| Research Abstract |
Phospholipids are main constituents of biomembranes. Therefore, attentions of many investigators have been paid to their roles in membrane functioning. From this viewpoint, structures of lipid molecule assemblies and their thermodynamic stabilities were investigated by negative stain electron microscopy and two calorimetries of differential scanning and isothermal reaction types. Neutral lipid, phosphatidylcholine, showed an unstability in small size, unilamellar vesicle prepared due to a high energy input by ultrasonication, which converted to large size, multilamellar vesicles by annealing of a gel phase temperature. On the other hand, negatively charged phospholipid, phosphatidylglycerol, showed a conversion of the MLV gel phase into the cylindrical crystalline phase in the presence of sufficient Na+. This indicates an enhancement of the cation binding to negative charge of the lipids, contrary to the situation of the liquid crystal phase. This result reveals the organization of superstructures of Na+-bound lipid, similar to Ca2+-bound lipid. Phosphatidylethanolamine is one of neutral lipids and has a small size of polar head group.In fact, the lipid formed multilamellar structures having a planar surface due to a close packing of the polar head groups. On annealing of a gel phase temperature, the lamellar structure converted to the crystal bilayer phase. These results indicated that aggregation states of lipid molecules are changed depending on external condition and temperature.
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