Anion transport across varying lipid membranes - The effect of lipophilicity

Article abstract of DOI:10.1039/c5cc00823a

The anion transport properties of a range of alkyl-substituted phenylthioureas were tested in vesicles of different lipid composition. Although changes in the bilayer affected the rate of transport for all compounds in the series, the 'ideal' log P for pe

Full text of DOI:10.1039/c5cc00823a

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Anion transport across varying lipid membranes –
the effect of lipophilicity†
Cite this: Chem. Commun., 2015,
51, 4883
Michael J. Spooner and Philip A. Gale*
Received 28th January 2015,
Accepted 13th February 2015
DOI: 10.1039/c5cc00823a
www.rsc.org/chemcomm
The anion transport properties of a range of alkyl-substituted phenyl- of a series of simple phenylthiourea molecules varied between
thioureas were tested in vesicles of different lipid composition. bilayers formed of different lipids.
Although changes in the bilayer affected the rate of transport for
A range of thioureas (Fig. 1) was synthesised with an increasing
all compounds in the series, the ‘ideal’ log P for peak activity did not length of alkyl substituent, such that the set of compounds spanned
change depending on the composition of the bilayers tested.
the full range of log P values from 2 to 8.5. Thioureas were chosen
as they are simple yet highly effective anion transporters.7,10 The
The development of small-molecule transmembrane anion carriers length of the alkyl chains was increased in a symmetrical manner,
has attracted significant attention recently, as these compounds in accordance with the recently described principle of lipophilic
have potential as future therapeutics for treatment of conditions balance.10 The compounds were synthesised from the appro-
such as cystic fibrosis in additional to being useful tools for the priate isothiocyanates and anilines and full synthetic details are
study of transport processes across cell membranes.1,2 Transport given in the ESI.† Compounds 1,11 2,11 312 & 107 have been
studies are often carried out using large unilamellar vesicles previously reported.
(LUVs) composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine
The association constants for anion binding were determined
(POPC).1 Biological membranes, however, are significantly more for compounds 1, 7 & 15 by 1H NMR titration in DMSO-d6/0.5%
complex, containing a wide range of phospholipids, sterols, H2O using the WINEQNMR2 program.13 The results are shown
proteins and other molecules.3,4 Little work has been done to in Table 1. As expected the binding affinities did not vary
directly compare the performance of synthetic anion transpor- significantly across the series, in line with previously reported
ters in different lipid environments.
thioureas.7,10 Any change in relative rate is therefore not likely to
The lipophilicity of an anion carrier is known to affect be attributable to differences in anion binding.
its transport activity5,6 and calculated log P (c log P) values (the
Lipid vesicles were prepared by previously reported methods.14–16
n-octanol/water partition coefficient) have been shown to be key Chloride efflux from the vesicles was measured using a chloride
in determining the transport activity of simple thioureas.7 ion-selective electrode. After 5 minutes, the vesicles were lysed
Additionally, Quesada et al. have shown that for the tambjamine to calibrate the readings to 100% efflux. Transport activity was
class of transporters, there exists an optimum log P for transport.8 quantified by a linear approximation, as described previously
It is known that energy barriers exist for solutes crossing lipid by Quesada et al.8
bilayers that vary in magnitude depending on lipid composition.
Initial transport rates were plotted against c log P values
In particular, computational studies have shown that the magnitude obtained from the ALOGP17 method (via the VCC Lab web
of these barriers affects the partitioning of solutes into the head applet,18,19 see Fig. 1). This model was chosen over a selection
or tail regions of the bilayer depend on the hydrophobic nature
of the solute.9 We wished to investigate whether similar effects
could influence the ideal log P for anion receptors in different
lipid bilayers therefore we investigated how the transport activity
Chemistry, University of Southampton, Southampton SO17 1BJ, UK.
E-mail: philip.gale@soton.ac.uk; Fax: +44 (0)2380 596805;
Tel: +44 (0)2380 593332
† Electronic supplementary information (ESI) available: Experimental details of
thiourea synthesis, transport studies and NMR titration data. See DOI: 10.1039/
c5cc00823a
Fig. 1 General structure and calculated log P values for the thiourea series.‡
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Table 1 Anion binding constants for 1, 7 & 15 and various TBA salts in
DMSO-d6/H2O (0.5%) at 298 K. Values calculated using WINEQNMR2
assuming a 1 : 1 binding mode. Errors r10%. See ESI for details
Host
Guest
ClÀ
1
7
15
16
o10
160
250
14
17
À
NO3
o10
180
200
o10
220
220
À
H2PO4
2À
SO4
of other computational methods as it gave the best correlation
with retention times obtained by RP-UHPLC (see ESI† for details).20
For POPC bilayers, the peak transport rate was observed for
Fig. 3 Initial rate of chloride efflux by 1–15 (1% loading wrt. lipid) from various
lipid vesicles (containing 489 mM NaCl, buffered to pH 7.2 with 5 mM sodium
phosphate salts), plotted against clog P of the transporter molecule. Vesicles
c log P = 5–6 (Fig. 3). The existence of the optimum log P range suspended in 489 mM NaNO3, buffered to pH 7.2 with 5 mM sodium
phosphate salts. Blue diamonds: 100% POPC; red squares: 100% POPG; green
has been rationalised as being due to a balance between
the receptor being lipophilic enough to cross the tail region
triangles: 3 : 1 POPE : POPC. Each point represents the average of 3 trials. Error
bars represent the standard error on the linear regression.
of the membrane, without being so lipophilic as for delivery to
the bilayer to be an issue.21 It might be expected that if the effect
were purely a partitioning effect then the optimum log P would be
POPE has a similar structure to POPC but lacks the three
the same for the same bilayer composition. This result suggests
methyl groups appended to the nitrogen atom (Fig. 2). Vesicles
otherwise as it is different from that found in studies of other sets
of a 3 : 1 POPE : POPC ratio were used as POPE will not form
of compounds,8 and presumably the effect is more complex and
vesicles on its own (due to the higher transition temperature for
the optimum log P range is specific to transporter class.
this lipid and the likelihood of forming a combination of gel
The experiments were also conducted in vesicles of 1-palmitoyl-2-
and liquid crystalline phases at the concentrations used in
oleoyl-sn-glycero-3-phosphoglycerol (POPG) and 1-palmitoyl-2-oleoyl-
these experiments22). In contrast to POPG, the transport rate
sn-glycero-3-phosphoethanolamine (POPE) (Fig. 2), both having the
across the mixed lipid bilayer was significantly faster than that
same tail groups as POPC. PG lipids have a glycerol head-group
across the pure POPC bilayer for the whole series (Fig. 3), where
in the place of the choline moiety in PC lipids and have an
it might be expected that increased head-group – head-group
overall negative charge in the head-group region. Across the
interactions for PE would increase the energy barrier in the
whole series, transport was slower across POPG bilayers (Fig. 3),
polar region of the membrane.9 Instead, the transporter mole-
possibly as the formation of the negatively charged chloride
cules may be relieving some potential energy introduced by
complex was disfavoured. The peak in activity with respect to
inclusion of the non-bilayer-forming lipid in the membrane.3
log P remained unchanged.
Again, the peak in activity did not appear to change with respect
to log P, although 6 & 10 also matched the rate of 7, 8 & 9,
suggesting that a different rate-determining step in transport
may be operating in this system.
To investigate the effect of changing the lipid tail group, the
screening was repeated using a fully-saturated PC lipid DPPC
(Fig. 2). These experiments were carried out at higher temperature
due to the higher phase transition temperature of DPPC. The rate
across the series was not significantly different from the POPC
control (Fig. 4). This would suggest that the rate-limiting step for
transport is not the diffusion across the tail region of the bilayer.
Experiments were also conducted using POPC vesicles
doped with cholesterol. The condensing and ordering effects
of cholesterol (Chol) on the membrane are well described,23
hence cholesterol assays have previously been used as evidence
for a mobile carrier mechanism in anion transport5,24 (as slower
diffusion across a more rigid membrane is evidence that supports a
mobile carrier over a channel mechanism).
For this series of thioureas, transport rate increased ubiqui-
tously in 7 : 3 POPC : Chol vesicles (Fig. 5). Crucially, the ideal
lipophilicity range remained constant. This provides further
Fig. 2 Structures of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine
(POPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG),
evidence that diffusion across the tail region is not the rate-
determining step, and in addition to other previous results21,25
1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) and
dipalmitoyl-sn-glycero-3-phosphocholine (DPPC).
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Fig. 4 Initial rate of chloride efflux by 1–15 (1% loading wrt. lipid) from
DPPC vesicles (containing 489 mM NaCl, buffered to pH 7.2 with 5 mM
sodium phosphate salts) at 55 1C, plotted against c log P of the transporter
molecule. Vesicles suspended in 489 mM NaNO3, buffered to pH 7.2 with
5 mM sodium phosphate salts. Blue diamonds: 100% POPC; red squares:
100% DPPC. Each point represents the average of 3 trials. Error bars
represent the standard error on the linear regression.
Fig. 6 Initial rate of chloride efflux by 1–15 (1% loading wrt. lipid) from
various lipid vesicles (containing 489 mM NaCl, buffered to pH 7.2 with
5 mM sodium phosphate salts) in the presence of external NaSO4, plotted
against c log P of the transporter molecule. Vesicles suspended in 162 mM
Na2SO4 buffered to pH 7.2 with 5 mM sodium phosphate salts. Transport
initiated with a pulse of NaNO3 in external buffer, to a final concentration
of 40 mM. Rate plotted against c log P of the transporter molecule. Blue
diamonds: 100% POPC; red squares: 100% POPG; green triangles: 3 : 1
POPE : POPC. Each point represents the average of 3 trials. Error bars
represent the standard error on the linear regression.
lipid interface. More work needs to be done to confirm the
origin of this effect.
A degree of ClÀ transport was observed in the 3 : 1 POPE :
POPC system before the nitrate spike addition. In further
assays, we found no direct evidence of sulfate transport or
vesicle leakage. A slight dependence of chloride transport on
the present cation was found however, indicating that MCl
co-transport may make a minor contribution to the overall
efflux of chloride. Slight de-acidification of the vesicle interior
was also seen for the most active compounds in an HPTS
pH assay, indicating a small amount of possible HCl transport.
We suspect these two mechanisms may explain this effect in
this system. This data is presented in full in the ESI.†
For the other systems, the compounds in the middle of the
log P range appear to transport faster in the sulfate system. We
attribute this to the longer equilibration time allowing majority
Fig. 5 Initial rate of chloride efflux by 1–15 (1% loading wrt. lipid) from 7 : 3
POPC : Chol vesicles (containing 489 mM NaCl, buffered to pH 7.2 with
5 mM sodium phosphate salts), plotted against c log P of the transporter
molecule. Vesicles suspended in 489 mM NaNO3, buffered to pH 7.2 with
5 mM sodium phosphate salts. Blue diamonds: 100% POPC; red squares:
7 : 3 POPC : Chol. Each point represents the average of 3 trials. Error bars
represent the standard error on the linear regression.
that cholesterol assays are not sufficient evidence of a mobile of transporters to be ideally located within the membrane when
carrier assay mechanism. transport is initiated, giving a boost to the initial rate. Conversely,
Finally, to explore any effect of the delivery of the com- those at the hydrophobic and hydrophilic extremes of the series
pounds into the membrane, experiments in POPC/POPG/ transport more slowly. The more polar compounds may be
POPE : POPC 3 : 1 were repeated in the presence of external involved in competitive binding to SO42À at the interface, whilst
NaSO4. The SO4 ion has a high energy of solvation26 and is the hydrophobic compounds are likely further confined to the
2À
generally considered not able to be transported through lipid tail region due to the effect of the presence of kosmotropic
bilayers mediated by small molecule transporters although sulfate anions at the lipid water interface.
there are compounds that have been shown to be capable of
We have shown that in the different bilayer systems tested,
transporting this anion.6,27 After 2 minutes allowing the trans- the ideal range of log P for maximum transport rate by this
2À
porter to equilibrate with the SO4 suspended vesicles, trans- series of compounds does not change. We note that the ideal
port was initiated with the addition of a pulse of NaNO3.
log P range appears to be a property of the transporter class and
In the three systems, the relative peak in optimal log P was not of the membrane.
retained in the presence of sulfate (Fig. 6). In the POPG system,
Optimising log P is still an important factor for maximising
transport rates were dramatically reduced for all compounds. It transport rate for a given compound series, although it is
has been proposed that glycerol –OH groups form hydrogen important to note that other design factors must determine
bonds to the lipid phosphate to screen the negative charge, the absolute rate in different lipid systems. We have shown that
packing lipid molecules closer together.28 It may be that different lipid environments have a significant influence on
the kosmotropic properties of sulfate are affecting the water transporter activity and it will be important in further work to
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4886 | Chem. Commun., 2015, 51, 4883--4886
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