PHASE-BEHAVIOR OF HETEROACID PHOSPHATIDYLSERINES AND CHOLESTEROL

The phase behavior of mixtures of cholesterol with 1-palmitoyl-2-oleoyl phosphatidylserine or with 1-stearoyl-2-oleoyl phosphatidylserine has been examined using differential scanning calorimetry and X-ray diffraction. We found that the miscibility of cholesterol in the bilayer is strongly affected by the small difference in length between the fully saturated fatty acid chains of the two phospholipids, Cholesterol crystallites are detected in the 1-palmitoyl-2-oleoyl-phosphatidylserine/cholesterol mixtures at a mol fraction of cholesterol of approximately 0.36, whereas for the mixtures with I-stearoyl-2-oleoyl-phosphatidylserine they appear at a cholesterol mol fraction of approximately 0.2. A phase diagram, based on differential scanning calorimetry experiments without cryoprotectant, is presented for the 1-stearoyl-2-oleoyl phosphatidylserine/cholesterol system. This phase diagram indicates the marked immiscibility of cholesterol with either the gel or the liquid crystalline phase of this phospholipid.