Effects of diacylglycerol on the structure and phase behaviour of non-bilayer forming phospholipid.
The phase behaviour of mixed aqueous dispersions of the monomethyl derivative of dioleoylphosphatidylethanolamine and dipalmitoylglycerol has been characterised by X-ray diffraction, differential scanning calorimetry and freeze-fracture electron microscopy for mixtures containing dipalmitoylglycerol in the concentration range 0-20 mol%. Dispersions prepared at temperatures where the phospholipid exhibits a liquid-crystalline lamellar phase show that dipalmitoylglycerol is completely phase separated into aggregates of stable crystal phase (beta'-phase). Heating mixed dispersions results in transformation of lamellar into hexagonal-II structure commencing at approximately 45 degrees C. This temperature coincides with a disappearance of beta'-phase of DPG which becomes incorporated into hexagonal-II phase. The pure phospholipid is transformed upon cooling from hexagonal-II into characteristic cubic phases; the formation of cubic phase is prevented by the presence of dipalmitoylglycerol and mixed dispersions initially form a lamellar liquid-crystalline phase in which the lipids are phase separated. The X-ray and thermal data suggest that relatively small domains of metastable crystal phase (alpha-phase) of DPG form initially on cooling and these subsequently coalesce and transform to beta'-phase.
Takahashi
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Department of Applied Physics, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan. takahashi@sephia.nuap.nagoya-u.ac.jp
Watanabe
M
M
Quinn
P J
PJ
Kato
S
S
Murayama
S
S
Ohki
K
K
Hatta
I
I