Edinburgh EHlO SDT, U.K.phospholipid bilayers with increasing cholesterol concentration has been well documented [ 11. The effects of low concentrations of cholesterol, and particularly its esters, have been less weU studied, particularly with regard to the pretransition. Apolar esters are poorly soluble in polar lipids but their concentration in membranes varies from negligible in Schwann cell membranes to 8% in lysosomal membranes [2], with high concentrations in malignant kidney cells [3] and in the pathogenesis of atherosclerosis [4]. Their presence affects the properties of membranes. Questions arise about the link between changes in lipid concentration and pathogenicity. To answer these one can use Differential Scanning Calorimetry @SC) to study the physical properties of model membranes i.e. Dimyristoylphosphatidylcholine (DMFT) liposomes containing varying amounts of cholesterol or its esters. This technique gives information on the enthalpy (AH) of phase transitions and the size of the cooperative unit (No), which is the average number of phospholipid molecules in microdomains of the non-dominant phase. No is given by dividing the van? Hoff enthalpy by AH. ester in chloroform-methanol (2: 1 v/v). The solvent was evaporated off in round-bottomed flasks under vacuum at 35OC.The vacuum was maintained for 3-4 hours after apparent evaporation with repeated flushing with N2 Liposome preparations were produced by mixing the lipid with an equal mass of 25 mM Tris-HCl, pH 7.2, containing 1 mM EDTA, at 35OC for 15 minutes in a rotary evaporator followed by agitation on a vortex mixer. For DSC measurements, a scan rate of 0.5 Wmin and sample mass of 15 mg were employed.At low concentrations of all four added lipids ( 4 . 5 mol%), the enthalpies of the main and pretransitions were increased while No was decreased. At higher concentrations (0.5-3 mol%), the enthalpies of both transitions were reduced while No showed less of an obvious trend. In the case of the pretransition, the greatest effect was observed with cholesterol while the cholesteryl esters, particularly the unsaturated oleate (18:l) and linoleate (18:2), had a greater effect on the main transition. It was also noted that for the unsaturated esters, concentrations above 1.5 mol% resulted in the main transition on the DSC curve being composed of two overlapping components, indicating phase separation and thus placing an upper limit on their solubility in DMPC. For the saturated ester, cholesteryl stearate (18:O). no such effect was observed and the upper limit of solubility is thought to be at least 2 mol%. disorder increases by ca. 20% during the pretransition while the acyl chains remain in the gel phase. Therefore, it may be that the pretransition is governed mainly by changes in head-group interactions at the bilayer surface which are modified by the presence of the polar -OH group of cholesterol while the less polar ester linkage (COO) will cause a less significantThe reduction of the main chain-melting transition in Liposomes were prepared by dissolv...