Effect of lipid admixtures on the L-dipalmitoylphosphatidylcholine subtransition

The effect of lipid admixtures on the properties of the L-dipalmitoylphosphatidylcholine (L-DPPC) subtransition is investigated by using high-sensitivity differential scanning calorimetry. The four admixtures used are D-DPPC, L-dipalmitoylphosphatidylethanolamine (L-DPPE), cholesterol, and palmitic acid. In all cases the subtransition decreases in enthalpy until disappearance with increase of the admixture concentrations. About 5-7 mol% of D-DPPC or palmitic acid are sufficient for abolishment (without position shifts) of the subtransition, while, on addition of L-DPPE or cholesterol, it persists up to about 20 mol% of the admixture and its disappearance is accompanied by a slight shift to higher temperatures. These data are tentatively interpreted in terms of lateral mixing of L-DPPC and admixture as indicating compound formation with D-DPPC and palmitic acid, and clustering of L-DPPE and cholesterol.