
. Mg2+ ∼ Li+, indicates a substantial improvement over that of
the Internet at http:/ / pubs.acs.org.
(32) Schaller, U.; Bakker, E.; Spichiger, U. E.; Pretsch, E. Anal. Chem. 1 9 9 4 ,
66, 391.
(33) Rosatzin, T.; Bakker, E.; Suzuki, K.; Simon, W. Anal. Chim. Acta 1 9 9 3 ,
280, 197-208.
(34) Armstrong, R. D.; Todd, M. J. Electroanal. Chem. 1 9 8 7 , 237, 181.
Received for review May 20, 2002. Accepted October 23,
2002.
(35) Armstrong, R. D.; Todd, M. J. Electroanal. Chem. 1 9 8 8 , 257, 161.
AC0257851
156 Analytical Chemistry, Vol. 75, No. 1, January 1, 2003