Thermal history alters cholesterol effect on transition of 1-palmitoyl-2-linoleoyl phosphatidylcholine

The effect of cholesterol on the bilayer phase behavior of heteroacid phosphatidylcholines with one unsaturated fatty acid depends on the nature of the unsaturated chain. Previous differential scanning calorimetry (DSC) studies showed that 1-palmitoyl-2-linoleoyl-sn-glycero-3-phosphocholine (16:0-18:2 PC) had a broad, weak transition at about -18 degrees C, which was effectively eliminated by less than 15 mol% cholesterol. Phospholipids with greater and lesser degrees of unsaturation displayed stronger phase transitions and less sensitivity to cholesterol. In this work, deuterium nuclear magnetic resonance has been used to examine the phase behavior of 1-perdeuteriopalmitoyl-2-linoleoyl-sn-glycero-3-phosphocholine (16:0-18:2 PC-d31) alone, and with 15 mol % cholesterol. The behavior is found to be sensitive to sample thermal history. Moderately fast cooling (1 degree/h) results in a continuous phase change from a fluid to an ordered phase in the pure lipid. Under similar cooling conditions, the sample containing cholesterol displays increased chain order and a continuous phase change with no apparent isothermal transition. However, when these systems are cooled at a reduced rate (0.3 degree/h), the continuous phase change is pre-empted by a sharp transition into a more ordered phase that gives a deuterium spectrum having intensity at a value of the quadrupole-splitting characteristic of a rigid lattice system. In the pure lipid, this transition effectively coincides with the center of the continuous phase change. Addition of 15 mol % cholesterol lowers the temperature of this sharp transition by about 3 degrees C. These observations provide some insights into the behavior of this system seen using differential scanning calorimetry. Results of deuteron transverse relaxation measurements under these conditions are also reported.