Requirement of charged lipids for the hexadecanol-induced gelation in the phospholipid bilayer system

We investigated the formation mechanism of the new type of hydrogel we found previously in the lipid system composed of a crude lecithin mixture (PC70) and hexadecanol (HD) [1] by examining purified lipid systems systematically and quantitatively. On the basis of a working hypothesis that the charged lipids included in PC70 play a crucial role in the formation of the homogeneous hydrogel, we analyzed the physicochemical properties and structures of the simplified ternary bilayer system composed of HD, distearoylphosphatidylcholine (DSPC) and distearoylphosphatidylglycerol (DSPG) as a negatively charged lipid by rheometry, freeze-fracture electron microscopy, synchrotron X-ray diffraction and differential scanning calorimetry. We found that the simplified system is able to form a hydrogel though the composition of the system must fall in a fairly narrow range. Structural analyses suggested that the necessary conditions for the gelation are that the fairly rigid bilayer sheets or vesicles with a flat shape are homogeneously distributed in the aqueous solution under an interaction potential without a secondary minimum.