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.
Highlights
The gel-liquid crystal transitions in lipid bilayers formed from synthetic dimyristoyl, dipalmitoyl and distearoyl L-W 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-r
All the results reported here were obtained with dimyristoyl lecithin, Lot 100822, dipalmitoyl lecithin, Lot 100653, and distearoyl lecithin, Lot 100296, all obtained from Calbiochem
The values for TInz and AH2 given in Table I agree moderately well with those reported by Phillips et al [9]
Summary
The gel-liquid crystal transitions in lipid bilayers formed from synthetic dimyristoyl, dipalmitoyl and distearoyl L-W 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-r. 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 degi (mole of lipid)-l 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 reached by Phillips et aI. In a previous communication [5] we have given the results of a calorimetric study of aqueous suspensions of mixtures of cholesterol with dimyristoyl lecithin and dipalmitoyl lecithin
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