A tripodal tris-selenourea anion transporter matches the activity of its thio- analogue but shows distinct selectivity*

Article abstract of DOI:10.1080/10610278.2018.1431394

We report the synthesis of a tripodal tris-selenourea transporter scaffold. The Cl^? and NO^?₃ transport activity of the compound has been compared extensively with the analogous oxo- and thiourea compounds. We found that the selenourea demonstrates remarkably similar transport efficacy and mechanistic properties to the equivalent thiourea, but demonstrates flipped selectivity for Cl^? over NO^?₃.

Full text of DOI:10.1080/10610278.2018.1431394

Supramolecular Chemistry
ISSN: 1061-0278 (Print) 1029-0478 (Online) Journal homepage: http://www.tandfonline.com/loi/gsch20
A tripodal tris-selenourea anion transporter
matches the activity of its thio- analogue but
shows distinct selectivity
Michael J. Spooner & Philip A. Gale
To cite this article: Michael J. Spooner & Philip A. Gale (2018): A tripodal tris-selenourea anion
transporter matches the activity of its thio- analogue but shows distinct selectivity, Supramolecular
Chemistry, DOI: 10.1080/10610278.2018.1431394
To link to this article: https://doi.org/10.1080/10610278.2018.1431394
View supplementary material
Published online: 08 Feb 2018.
Submit your article to this journal
Article views: 8
View related articles
View Crossmark data
Full Terms & Conditions of access and use can be found at
http://www.tandfonline.com/action/journalInformation?journalCode=gsch20

Supramolecular chemiStry, 2018
ꢁꢁꢂs://dꢃꢄ.ꢃꢅg/10.1080/10610278.2018.1431394
ꢀ
A tripodal tris-selenourea anion transporter matches the activity of its
thio- analogue but shows distinct selectivity*
Michael J. Spooner and Philip A. Galeb
a
ꢆ
b
cꢀꢇꢈꢄsꢁꢅꢉ, unꢄvꢇꢅsꢄꢁꢉ ꢃf Sꢃꢊꢁꢀꢆꢈꢂꢁꢃn, Sꢃꢊꢁꢀꢆꢈꢂꢁꢃn, uK; Sꢋꢀꢃꢃꢌ ꢃf cꢀꢇꢈꢄsꢁꢅꢉ, tꢀꢇ unꢄvꢇꢅsꢄꢁꢉ ꢃf Sꢉdnꢇꢉ, Sꢉdnꢇꢉ, aꢊsꢁꢅꢆꢌꢄꢆ
ABSTRACT
ARTICLE HISTORY
rꢇꢋꢇꢄvꢇd 2 Nꢃvꢇꢈbꢇꢅ 2017
aꢋꢋꢇꢂꢁꢇd 17 Jꢆnꢊꢆꢅꢉ 2018
−
−
We report the synthesis of a tripodal tris-selenourea transporter scaffold. The Cl and NO transport
3
activity of the compound has been compared extensively with the analogous oxo- and thiourea
compounds. We found that the selenourea demonstrates remarkably similar transport efficacy and
KEYWORDS
−
mechanistic properties to the equivalent thiourea, but demonstrates flipped selectivity for Cl over
Sꢊꢂꢅꢆꢈꢃꢌꢇꢋꢊꢌꢆꢅ ꢋꢀꢇꢈꢄsꢁꢅꢉ;
ꢆnꢄꢃn ꢁꢅꢆnsꢂꢃꢅꢁ; ꢀꢉdꢅꢃgꢇn
bꢃndꢄng; sꢇꢌꢇnꢃꢊꢅꢇꢆ
−
NO3.
Introduction
of transporters are being reported (4,5), so the rules gov-
erning the efficacy of this class of compounds are better
understood and the number of tools available for trans-
porter design increases.
Ureas have long been used as a hydrogen bond donor
motif in anion recognition, the two parallel N–H bonds
being ideally arranged for the chelation of halides and
Y-shaped oxoanions (6). Their ease of synthesis and
Supramolecular anion transport across lipid bilayers by
synthetic carriers is a rapidly growing field, driven by the
potential application of these compounds as therapeu-
tics in the treatment of diseases such as cystic fibrosis or
cancer (1). New assays have been developed that allow
insight into the activity of these molecules in live cells (2)
and the anion transport mechanism (3). As more families
CONTACT pꢀꢄꢌꢄꢂ a. Gꢆꢌꢇ
Dꢇdꢄꢋꢆꢁꢇd ꢁꢃ pꢅꢃf. Jꢇꢅꢅꢉ aꢁwꢃꢃd ꢃn ꢁꢀꢇ ꢃꢋꢋꢆsꢄꢃn ꢃf ꢀꢄs 75ꢁꢀ bꢄꢅꢁꢀdꢆꢉ.
ꢂꢀꢄꢌꢄꢂ.gꢆꢌꢇ@sꢉdnꢇꢉ.ꢇdꢊ.ꢆꢊ
*
Sꢊꢂꢂꢌꢇꢈꢇnꢁꢆꢌ dꢆꢁꢆ fꢃꢅ ꢁꢀꢄs ꢆꢅꢁꢄꢋꢌꢇ ꢋꢆn bꢇ ꢆꢋꢋꢇssꢇd ꢀꢁꢁꢂs://dꢃꢄ.ꢃꢅg/10.1080/10610278.2018.1431394.
2018 infꢃꢅꢈꢆ uK lꢄꢈꢄꢁꢇd, ꢁꢅꢆdꢄng ꢆs tꢆꢉꢌꢃꢅ & Fꢅꢆnꢋꢄs Gꢅꢃꢊꢂ
©

2
M. J. SPOONER AND P. A. GALE
tunability of the N–H bond acidity through variation of
the N-substituents make them an attractive moiety to work
with in the development of anion receptors and organo-
catalysts (6–8). Hence they have often been used as the
anion binding motif in the design of compounds for supra-
molecular anion transport (4).
There are however a number of moieties that are struc-
turally-related to ureas that can be employed to boost the
efficacy of anion receptors. This has been shown, for exam-
ple, with the squaramide binding group, which improves
anion recognition or catalytic activity over urea with its
higher acidity and more convergent N–H hydrogen bond
donors (9–12). More recently, croconamide and deltamide
structures have been suggested as potential donor units
for anion receptors (13).
colleagues have recently reported a selenourea-based sil-
−
2−
ica-supported chemosensor for CN and S which relies
on the formation of diselenide bonds between selenourea
groups for its selective sensing mechanism (21). The same
group has also exploited diaryl-substituted selenoureas as
molecular logic gates, through their ability to form either
mono- or bi-coordinated adducts based on the shape of
the anion complexed (22). The recent report of a facile
synthesis of an alkyl isoselenocyanate (23) allowed us to
expand a previously reported series of alkyl-substituted
tripodal chloride transporters (15) to include the sele-
nourea analogue, and thus carry out an extensive evalua-
tion of its transport abilities (Figure 1).
Results and discussion
Most commonly however, the urea group has been
directly replaced by a thiourea binding moiety. This has
been shown in several studies to increase transport effi-
cacy in many scaffolds including steroidal cholapods (14),
tren-based tripodal structures (15) and simple small urea-
based molecules (16–18). Generally, it is accepted that
the thiourea confers two advantages over urea; firstly the
greater acidity of the thiourea group gives stronger inter-
action with the anions being studied, most commonly
chloride, and secondly the larger, more charge-diffuse sul-
fur atom gives a lipophilic advantage, reducing the energy
barrier for translocation of the receptor and its complexes
through the lipid bilayer.
Compounds 1 & 2 were prepared by variations on the lit-
erature methods (15) (Figure 2). The tripodal selenourea
3 was prepared from n-butylisoselenocyante 4 prepared
via an adapted literature procedure (23). The crude isose-
lenocyanate was dissolved in dichloromethane and
treated with 0.33 equivalents of tris-(2-aminoethyl)amine.
Compound 3 was obtained after stirring at room tempera-
ture for 4 h and purification by flash chromatography. Full
synthetic details and characterisation data are available in
the Supplementarty Materials.
The anion binding properties of 3 were investigated
1
by H NMR titration in DMSO-d (0.5% H O). Titration of 3
6
2
With this in mind, we wished to determine whether con-
tinuing the chalcogen series and incorporating selenourea
in an anionophore could confer any further advantage
over thiourea. Limited data on the electronic structure of
selenourea suggests that it is electronically is very similar
to thiourea, with the larger, more polarisable chalcogen
affording slightly more delocalisation of electron density
with TBACl led to downfield shift of both selenourea NH
protons, confirming Cl binding to 3. However, the titration
−
data was complicated by the broadening of the NH peak at
7.9 ppm (see Figure S3.2 in the Supplementarty Materials),
−
preventing reliable determination of Cl binding constant
in DMSO. To aid discussion of anion selectivity in transport
(below), the nitrate and chloride binding constants for 2
& 3 were determined in acetonitrile by UV–vis titration
(24,25). Solutions of the receptors (20 μM) were titrated
(
19,20).
Until recently, there have been very few reports of sele-
noureas as receptors in anion recognition. Caltagirone and
with TBACl (0 to ~200 μM) and TBANO (0 to ~3 mM) and
3
the absorbance spectrum between 220 and 320 nm was
−
recorded for each titre. Binding constants for Cl were very
5
−1
5
−1
high, 9.3 × 10 M for 2 and 4.9 × 10 M for 3, as would
be expected compared to values obtained in the more
competitive DMSO.
−
The interaction with NO was weak, nevertheless a
3
:1 binding constant of 1550 M− (±3%) for 3 could be
1
1
−
obtained. In the case of 2, background NO absorbance
3
appeared to overlap with the thiourea peak. The interfer-
ences was quantified in by taking the spectra of TBANO3
acetonitrile solution and subtracting from the spectra
of 2·NO before calculating binding constants. Fitting
3
to a 1:1 binding model yielded a binding constant of
-
1
7
821 ± 4% M . Full spectra and fittings are detailed in the
Figure 1. (cꢃꢌꢃꢊꢅ ꢃnꢌꢄnꢇ) Gꢇnꢇꢅꢆꢌ sꢁꢅꢊꢋꢁꢊꢅꢇ ꢃf ꢁꢀꢇ ꢁꢅꢄꢂꢃdꢆꢌ
ꢋꢃꢈꢂꢃꢊnds sꢁꢊdꢄꢇd ꢄn ꢁꢀꢄs wꢃꢅk.
Supplementarty Materials.

SUPRAMOLECULAR CHEMISTRY
3
Figure 2. (cꢃꢌꢃꢊꢅ ꢃnꢌꢄnꢇ) Sꢉnꢁꢀꢇsꢄs ꢃf ꢁꢀꢇ ꢁꢅꢄꢂꢃdꢆꢌ sꢇꢌꢇnꢃꢊꢅꢇꢆ 3, bꢆsꢇd ꢃn ꢁꢀꢇ ꢅꢇꢂꢃꢅꢁꢇd sꢉnꢁꢀꢇsꢄs ꢃf bꢊꢁꢉꢌꢄsꢃsꢇꢌꢇnꢃꢋꢉꢆnꢆꢁꢇ 4.
The transport ability and mechanism of the compounds
were assayed using a variety of techniques. Firstly, the abil-
are within the margin in experimental error of each other.
−
−
Hence in terms of the Cl /NO exchange ability, the sele-
3
−
−
ity of the compounds to perform Cl /NO exchange from
nourea appears to have the same activity as the thiourea,
offering the same advantage over the oxourea, but no
additional increase in activity over the thiourea. It should
be noted however that related tripodal compounds have
3
synthetic vesicles was determined using the previously
reported chloride ion selective electrode (ISE) method.
Briefly, synthetic POPC vesicles loaded with 500 mM NaCl
buffered to pH 7.2 (5 mM phosphate salts) were suspend-
−
−
demonstrated Cl overNO3 selectivity previously (26), thus
it is unknown whether this value is limited by Cl or NO
−
−
ing in a solution of buffered NaNO (500 mM). The % efflux
3
3
−
over 5 min of the total Cl from the vesicles was recorded
using a Cl ISE at a variety of compound concentrations,
transport. The selectivity of these compounds has been
explored in depth below.
−
and the values at 270 s fitted to the Hill Equation to obtain
an EC50 (270 s) for each compound (EC50 (270 s) ≡ concentration
required to obtain 50% efflux after 270 s).
Evidence of the similar nature of the thio- and seleno-
analogues was obtained by calculating the electrostatic
potential of the Cl complexes of the three compounds
−
Compound 1 was too inactive in these experiments for
Hill analysis, as has been previously reported (15), reaching
a maximum of 1% Cl efflux after 270 s at 5 mol % load-
ing (with respect to lipid, Figure S4.1). The EC50 (270 s) for
thiourea 2 (0.138 mol %) and selenourea 3 (0.139 mol %),
from DFT minimised structures (Figure 3). The charge
distributions of 2 and 3 are remarkably similar, with the
negative charge (red/orange) spread over the larger S/
Se atoms and a larger spread of more neutral blue colour
across the molecule. This is also demonstrated numerically
−
Figure 3. (cꢃꢌꢃꢊꢅ ꢃnꢌꢄnꢇ) cꢆꢌꢋꢊꢌꢆꢁꢇd ꢇꢌꢇꢋꢁꢅꢃsꢁꢆꢁꢄꢋ ꢂꢃꢁꢇnꢁꢄꢆꢌ ꢈꢆꢂs fꢅꢃꢈ DFt ꢈꢄnꢄꢈꢄsꢆꢁꢄꢃns (m06/6-31G* ꢌꢇvꢇꢌ ꢃf ꢁꢀꢇꢃꢅꢉ ꢄn vꢆꢋꢊꢊꢈ) fꢃꢅ
ꢋꢃꢈꢂꢃꢊnds 1–3 ꢊsꢄng Sꢂꢆꢅꢁꢆn ‘14 fꢃꢅ Wꢄndꢃws (27).
Nꢃꢁꢇ: Vꢆꢌꢊꢇs ꢃf ꢇꢌꢇꢋꢁꢅꢃsꢁꢆꢁꢄꢋ ꢂꢃꢁꢇnꢁꢄꢆꢌ ꢆꢅꢇ gꢄvꢇn ꢄn kJ ꢈꢃꢌ−
1
.

4
M. J. SPOONER AND P. A. GALE
−
Figure 4. (cꢃꢌꢃꢊꢅ ꢃnꢌꢄnꢇ) cꢌ ꢇfflꢊx fꢅꢃꢈ popc vꢇsꢄꢋꢌꢇs ꢌꢃꢆdꢇd wꢄꢁꢀ 300 ꢈm Kcꢌ ( sꢊsꢂꢇndꢇd ꢄn 300 ꢈm KGꢌꢊꢋ (ꢆq) fꢃꢅ ꢁꢀꢄꢃꢊꢅꢇꢆ 2 (A) ꢆnd
ꢆq)
sꢇꢌꢇnꢃꢊꢅꢇꢆ 3 (B) ꢆꢁ 1 ꢈꢃꢌ % ꢌꢃꢆdꢄng ꢄn ꢁꢀꢇ ꢂꢅꢇsꢇnꢋꢇ ꢃf 0.1 ꢈꢃꢌ % vꢆꢌꢄnꢃꢈꢉꢋꢄn (ꢅꢇd sqꢊꢆꢅꢇs) ꢃꢅ 0.1 ꢈꢃꢌ % ꢈꢃnꢇnsꢄn (bꢌꢊꢇ ꢁꢅꢄꢆngꢌꢇs). Bꢌꢆꢋk
ꢈꢆꢅkꢇꢅs: 5 μl DmSo ꢋꢃnꢁꢅꢃꢌ.
−
by the differences between the electrostatic potential
(detected by Cl ISE) determines the ability of the aniono-
−1
−
minima being minimal (−381.65 kcal mol for 2 and
phore to effect electrogenic Cl transport or electroneutral
HCl cotransport.
Valinomycin (Vln) is a strict electrogenic K uniporter,
thus compounds must facilitate electrogenic Cl transport
to couple to this process and effect KCl efflux without the
build-up of a pH gradient or membrane potential. Both
thiourea 2 and selenourea 3 couple strongly toVln (Figure 3),
indicating they are both good electrogenic Cl transporters.
Monensin (Mon) is a H /K antiporter, thus aniono-
phores being tested must facilitate HCl cotransport (or
functionally equivalent Cl /OH exchange) in order to avoid
building up a pH gradient to facilitate overall KCl efflux.
Again, both thiourea 2 and selenourea 3 couple to Mon,
indicating that they are also capable of HCl co-transport,
although the efflux is markedly reduced in comparison to
that achieved with Vln, indicating they may be limited by
−
1
−
373.86 kJ mol for 3). As similar electrostatic potential
+
surfaces would be presented to the bilayer, the energy
barriers to crossing the tail region are likely to be very
similar. In contrast, 1 demonstrates a high polarisation
of the C=O bond (electrostatic potential minimum of
−
−
1
−
440.58 kJ mol ), with a high negative charge density on
−
the oxygen atom, which would disfavour interaction with
the tail region of the bilayer.
+
+
The similar lipophilicity of 2 & 3 is also clearly demon-
strated in their calculated log P values (4.58 and 4.94
respectively, cf. 2.96 for 1 (28)). This is confirmed exper-
imentally by considering the retention factor (k`) of the
compounds on a C18 HPLC column. Under isocratic elu-
tion conditions, log k` is directly proportional to log P
−
-
(29,30). Compounds 2 & 3 have the same log k` values, (log
+
-
k` = −0.14, their retention time under the employed condi-
tions was exactly the same, and the compounds co-eluted
when run together) indicating a very similar lipophilicity.
This is in stark contrast to 1, which is not retained by the
column (retention time equal to the column dead time,
hence log k` is undefined) as it is significantly more polar.
These values demonstrate a large difference in lipophilic-
their H /OH transport ability.
Oxourea 1 was also tested using this experiment, but its
limited activity necessitated increasing its concentration
to 5 mol % with respect to lipid molecules. In this case, it
appeared to be able to couple to Mon but not Vln (Figure
S4.4) suggesting limited ability to facilitate electrogenic
transport, although the efflux achieved after 5 min reached
only 6%.
−
−
ity, and hence Cl /NO3 transport activity, between oxourea
1
and thio- and seleno- analogues 2 & 3.
The anion shuttling mechanism of the anionophores
As previously mentioned, due to the potential
−
−
+
selectivity of these compounds, the Cl , NO and H /
3
-
was probed using a valinomycin-/monensin-coupled assay
3). POPC vesicles are loaded with KCl and suspended in
OH selectivity of the compounds was probed using a
(
pH-driven HPTS assay (3,26). POPC vesicles were sus-
a solution of potassium gluconate (KGluc; gluconate is a
large, polar anion that we assume cannot be transported).
Experiments are run in the presence of valinomycin or
monensin, which transport K through the membrane
by different mechanisms. The ability of the anion trans-
porters to couple to these processes to facilitate KCl efflux
pended in a solution of NMDG·HCl or NMDG·HNO at
3
pH 7 (NMDG = N-methyl-D-glucamine, protonated at phys-
iological pH forming a large hydrophilic cation that cannot
be transported) and the anionophores to be tested were
added to the membrane as a solution in DMSO. The exper-
iment was commenced by the addition of the free NMDG
+

SUPRAMOLECULAR CHEMISTRY
5
Table 1. ec50 (270 s) d−ꢆꢁꢆ ꢃbꢁꢆꢄnꢇd fꢃꢅ ꢋꢃꢈꢂꢃꢊnds 2 & 3 ꢄn ꢁꢀꢇ NmDG·hcꢌ ꢆnd NmDG·hNo ꢆssꢆꢉs, ꢁꢃgꢇꢁꢀꢇꢅ wꢄꢁꢀ ꢋꢆꢌꢋꢊꢌꢆꢁꢇd ꢁꢅꢆnsꢂꢃꢅꢁ
3
−
−
−
sꢇꢌꢇꢋꢁꢄvꢄꢁꢄꢇs, ꢆnd cꢌ ꢃvꢇꢅ NO3 bꢄndꢄng sꢇꢌꢇꢋꢁꢄvꢄꢁꢉ ꢋꢆꢌꢋꢊꢌꢆꢁꢇd bꢉ ꢁꢀꢇ ꢅꢆꢁꢄꢃ ꢃf cꢌ ꢁꢃ NO3 1:1 bꢄndꢄng ꢋꢃnsꢁꢆnꢁs fꢅꢃꢈ uV–vꢄs ꢁꢄꢁꢅꢆꢁꢄꢃns ꢄn
ꢆꢋꢇꢁꢃnꢄꢁꢅꢄꢌꢇ (20 μm ꢋꢃꢈꢂꢃꢊnd ꢋꢃnꢋꢇnꢁꢅꢆꢁꢄꢃn).
EC50 (270 s)/mol %
NMDG·HCl + Gr NMDG·HNO3
Transport selectivity (S)
−
Binding Selectivity
−
−
‒
+
+
‒
‒
Compound
NMDG·HCl
NMDG·HNO + Gr
Cl /H
NO /H
Cl /NO3
Cl /NO3
3
3
2
3
0.032 ± 0.0095 0.0039 ± 0.00073 0.022 ± 0.0023
0.031 ± 0.0037 0.0024 ± 0.00021 0.034 ± 0.0089
0.0010 ± 0.00017
0.0054 ± 0.000021
8.3
13.0
21.4
6.1
0.26
2.4
119
316
base to raise the external pH to pH 8, the anionophore
is able to dissipate the pH gradient by performing HCl
As discussed above, 2 binds NO much more strongly
3
−
−
than 3, thus its selectivity for Cl overNO binding is much
3
−
-
−
co-transport (or functionally equivalent Cl /OH exchange).
The experiment is repeated in the presence of 0.1 mol %
of the proton channel gramicidin (Gr), which provides a
fast flux of protons through the membrane. Anionophores
lower despite having a higher absolute affinity for Cl . The
100-fold binding selectivity is not enough to override the
−
Hofmeister bias towardsNO3 selectivity in transport, hence
the compound’s selectivity. In contrast, the binding selec-
tivity of 3 is 3 times higher, significant enough to flip the
selectivity. This is in agreement with previous studies, where
compounds that exhibited 100-fold selectivity or less were
+
-
which are limited by their H /OH transport ability, should
be enhanced under these conditions as they only need to
−
carry out electrogenic Cl transport to balance the charge
+
3
as fast H flux to dissipate the pH gradient is provided by
selective for nitrate, whereas those with >10 fold selectiv-
−
the channel.
ity are able to mediate Cl selective transport. It should be
The HPTS assays were repeated at multiple concentra-
tions of receptors, and Hill fittings were used to obtain
noted that previous studies on a pentyl-substituted tripodal
thiourea suggested that this compound showed the oppo-
−
−
−
−
EC50 (270 s) values as with the Cl /NO exchange assay
site selectivity (Cl overNO3 preference) compared to 2 (26),
suggesting a single carbon difference in the length of the
chain is sufficient to increase the encapsulation of the anion
enough to flip the selectivity of the thiourea analogues.
Finally, it is interesting to consider the EC for the elec-
3
(
Table 1). Note that these values are significantly lower
due to the increased sensitivity of this assay caused by
the much lower concentration of ions that must be trans-
ported to achieve 100% response. Selectivity (S) values for
5
0
-
+
-
X over H over OH were obtained by dividing the EC for
trogenic uniport process from the NMDG assays which
5
0
−
−
that anion in the absence of Gr to that in its presence. The
would be the rate-limiting in the Cl /NO assays (i.e.
3
−
−
−
S value for Cl overNO was obtained by dividing the EC
NMDG·HCl + Gr for NO selective 2 and NMDG·HNO + Gr
3
50
3
3
−
from the NMDG·HNO + Gr by that from NMDG·HCl + Gr.
for Cl selective 3). These values are the same within error
3
These data are summarised in Table 1, with the Hill plots
reproduced in the Supplementarty Materials. Compound
(Table 1), hence the same EC values for both compounds
50
−
−
are observed for Cl /NO exchange.
3
1
was again too inactive for Hill analysis, a single-point
−
−
screen suggested slight Cl /NO selectivity, with little
3
Conclusions
gramicidin enhancement in either case suggesting poor
electrogenic transport ability, in agreement with the Vln/
Mon experiments (Figure S4.5).
We have extended the series of alkyl tripodal tris-ureas
with the synthesis of a selenourea analogue 3. In exchange
assays, it matches the performance of the previously
reported thiourea analogue 2, maintaining a significant
lipophilic advantage over oxourea 1. On more specific
analysis of the compounds’ selectivity it was shown that
the replacing the thiourea group with a selenourea, flipped
Firstly it should be noted that all the EC values meas-
5
0
ured in the absence of Gr are the same within error. Thus in
these conditions the compounds are rate-limited by their
+
-
H /OH transport ability, which is equal for 2 & 3. This is in
agreement with the Vln/Mon experiments which showed
that the compounds’electroneutral HCl cotransport ability
−
−
the selectivity of the scaffold from a NO to a Cl prefer-
3
−
was lower than their electrogenic Cl uniport ability.
ence. Thus, despite the additional synthetic difficulty, sele-
nourea may offer an additional advantage in the design of
more specific transporter scaffolds in applications where
The EC values obtained for the electrogenic (with Gr)
5
0
−
−
transport of Cl and NO show some interesting trends. 2
demonstrates a selectivity for NO over Cl (S = 0.26). In
3
−
−
−
3
selectivity for Cl is key.
contrast, 3 demonstrates faster electrogenic transport for
−
−
Cl overNO (S = 2.4). It has been previously reported that
3
−
−
−
−
Cl overNO3 selectivity is well correlated with Cl overNO3
binding selectivity (26). This can be calculated by dividing
the 1:1 binding constants for Cl by that for NO from the
Acknowledgements
MJS thanks the EPSRC and the University of Southampton for a
Doctoral Prize postdoctoral fellowship. PAG thanks the Universi-
ty of Sydney and the ARC for funding.
−
−
3
UV–vis titration data; these values are also shown inTable 1.

6
M. J. SPOONER AND P. A. GALE
Disclosure statement
(15) Busschaert, N.; Gale, P.A.; Haynes, C.J.E.; Light, M.E.;
Moore, S.J.; Tong, C.C.; Davis, J.T.; Harrell, W.A. Tripodal
Transmembrane Transporters for Bicarbonate. Chem.
Commun. 2010, 46 (34), 6252–6254.
No potential conflict of interest was reported by the authors.
(
16) Andrews, N.J.; Haynes, C.J.E.; Light, M.E.; Moore, S.J.; Tong,
C.C.; Davis, J.T.; Harrell, W.A., Jr.; Gale, P.A. Structurally
Simple Lipid Bilayer Transport Agents for Chloride and
Bicarbonate. Chem. Sci. 2011, 2 (2), 256–260.
17) Moore, S.J.; Wenzel, M.; Light, M.E.; Morley, R.; Bradberry,
S.J.; Gómez-Iglesias, P.; Soto-Cerrato, V.; Pérez-
Tomás, R.; Gale, P.A. Towards “drug-like” Indole-based
Transmembrane Anion Transporters. Chem. Sci. 2012, 3
Funding
This work was supported by the EPSRC; the University of South-
ampton; University of Sydney; and the ARC [grant number
DP170100118].
(
References
(
8), 2501–2509.
(1) Gale, P.A.; Davis, J.T.; Quesada, R. Anion Transport and
(
18) Berry, S.N.; Soto-Cerrato, V.; Howe, E.N.W.; Clarke, H.J.;
Mistry, I.; Tavassoli, A.; Chang, Y.-T.; Pérez-Tomás, R.; Gale,
P.A. Fluorescent Transmembrane Anion Transporters:
shedding light on anionophoric activity in cells. Chem. Sci.
Supramolecular Medicinal Chemistry. Chem. Soc. Rev.
2
017, 46 (9), 2497–2519.
(
2) Li, H.; Valkenier, H.; Judd, L.W.; Brotherhood, P.R.; Hussain,
S.; Cooper, J.A.; Jurček, O.; Sparkes, H.A.; Sheppard, D.N.;
Davis, A.P. Efficient, Non-toxic Anion Transport by Synthetic
Carriers in Cells and Epithelia. Nat. Chem. 2016, 8, 24–32.
3) Wu, X.; Judd, L.W.; Howe, E.N.W.; Withecombe, A.M.; Soto-
Cerrato, V.; Li, H.; Busschaert, N.; Valkenier, H.; Pérez-Tomás,
R.; Sheppard, D.N.; et al. Nonprotonophoric Electrogenic
Cl− Transport Mediated by Valinomycin-like Carriers. Chem
2016, 7 (8), 5069–5077.
(
(
19) Hampson, P.; Mathias, A., Nitrogen-14 Chemical Shifts in
Ureas. J. Chem. Soc. B 1968, 673–675.
20) Bharatam, P.V.; Moudgil, R.; Kaur, D. Electron Delocalization
in Isocyanates, Formamides, and Ureas: Importance of
Orbital Interactions. J. Phys. Chem. A 2003, 107 (10), 1627–
(
(
1
634.
2
016, 1 (1), 127–146.
(
21) Casula, A.; Llopis-Lorente, A.; Garau, A.; Isaia, F.; Kubicki, M.;
Lippolis, V.; Sancenón, F.; Martínez-Máñez, R.; Owczarzak,
A.; Santi, C.; Scorciapino, M.A.; Caltagirone, C. A New Class
of Silica-supported Chromo-fluorogenic Chemosensors
for Anion Recognition based on a Selenourea Scaffold.
Chem. Commun. 2017, 53 (26), 3729–3732.
4) Busschaert, N.; Caltagirone, C.; Rossom, W.V.; Gale, P.A.
Applications of Supramolecular Anion Recognition. Chem.
Rev. 2015, 115 (15), 8038–8155.
(
(
5) Gale, P.A.; Howe, E.N.W.; Wu, X. Anion Receptor Chemistry.
Chem 2016, 1 (3), 351–422.
6) Amendola, V.; Fabbrizzi, L.; Mosca, L. Anion Recognition by
Hydrogen Bonding: Urea-based Receptors. Chem. Soc. Rev.
(
(
22) Casula, A.; Begines, P.; Bettoschi, A.; Fernandez-Bolaños,
J.G.; Isaia, F.; Lippolis, V.; López, Ó.; Picci, G.; Scorciapino,
M.A.; Caltagirone, C. Selenoureas for Anion Binding as
Molecular Logic Gates. Chem. Commun. 2017, 53 (87),
2
010, 39 (10), 3889–3915.
(
(
(
7) Amendola, V.; Esteban-Gómez, D.; Fabbrizzi, L.; Licchelli, M.
What Anions Do to N−H-Containing Receptors. Acc. Chem.
Res. 2006, 39 (5), 343–353.
8) Zhang, Z.; Schreiner, P.R., (Thio)urea Organocatalysis –
What can be Learnt from Anion Recognition? Chem. Soc.
Rev. 2009, 38, (4), 1187–1198.
9) Prohens, R.; Tomàs, S.; Morey, J.; Deyà, P.M.; Ballester,
P.; Costa, A. Squaramido-based Receptors: Molecular
Recognition of Carboxylate Anions in Highly Competitive
Media. Tetrahedron Lett. 1998, 39 (9), 1063–1066.
1
1869–11872.
23) Chang, W.-J.; Kulkarni, M.V.; Sun, C.-M. Regioselective
One-pot Three Component Synthesis of Chiral
2
(
-Iminoselenazolines under Sonication. RSC Adv. 2015, 5
118), 97113–97120.
24) BindFit v0.5. http://app.supramolecular.org/bindfit/ (May
016).
25) Thordarson, P. Determining Association Constants from
Titration Experiments in Supramolecular Chemistry. Chem.
Soc. Rev. 2011, 40, 1305–1323.
(
(
2
(
10) Rostami, A.; Colin, A.; Li, X.Y.; Chudzinski, M.G.; Lough,
A.J.; Taylor, M.S. N, N′-Diarylsquaramides: General, High-
Yielding Synthesis and Applications in Colorimetric Anion
Sensing. J. Org. Chem. 2010, 75 (12), 3983–3992.
(
26) Yang, Y.; Wu, X.; Busschaert, N.; Furuta, H.; Gale, P.A.
Dissecting the Chloride–Nitrate Anion Transport Assay.
Chem. Commun. 2017, 53 (66), 9230–9233.
(
11) Busschaert, N.; Kirby, I.L.; Young, S.; Coles, S.J.; Horton,
P.N.; Light, M.E.; Gale, P.A. Squaramides as Potent
Transmembrane Anion Transporters. Angew. Chem. Int. Ed.
(
(
27) Spartan ‘14, 1.1.4; Irvine, CA: Wavefunction.
28) Values obtained from the MiLogP model using the
Molinspiration Property Calculation Service, http://www.
molinspiration.com. Many computational methods
were attempted for ClogP but yielded unsatisfactory
results, including ALOGPS, ACD/logP, Spartan ‘14 (Ghose-
Crippen), most likely as these methods seem to be poorly
parameterised for Se and in some cases fail altogether
when Se is present.
2
012, 51 (18), 4426–4430.
(
(
12) Alemán, J.; Parra, A.; Jiang, H.; Jørgensen, K.A. Squaramides:
Bridging from Molecular Recognition to Bifunctional
Organocatalysis. Chem. Eur. J. 2011, 17 (25), 6890–6899.
13) Ho, J.; Zwicker, V.E.; Yuen, K.K.Y.; Jolliffe, K.A. Quantum
Chemical Prediction of Equilibrium Acidities of Ureas,
Deltamides, Squaramides, and Croconamides. J. Org.
Chem. 2017, 82 (19), 10732–10736.
(
(
29) OECD, Test No. 117: Partition Coefficient (n-octanol/
water), HPLC Method, OECD Guidelines for the Testing of
Chemicals, Section 1 2004.
30) Berthod, A.; Carda-Broch, S. Determination of Liquid–
Liquid Partition Coefficients by Separation Methods. J.
Chromatogr. 2004, 1037 (1–2), 3–14.
(
14) Valkenier, H.; Judd, L.W.; Li, H.; Hussain, S.; Sheppard, D.N.;
Davis, A.P. Preorganized Bis-Thioureas as Powerful Anion
Carriers: Chloride Transport by Single Molecules in Large
Unilamellar Vesicles. J. Am. Chem. Soc. 2014, 136 (35),
12507–12512.