Novel properties of cholesterol-dioleoylphosphatidylcholine mixtures.

We have studied the properties of mixtures of cholesterol with dioleoylphosphatidylcholine (DOPC), and with several other phospholipids, including 1-stearoyl-2-oleoylphosphatidylcholine (SOPC) and dioleoleoylphosphatidylserine (DOPS), as a function of cholesterol molar fraction and of temperature. Mixtures of DOPC with a cholesterol molar fraction of 0.4 or greater display polymorphic behavior. This polymorphism includes the formation of structures that give rise to isotropic peaks in 31P NMR at cholesterol molar fractions between 0.4 and 0.6, dependent on the thermal history of the sample. Cryo-electron microscopy studies demonstrate the formation of small globular aggregates that would contribute to a narrowing of the 31P NMR powder pattern. At molar fraction cholesterol 0.6 and higher and at temperatures above 70 degrees C, the mixtures with DOPC convert to the hexagonal phase. Lipid polymorphism is accompanied by the phase separation of cholesterol crystals in the anhydrous form and/or the monohydrate form. The crystals that are formed have substantially altered kinetics of hydration and dehydration, compared with both pure cholesterol monohydrate crystals and with crystals formed in the presence of the other phospholipids that do not form the hexagonal phase in the presence of cholesterol. This fact demonstrates that these cholesterol crystals are in intimate contact with the DOPC phospholipid and are not present as morphologically separate structures.