Angewandte
Chemie
compared to conventional cis double bond-containing lipids
used so far. The target lipid, MDS, contains an identical
headgroup to that of MO, and a rigid cyclopropyl moiety
replacing the cis double bond between C and C , giving rise
9
10
to a different curvature and different dihedral angles of the
hydrophobic tail.
The design of the novel lipid monodihydrosterculin
(
MDS) implements a geometrically confined cyclopropyl
ring as a saturated “cis-kink” surrogate (Figure 1b). The
resultant LCP phase behavior of MDS supports the use of
a small-ring replacement principle, from which other novel
LCP lipids can be designed and their properties anticipated.
An additional parameter that could influence phase
behavior is the equilibrium composition of esters formed at
[
22]
the 1- and 2-position on the glycerol head group. Indeed,
hydrated monoacylglycerols undergo spontaneous acyl
migration at moderate temperatures, yielding a mixture of
[
23]
about 9:1 of the 1- and 2-isomers. This balance seems to be
overlooked in previous discussions of MO-LCP and may
account for material variability. To ascertain that the data
obtained are reproducible with respect to composition all
measurements were performed with a pre-equilibrated mix-
ture of (9:1) 1-MDS to 2-MDS (see the Materials and
Methods section in the Supporting Information).
Figure 2. 1D SAXS spectra of the scattered intensities versus scattering
vector q, depicting the diffraction patterns of MDS mesophases
containing 50% (w/w) water at the selected temperatures. The double
diamond Pn3m cubic phase exists in the temperature range of 4–
2
28C, as revealed by the SAXS diffraction peaks in the ratio:
2: 3: 4: 6: 8: 9.
pffiffi pffiffi pffiffi pffiffi pffiffi pffiffi
The binary MDS:H O phase diagram, established by
2
small-angle X-ray scattering (SAXS) and melting point
determination features order-to-order transitions from the
La to Ia3d, and from Ia3d to Pn3m upon hydration, and
formation of the fully hydrated Pn3m cubic phase above
a characteristic boundary of full hydration (Figure 1c,d). At
high temperature, all phases transform into the isotropic L2
phase. The sequence of transitions is the same as that of MO,
but the resulting phase behavior is different because of the
the cubic phase for MDS is larger in the composition space,
and smaller in the temperature space when compared to MO.
Significantly, MDS exhibits a thermodynamically stable
cubic phase at low temperature (Figure 2). This observation is
in accord with the finding that the high-temperature bounda-
ries of MDS phases are lower than those of MO. Currently
available LCPs are often limited in the range of temperature
over which they can be used, and are generally unstable at low
temperatures (0–48C), undergoing phase transition into
a crystalline phase. Therefore, formation of a stable cubic
phase at low temperature would have significant implications
in biomaterial science and technology in general, and in
membrane structural biology in particular.
stability of each phase. Significantly, the high-temperature H
phase that appears in other monoacylglycerol systems is
II
[13]
[
13]
absent in the new system. The highly curved H phase has the
II
highest packing frustration energy, and its absence may be
correlated with a difference in the chain stretching energy
[24]
because of the new hydrophobic motif.
Overlay of the binary phase diagrams of MDS and MO
Figure 1e) reveals that at a given composition, the collapsing
For a given phase at constant temperature, an increase in
water content results in an increase of the lattice size
(Figure 3). For the lamellar phase, this corresponds to the
inter-bilayer distance, that is, thickness of the aqueous
compartment. For the cubic phases, this corresponds to the
aqueous channel size. At a given temperature there is
a maximal hydration for a certain phase, above which
coexistence with a higher hydrated phase sets in. The
coexistence region eventually transforms into the pure
higher hydrated phase upon further increase of water
(Figure 3). In the coexistence regime, the lattice parameter
remains constant until complete phase transition is accom-
plished (Figure 3b and Figure S3 in the Supporting Informa-
tion). The curvature and the size variation of the unit cell are
phase-dependent: The La and the cubic Pn3m have smaller
lattice parameters and the cubic Ia3d has larger lattice
parameters (Figure 3b, Figure S3).
(
temperature of any phase to the high-temperature L phase is
2
lower in the MDS than in the MO system. The boundary lines
between L and the lower-temperature phases in these two
2
systems are not parallel, and form a wedge that broadens with
increasing hydration: at low water content (5%) the tran-
sition from La to L is isothermal (358C) in both systems.
2
Upon increasing the water content the MDS mesophases
become increasingly less stable at high temperature, which is
reflected in a relatively low collapsing temperature: thus at
1
8
5% water the Ia3d to L transition temperatures are 65 and
58C for the MDS and MO systems, respectively.
2
The cubic-Ia3d is the most pronounced phase in both MO
and MDS phase diagrams. For MDS, the La-Ia3d boundary is
at lower water content than in MO, whereas the Ia3d-Pn3m
boundary starts at lower hydration at low temperature, and
ends at higher hydration at high temperature (Figure 1e). At
even higher water content, the fully hydrated Pn3m phase is
shifted to higher water content in MDS. Thus, the existence of
The phase diagram of the MDS:H O system (Figure 1)
2
demonstrates a number of novel features that can be directly
related to the molecular geometry of the hydrophobic chain,
Angew. Chem. Int. Ed. 2015, 54, 1027 –1031
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Salvati Manni, Livia
