Complementary Packing of Phosphoglyceride and Cholesterol Molecules in the Bilayer

Abstract

It is well established for phospholipids that the molecular structure and packing properties of the hydrophobic region of the molecule are intimately coupled to the associated structural properties of the polar head group of the phospholipid. For example, phosphatidylethanolamine molecules extracted from biological sources such as hen egg yolk form both lamellar (or bilayer) and hexagonal type II phases when exposed to excess water at physiological temperatures (Reiss-Husson, 1967; Junger and Reinauser, 1969). In contrast, phosphatidylcholine molecules form only the bilayer structure under the same conditions (Luzzati and Husson, 1962; Luzzati, 1968). The large shift in the main endothermic transition temperature, T _m, of the lipid hydrocarbon chains from the crystalline gel to the liquid-crystalline state serves as another example. The value of T _m for synthetic [C_{16 : 0}–C_{16 : 0}] dipalmitoylphosphatidylcholine is 41.4°C (Mabrey and Sturtevant, 1976), whereas phosphatidylethanolamine with the same saturated hydrocarbon chains has a T _m of about 63.8°C (Mabrey and Sturtevant, 1977). Even within the same phospholipid such as dipalmitoylphosphatidylcholine, an alteration in polar-head-group conformations or properties can be shown to affect the structural and packing properties of the hydrophobic portion of the lipid bilayers. For example, upon the addition of about 4 molecules of water per molecule of dipalmitoylphosphatidylcholine, the conformation and the mobility of the lipid polar head group in bilayers are found to be distinctly different from those in anhydrous solids (Griffin, 1976; Bush et al., 1980a). Any marked change in the structural motional properties of the polar head group of phospholipids upon hydration is not unexpected, for it is well known that the water molecules can associate with the polar head group of phospholipid molecules.