The influence of phenyl and phenoxy modification in the hydrophobic tails of di-n-alkyl phosphate amphiphiles on aggregate morphology

Article abstract of DOI:10.1039/b514285g

A series of di-n-alkyl phosphate amphiphiles containing phenyl and phenoxy groups in the hydrophobic tails were synthesised, and their aggregation behaviour was investigated using fluorescence spectroscopy, differential scanning calorimetry, and cryo-electron microscopy. The aggregates displayed a wide variety of aggregate morphologies. The incorporation of a phenyl group into the end or in the middle of the alkyl chain lowered the main phase transition temperature, resulting in closed vesicles only above the phase transition temperature. Introducing a phenoxy group at the end of the alkyl chain resulted in open bilayer structures and bicelles. The Royal Society of Chemistry.

Full text of DOI:10.1039/b514285g

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www.rsc.org/obc | Organic & Biomolecular Chemistry
The influence of phenyl and phenoxy modification in the hydrophobic tails of
di-n-alkyl phosphate amphiphiles on aggregate morphology†
Inge Visscher, Marc C. A. Stuart* and Jan B. F. N. Engberts*
Received 10th October 2005, Accepted 15th December 2005
First published as an Advance Article on the web 19th January 2006
DOI: 10.1039/b514285g
A series of di-n-alkyl phosphate amphiphiles containing phenyl and phenoxy groups in the hydrophobic
tails were synthesised, and their aggregation behaviour was investigated using fluorescence
spectroscopy, differential scanning calorimetry, and cryo-electron microscopy. The aggregates displayed
a wide variety of aggregate morphologies. The incorporation of a phenyl group into the end or in the
middle of the alkyl chain lowered the main phase transition temperature, resulting in closed vesicles
only above the phase transition temperature. Introducing a phenoxy group at the end of the alkyl chain
resulted in open bilayer structures and bicelles.
Introduction
The finding that sodium di-n-hexadecyl phosphate forms vesicles1
led to various investigations of the properties of di-n-hexadecyl
phosphate aggregates2–4 Also the influence of changing the molec-
ular structure of the di-n-alkyl phosphate on the morphology
and properties of the aggregates in water has been investigated.
The alkyl chain length was varied and also asymmetric di-
n-alkyl phosphates, i.e. phosphates with two dissimilar alkyl
chains, have been prepared.5–7 Substituting the saturated chains by
unsaturated ones changed the vesicular properties considerably.8
Also less obvious structural changes have been applied. Various
di(polyprenyl) phosphates,9 macrocyclic phosphates,10 and bo-
laform phosphates11,12 have been studied.
Incorporating aromatic moieties into the alkyl chains of am-
phiphiles is expected to change the packing of the hydrophobic
tails. Conformational changes of the hydrocarbon chain and p–p
Fig. 1 Molecular structures of the di-n-alkyl phosphate derivatives.
interactions are the most obvious effects to affect tail packing.
to that of di-n-tetradecyl phosphate. We find that incorporating a
Previous research has been performed on amphiphiles containing
phenyl group at the end of the chain significantly disturbs the pack-
cyanobiphenyl units in both chains. The ammonium amphiphiles
ing of the bilayer, resulting in a drastic reduction of the main phase
transition temperature (Tm) and an increased permeability of the
bilayer. This disturbance is even larger when a phenyl or phenoxy
group is positioned in the middle of the chain. This disturbance is
similar to the disturbance caused by a cis double bond.8
containing cyanobiphenyl units have a higher main phase tran-
sition temperature (Tm) than their aliphatic chain analogue di-
octadecylammonium bromide (Tm = 45 ◦C)13,14 and UV-VIS spec-
tra of vesicular solutions showed the presence of H-aggregates.15
Also azobenzene-containing phosphate amphiphiles have been
studied. These amphiphiles form bilayer sheets instead of vesicles
Results
when dispersed in water and exhibit a blue shift in the UV-VIS
spectrum, indicating H-aggregation.16 A phosphate amphiphile
containing a cyanobiphenyl moiety did not form vesicles.15 The
photophysical behaviour17 and the phase behaviour18 of phospho-
lipid derivatives containing aromatic moieties have been studied.
In this article the synthesis, and the aggregation behaviour of di-
n-alkyl phosphates with various structural modifications in their
alkyl chains (Fig. 1) will be described. Care has been taken that
the tail lengths of all these derivatives were approximately similar
Synthesis
Di-n-alkyl phosphates can be synthesised from the corresponding
alkanol and P2O5 or POCl3.19 Alternatively, the mono-n-alkyl
phosphate can be alkylated with the corresponding bromide using
tetramethylammonium hydroxide.6 A mild method to synthesise
dialkyl phosphates was published recently.16 We used PCl3 for the
synthesis of the phosphoric acid esters, because it is more reactive
than POCl3 and it was expected that the functional groups in the
chains would withstand the subsequent oxidation step.
A slightly modified literature procedure9 was used for the syn-
thesis of the phenyl and phenoxy substituted di-n-alkyl phosphates
(Scheme 1). First a di-n-alkyl phosphite was synthesised from two
Physical Organic Chemistry Unit, Stratingh Institute, University of Gronin-
gen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands. E-mail:
m.c.a.stuart@rug.nl, j.b.f.n.engberts@rug.nl; Fax: +31 (0)503634296
† Electronic supplementary information (ESI) available: Further experi-
mental details. See DOI: 10.1039/b514285g
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Scheme 1 Synthesis of sodium di-n-alkylphosphate analogues.
equivalents of alkanol and pyridine and one equivalent of PCl3 in
tetrahydrofuran. After evaporation of the solvent, the ester was
oxidised in 98% (v/v) aqueous pyridine with an excess of iodine.
This usually gave overall yields of around 30%.
Alcohols with an aromatic moiety connected via an ether linkage
were synthesised by refluxing the corresponding aromatic alcohol,
and the x-bromoalkanol, or x-bromoalkanoic acid in acetone
with K2CO3 followed in the case of the alkanoic acid by reduction
with NaBH4–I2 in THF. 11-Phenylundecan-1-ol was synthesised
by attaching a phenyl group to 11-bromoundecan-1-oyl chloride
via a Friedel–Crafts acylation in benzene, followed by converting
the bromide into the acetate ester and, subsequently removing
the carbonyl group by a Wolf–Kishner reduction, during which
hydrolysis of the acetate ester to form 11-phenylundecan-1-ol
took place (Scheme 2). The synthesis of the derivatised alcohols is
described in the supplementary material.†
Fig. 2 DSC enthalpograms of C11Phen, C14P and C11Ophen.
C6OphenC5 (Fig. 3) to obtain more information on the phase
behaviour and orientational order in the bilayer aggregates.20,21
C6phenC5 and C6OphenC5 showed no sudden increase in
anisotropy, indicating that no cooperative transition from the
liquid-crystalline to the gel phase took place. Instead a gradual
increase of polarisation (P) from 0.12 to 0.34 (for C6OphenC5) and
from 0.12 to 0.28 (for C6phenC5) was observed, which has been
described as a non-cooperative phase transition.8 An alternative
description is that the phase transition occurs below 0 ◦C, similar
to di-oleylphosphatidylcholine (DOPC). The polarisation of DPH
in DOPC bilayers also shows a gradual increase, although the
values are lower and range from 0.05 to 0.18.22 The anisotropy of
C6OphenC5 is higher over the whole temperature range than that
for C6phenC5, which might point to a more ordered bilayer for
C6OphenC5.
Scheme 2 Synthesis of 11-phenylundecan-1-ol.
Aggregation behaviour and colloidal stability
The colloidal stability of aggregates formed from these compounds
(Fig. 1) was compared by monitoring the absorbance of a 0.5 mM
amphiphile solution at 400 nm as a function of time.
Aggregate solutions in pure water or at 50 mM ionic strength
were stable over two days. Solutions of aggregates formed from
C11phen in 150 mM NaCl precipitated within one hour, but
solutions of C6phenC5, C6OphenC5, and C14P in 150 mM NaCl
were stable for more than two days.
Phase behaviour
The aggregate solutions were studied using differential scanning
calorimetry (DSC). The main phase transition temperature (Tm)
measured for C14P is 54.5 ◦C, in agreement with previous
studies.7 Replacement of three methylene units at the end of the
hydrophobic tail by a phenoxy group leads to a slight increase
of Tm to 56.3 ◦C. When a phenyl group replaces the terminal
methylene groups, the Tm is drastically lowered to 28.5 ◦C (Fig. 2).
Aggregates formed from C6OphenC5 and C6phenC5 do not show
a phase transition between 0 and 100 ◦C. Phosphatidylcholine
derivatives with chains that contain aromatic units also do not
exhibit a phase transition.18
Fig. 3 Temperature dependent fluorescence depolarisation of DPH in
C11phen (ꢀ), C6phenC5 (ꢀ) and C6OphenC5 (᭹) aggregates.
The Tm found for C11phen is 22 ◦C, which is in reasonable
agreement with the value determined with DSC (28.5 ◦C).
When the DPH polarisation values of C11phen aggregates at
temperatures below the Tm (0.38) and above the Tm (0.08), are
compared to those of C14P (0.38 and 0.13), it seems that the
liquid crystalline phase of C14P is slightly more ordered than
that of C11phen. The depolarisation values in sodium di-dodecyl
The change in steady-state fluorescence anisotropies upon vari-
ation of temperature was measured for 1,6-diphenyl-(E),(E),(E)-
1,3,5-hexatriene (DPH) in the bilayers of C11phen, C6phenC5, and
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phosphate vesicles (0.39 and 0.09)23 agree more with those of
C11phen, also its Tm, 33 ◦C,7 is more comparable to that of C11phen
than the Tm of C14P. Bearing in mind that a linear relationship
exists between the Tm and chain length, the ‘effective chain length’
of C11phen is somewhat less than 12 methylene moieties.
Encapsulation and leakage
A vesicle can encapsulate a water-soluble dye in its aqueous
compartment. The ability of the aggregates of the different dialkyl
phosphates to entrap the fluorescent dye carboxyfluorescein
(CF) was investigated to determine whether the amphiphiles
formed effectively closed vesicular aggregates. The relation be-
tween amphiphile structure and leakage over the membrane was
simultaneously studied.
C11Ophen aggregates did not encapsulate CF. Aggregates
formed from C6OphenC5, C6phenC5, and C11phen did encapsulate
CF. Leakage studies of C6OphenC5 and C6phenC5 could be per-
formed at 25 ◦C (Fig. 4). For C11phen a temperature of 70 ◦C, well
above Tm, was needed to perform the measurements. Below this
temperature instantaneous leakage occurred, indicating rupture
of the membrane. Also the encapsulations and the separation of
the extra-vesicular CF had to be carried out at this temperature.
Fig. 4 shows that C11phen vesicles at 70 ◦C are more permeable
towards carboxyfluorescein than vesicles formed from C6OphenC5
Fig. 5 Cryo-EM micrographs of vesicles composed of C6phenC5 after
sonication. Bar represents 100 nm.
The samples of C6OphenC5 and C6phenC5, prepared after son-
ication and repeated freeze–thaw and extrusion through 200 nm
polycarbonate membranes, showed bilayer vesicles with various
inclusions (Fig. 6). When this preparation method is used for
phospholipids, usually unilamellar vesicles are observed.24
◦
and C6phenC5 at 25 C. Apparently the phenyl group at the end
disturbs the packing more significantly, leading to an increased
trafficking of the fluorescent probe across the bilayer.
The inclusions in the vesicles composed of both C6OphenC5
and C6phenC5 consist of small vesicles and non-spherical bilayer
fragments. In giant liposomes, the formation of endo-vesicles has
been observed previously and is usually induced by temperature
changes or addition of surfactant molecules that can influence
the membrane curvature.25,26 Also as a result of osmotic effects
invagination can be found in unilamellar vesicles.27 Here the lateral
mobility of the phosphate molecules seems to be hindered in the
bilayers, due to the phenyl or phenoxy substituents.
Samples from an aggregate solution of C11phen (prepared after
stirring the amphiphile in water above Tm followed by extrusion
through 200 nm polycarbonate filters) were prepared for cryo-EM
by vitrification above Tm (Fig. 7, left) and below Tm (Fig. 7, right).
Micrographs of aggregate solutions of C11phen above Tm show
closed bilayer vesicles with inclusions similar to those observed
with phenyl and phenoxy substituents in the middle of the alkyl
chain. Upon cooling below the phase transition temperature the
vesicles become faceted and burst open. Below Tm the hydrophobic
chains loose part of their flexibility, which makes the bilayer
apparently too rigid to allow accommodation of the required
curvature to form a vesicle. As a result the vesicle bursts open
and forms faceted open bilayer fragments with a low curvature.
This is in contrast to phospholipid vesicles, which change from
spherical to faceted upon going from a temperature above to
a temperature below Tm, but stay in one piece.28 The hindered
mobility of the phenyl containing phosphate in combination with
increased rigidity, likely causes the breakage of the vesicles. It is
known that vesicles prepared from certain synthetic amphiphiles,
including di-n-hexadecyl phosphate, do not form closed spherical
vesicles below Tm.4,29,30 The current observations are in accordance
with the leakage studies (Fig. 4). Aggregates formed from C11phen
Fig.
4
CF leakage from vesicles composed of C6OphenC5 (ꢀ),
C6phenC5 (ꢁ), and from vesicles and C11phen (ꢂ) at 70 ◦C.
Aggregate morphology
In order to obtain more insight into the morphology of the ag-
gregates, cryo-electron microscopy (cryo-EM) was performed on
aqueous aggregate solutions of C6OphenC5, C6phenC5, C11phen,
and C11Ophen.
In sonicated aqueous solutions of C6phenC5 (Fig. 5) smooth
unilamellar spherical vesicles are observed. The vesicles display a
large variation in size, from many small vesicles with diameters of
15 to 35 nm to a smaller number of larger vesicles. These larger
vesicles have diameters ranging between 65 nm and 105 nm.
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Fig. 6 Cryo-electron micrographs of vesicles (after freeze–thawing and extrusion through 200 nm polycarbonate filters) composed of C6OphenC5 (left),
C6phenC5 (right). Bar represents 100 nm.
Fig. 7 Cryo-electron micrographs of vesicles (after extrusion through 200 nm polycarbonate filters) composed of C11phen above Tm (left), and below Tm
(right). Bar represents 100 nm.
could not entrap carboxyfluorescein at temperatures below Tm, in
contrast to vesicles of C11phen above Tm.
and CHAPSO both form spherical micelles.34,35 It is believed that in
bicelles DMPC forms the bilayered part of the bicelle while DHPC
or CHAPSO is predominantly situated at the rim of the bicelle.
This rim has a high curvature. To accommodate this curvature, the
lipids must have a small packing parameter. This is presumably not
the case for C11Ophen, so the reason for C11Ophen to aggregate
into bicelles is not obvious (vide supra).
In aggregate solutions of C11Ophen curved bilayer fragments
were observed (Fig. 8). From these structures it is not sur-
prising that C11Ophen cannot entrap carboxyfluorescein, even
at temperatures above Tm. Surprisingly, bicelles were also ob-
served in C11Ophen suspensions. Bicelles are bilayered aggregates
that have a discoid shape. Usually bicelles are observed in
lipid mixtures, often composed of 1,2-dimyristoyl-sn-glycero-
3-phosphocholine (DMPC) and 1,2-dihexanoyl-sn-glycero-3-
phosphocholine (DHPC), or DMPC and 3-[(3-cholamidopropyl)-
dimethylammonio]-2-hydroxypropanesulfonic acid (CHAPSO), a
zwitterionic bile salt derivative.31,32 Recently it was found that
bicelles are only formed below the Tm of DMPC.33 Pure DHPC
Discussion and conclusions
The cryo-EM results show that the phosphates with phenyl and
phenoxy substituents in the middle of the alkyl chain form
spherical, closed bilayer vesicles at room temperature when they
are in the liquid-crystalline phase. Above Tm, also the phosphate
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Micrographs of aggregate solutions of C11Ophen showed only
curved bilayer structures and bicelles. Apparently this amphiphile
is able to accommodate the high curvature at the rim of the
bicelle. This could occur by bending the less hydrophobic phenoxy
back to the interface, thereby changing the packing parameter.
Back bending has been recently also proposed for single-tailed
surfactants with two ester modifications in the alkyl tail.39 Another
possibility is that this bending is not necessary, because the
substituted tails could better interact with water at the edge of the
flat bilayer than the corresponding alkyl tails. The polar phenoxy
functionality will also decrease the hydrophobic interactions. The
hydration sphere of the polar functionality prevents a complete
development of hydrophobic hydration shells of the CH2-groups
next to the polar moiety, and therefore limits the availability of
these CH2-groups for hydrophobic interactions. The influence of
hydrophilic hydration on the apparent hydrophobicity has been
investigated using kinetic medium effects on hydrolysis reactions
in dilute aqueous solution.40,41
Overall, it can be concluded that the phenyl rings that are
incorporated in the alkyl chain disrupt the packing of the
hydrophobic tails in a bilayer. A phenyl or phenoxy substituent
in the middle of the alkyl chain results in a drastic lowering
of the phase transition temperature and leads to the formation
of bilayered vesicles with hindered lateral movement. A phenyl
or phenoxy substitution at the end of the alkyl chain results
in a less drastic lowering (phenyl) or no change in the phase
transition temperature (phenoxy). The alkyl phosphates with a
terminal phenoxy substituent do not form vesicles but curved
bilayer fragments and bicelles.
Fig. 8 Cryo-electron micrographs of aggregates (after extrusion through
200 nm polycarbonate filters at a temperature above Tm) composed of
C11Ophen. White arrow indicates top view of the bicelle, black arrow
indicates side view of a bicelle. Bar represents 100 nm.
with a phenyl substituent at the end of the alkyl chain forms
closed vesicles. This is consistent with the inability of C11phen
to entrap CF below their Tm and also with cryo-EM results
for di-n-hexadecyl phosphate aggregates below Tm.4 Striking is
the presence of inclusions in the larger vesicles. The preparation
method that was deployed usually yields unilamellar vesicles in
the case of phospholipids.24 The shapes of the vesicles and the
inclusions bear a resemblance to vesicular shapes observed in giant
liposomes.26 The cause of these inclusions is not clear. The inclu-
sions are probably inherent to the amphiphiles used. We believe
that the tails in the substituted di-n-alkyl phosphate molecules
in the inner and outer leaflet cannot move as independently
from each other as compared to phospholipid molecules in their
bilayer environment. This hampers the ability to accommodate the
required bilayer curvature. In a phospholipid molecule the tails
are connected to each other via three bonds. In many synthetic
amphiphiles the tails are connected via two bonds. This means
that a phospholipid molecule has one rotational degree of freedom
more than the synthetic amphiphiles. During preparation of the
vesicles a certain curvature stress and a certain ratio between the
surface and volume of the vesicle is induced. In phospholipid
vesicles relaxation takes place, but in our vesicles this apparently
does not occur.36 Mathematical modelling studies predicted vesicle
shapes corresponding to minimal isotropical bending energies for
different values of the normalised average mean curvature.4 We
have observed these vesicular shapes in our electron microscopic
studies. We believe that due to the inflexibility of the chains, no
unilamellar vesicles can be formed.
Experimental
Syntheses are described in the supplementary material.† All water
used in the experiments was bidistilled in an all quartz apparatus
or purified by a Millipore system.
Colloidal stability
Vesicles were prepared by dissolving the appropriate amount of
amphiphile in water by heating above Tm and vigorous vortexing.
Then the amphiphile solution was extruded 21 times through
a polycarbonate filter of 200 nm. The time dependence of the
optical density at 400 nm was monitored using a Perkin Elmer k5
spectrophotometer.
Differential scanning calorimetry
Vesicles were prepared by dissolving the appropriate amount of
amphiphile in water by heating above Tm and vigorous vortexing.
Then the amphiphile solution was extruded 21 times through a
polycarbonate filter of 200 nm. The DSC enthalpograms were
recorded on a VP-DSC apparatus (Microcal, Northhampton,
MA). The reference cell was filled with water. The concentration
of the amphiphiles was 2.0 mM. The samples were degassed before
measurements. Heating scans were performed fro◦m 20 to 100 ◦C
at a rate of 1 ◦C min−1 (C11Ophen), from 10 to 100 C (C11phen) or
from 0 to 100 ◦C (C6phenC5 and C6OphenC5), the time in between
runs was 60 minutes.
From NMR and molecular dynamics simulations, it is known
that the conformational order at the interface is high, and that
the order in the hydrophobic core is low.37,38 An already low order
cannot be perturbed much further, whereas an ordered situation
can be perturbed more easily.
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Fluorescence polarisation
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The measurements were performed with an SLM Aminco SPF
500 C spectrofluorometer equipped with a thermostated cell
holder. 1,6-Diphenyl-E,E,E-1,3,5-hexatriene (DPH) (5 × 10−8 M)
was excited at 360 nm. The emission wavelength was 428 nm
(bandpass 5 nm). The fluorescence polarisation was calculated
from the intensities of the emitted light parallel and perpendicular
to the direction of the excitation radiation using P = (Iꢁ − I⊥)/(Iꢁ +
I⊥). The amphiphile concentration was 5 × 10−5 M. Measurements
involved heating scans with 3 ◦C intervals. The samples were
allowed to equilibrate for 10 minutes after each temperature in-
crease. Reported values for P are average values of six independent
measurements.
Carboxyfluorescein leakage assay
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250 lL of a 10 mM solution of the appropriate amphiphile in
methanol was evaporated to form a film on the wall of a glass
tube. This was kept at vacuum for several hours. 100 lL of
a stock solution of carboxyfluorescein (100 mM) and 0.49 mL
of a buffer solution (5 mM HEPES, 5 mM sodium acetate,
1 mM EDTA) were added. This solution was heated to 70 ◦C
(C11phen) and sonicated for 30 seconds at this temperature. The
non-encapsulated carboxyfluorescein was removed by passing
the vesicle solution over a column of Sephadex G-75 at 65 ◦C
(C11phen) or at 25 ◦C (C6phenC5 and C6OphenC5) with an
elution buffer (5 mM HEPES, 5 mM sodium acetate, 1 mM
EDTA and 20 mM NaCl, preheated to 65 ◦C or 25 ◦C). The
fraction containing the vesicles was collected in a preheated glass
tube. 20 lL of this solution was injected into a vigorously stirred
cuvet containing 3 mL elution buffer at 70 ◦C (C11phen) or
at 25 ◦C (C6phenC5 and C6OphenC5). The carboxyfluorescein
emission intensity (kex = 490 nm, bandpass 2 nm, kem = 520 nm,
bandpass 4 nm) was measured on a SLM-Aminco SPF-500 C
spectrofluorometer equipped with a thermostatted cell holder and
magnetic stirring device. CF leakage was calculated using % =
(I − I0)/(Imax − I0) × 100, where % is the leakage in percentage
of the maximum leakage, I0 is the emission intensity at t = 0, It is
the emission intensity at t = t and Imax is the maximum emission
intensity after disruption of the vesicles with 150 ll of a Triton
X-100 solution (10% w/v).
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Cryo-electron microscopy
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in a temperature controlled environment at 100% humidity and
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transferred to a Gatan model 626 cryo holder at −170 ◦C in a
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