Calorimetric Studies of Dilute Aqueous Suspensions of Bilayers Formed from Synthetic l-α-Lecithins

Abstract

Abstract The gel-liquid crystal transitions in lipid bilayers formed from synthetic dimyristoyl, dipalmitoyl and distearoyl l-α-lecithins have been studied by high sensitivity differential scanning calorimetry in dilute aqueous suspensions ranging in concentration from 0.4 to 6.6 mg ml-1. Each lipid shows two endothermic transitions, an extremely sharp main transition and a broader transition accompanied by a smaller heat absorption at a temperature 5–10° below the main transition. The enthalpy increases in the main transition are respectively 6.3, 9.7, and 10.8 kcal per mole of monomeric lipid. The main transition widths, which are not very reproducible, reach a minimum in the case of dimyristoyl lecithin of as little as 0.2° for 10% to 90% conversion, suggesting that these transitions would be truly isothermal with completely pure lipids. The apparent heat capacities of the lipids in the liquid crystal state are, with the possible exception of dipalmitoyl lecithin, no more than 5 cal deg-1 (mole of lipid)-1 larger than in the gel state, indicating that the hydrocarbon chains have much less mobility in the liquid crystalline state than in the corresponding liquid normal paraffins. This conclusion was also reached by Phillips et al. (Phillips, M. C., Williams, R. M., and Chapman, D. (1969) Chem Phys. Lipids 3, 234) on the basis of entropy comparisons.