DSC OF DMPC LIPOSOMES CONTAINING LOW CONCENTRATIONS OF CHOLESTERYL ESTERS OR CHOLESTEROL

Using differential scanning calorimetry (DSC) the effects of the addit ion of 0-3 mol% cholesterol, cholesteryl stearate (18:0), cholesteryl oleate (18:1), cholesteryl linoleate (18:2) or cholesteryl linolenate (18:3) upon the main transition and pretransition of fully hydrated di myristoylphosphatidylcholine (DMPC) multilamellar liposomes have been measured. The results are interpreted in terms of the standard two-sta te model for thermal phase transitions. With the exception of choleste ryl stearate all steroids caused, overall, marked changes in the entha lpy and cooperativity of the bilayer main transition and in the enthal py of the bilayer pretransition. For the esters, the precise nature of the effects and the maximum solubility in the bilayers were found to be dependent on the degree of unsaturation in the fatty acyl chain. Mo re specifically, it was observed that the solubilities of the eaters i n the bilayers increased with increasing fatty acyl chain unsaturation , but were still lower than that of cholesterol. Additionally, it was observed that while eater incorporation was enhanced in the bilayer li quid crystalline phase, some incorporation also occurred in the gel ph ase.