The thermotropic phase behavior of cationic lipids: calorimetric, infraredspectroscopic and X-ray diffraction studies of lipid bilayer membranes composed of 1,2-di-O-myristoyl-3-N,N,N-trimethylaminopropane (DM-TAP)

The thermotropic phase behavior of lipid bilayer model membranes composed o f the cationic lipid 1,2-di-O-myristoyl-3-N,N,N-trimethylaminopropane (DM-T AP) was examined by differential scanning calorimetry, infrared spectroscop y and Xray diffraction. Aqueous dispersions of this lipid exhibit a highly energetic endothermic transition at 38.4 degreesC upon heating and two exot hermic transitions between 20 and 30 degreesC upon cooling. These transitio ns are accompanied by enthalpy changes that are considerably greater than n ormally observed with typical gel/liquid-crystalline phase transitions and have been assigned to interconversions between lamellar crystalline and lam ellar liquid-crystalline forms of this lipid. Both infrared spectroscopy an d X-ray diffraction indicate that the lamellar crystalline phase is a highl y ordered, substantially dehydrated structure in which the hydrocarbon chai ns are essentially immobilized in a distorted orthorhombic subcell. Upon he ating to temperatures near 38.4 degreesC, this structure converts to a liqu id-crystalline phase in which there is excessive swelling of the aqueous in terlamellar spaces owing to charge repulsion between, and undulations of, t he positively charged lipid surfaces. The polar/apolar interfaces of liquid -crystalline DM-TAP bilayers are not as well hydrated as those formed by ot her classes of phospho- and glycolipids. Such differences are attributed to the relatively small size of the polar headgroup and its limited capacity for interaction with moieties in the bilayer polar/apolar interface. (C) 20 01 Elsevier Science B.V. All rights reserved.