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5.7
DIRECT LIPOSOMES-CUBOSOMES TRANSITION
In a recent study, synchrotron SAXS and Cryo-TEM were used to characterize
the temperature-induced structural transitions of ME-based aqueous disper-
sion in the presence of the polymeric stabilizer F127 (Yaghmur et al., 2008).
ME is neutral monoacylglycerol having the same molecular weight as its con-
gener MO but with different molecular shape. ME has trans double bonds
located at the 9,10 position in its straight acyl chain (C18:1t9), whereas MO
has a different confi guration (cis double bond in the carbon atom backbone,
C18:1c9), which causes a “kink” in the middle of the molecule and reduces its
effective length (Fig. 5.17a). This explains the different thermotropic behavior
of the ME-water system from the observed temperature dependence of its
counterpart MO-water system. One interesting feature of the nondispersed
ME-water system is the unique direct transformation under full hydration
conditions from L
α
to V
2
. Therefore, it is well suited to serve as a model system
mimicking the fi rst steps of membrane fusion involving lamellar-nonlamellar
transitions. Our main focus was to check the possibility of inducing analogous
temperature-dependent structural order in the interior of ME-based disper-
sions. This means to prove that the direct transition from vesicles to cubosomes
by heating this dispersion is possible. Our study revealed indeed a direct
80
Monoolein (MO)
70
60
50
40
Monoelaidin (ME)
L
α
30
0.05
0.10 0.15
0.20
0.25 0.30
0.35 0.40
q (Å)
(a)
(b)
Figure 5.17
(a) Molecular structure of the lipids: MO and ME. (b) Contour X-ray
diffraction plot covering the regime of the L
α
, Im3m, and Pn3m phases as a function
of
q
and temperature. In a temperature scan from 25 to 80°C with a rate of 1° C/min,
the ME aqueous dispersion underwent the internal L
α
via Im3m to Pn3m structural
transition. [Reprinted from Yaghmur et al. (2008).]
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