COMPLEX PHASE MIXING OF PHOSPHATIDYLCHOLINE AND PHOSPHATIDYLSERINE IN MULTILAMELLAR MEMBRANE-VESICLES

The phase mixing in dipalmitoyl phosphatidylcholine and bovine brain phosphatidylserine mixtures suspended in aqueous salt solutions was studied by differential scanning calorimetry, freeze-fracture electron microscopy and X-ray and electron diffraction. The pure dipalmitoyl phosphatidylcholine has two well-defined solidus phases Pβ′ and Lβ′ and a liquidus phase Lα while the pure phosphatidylserine has a broad transition from Lβ to Lα. The mixture exhibits some dipalmitoyl phosphatidylcholine characteristics up to 30% phosphatidylserine when pure Pβ′ phase no longer exists. Phase mixing is observed at all compositions. An addition of 3% phosphatidylserine is found to be sufficient to destroy the sharp Lβ′-Pβ′ transition observed in pure dipalmitoyl phosphatidylcholine. The transition between Lβ′ and Pβ′ in these mixtures is displaced to lower temperatures and becomes gradual as depicted by all three techniques, but freeze-fracture gives more definite information of the continuous transition. The most striking observation on the morphology of the mixed membranes (3-30% phosphatidylserine in dipalmitoyl phosphatidylcholine) is the presence of banded patterns (Pβ′) at temperatures well below the main transition peak as detected by differential scanning calorimetry. © 1979.