DIFFERENTIAL SCANNING CALORIMETRY ON MIXTURES OF LECITHIN, LYSOLECITHIN AND CHOLESTEROL

The effect of increasing concentrations of lysolecithin (1-palmitoyl-sn-glycerol-3-phosphorylcholine) on the gel → liquid crystal thermal transition of lecithin (1,2-dipalmitoyl-sn-glycerol-3-phosphorylcholine) in the aqueous phase was studied by differential scanning calorimetry (DSC). Lysolecithin showed an endothermic transition at 3.4°C whereas the transition of the lecithin occurred at 42°C. No phase separation could be observed calorimetrically at lysolecithin concentrations up to 60 mol%. Freeze etch electron microscopy showed that mixtures containing as much as 50 mol% lysolecithin exist in a lamellar phase. The lysolecithin was found to cause an initial slight increase in the enthalpy of transition followed by a gradual decrease. The enthalpy increased again at very high lysolecithin concentrations. The lysolecithin also caused a non-linear decrease in the temperature at which the lecithin transition took place. Cholesterol was found to decrease the enthalpy of transition of the lysolecithin, eliminating it at a concentration of 50 mol%. Cholesterol caused an increase in the temperature at which the lysolecithin transition took place.