title: The thermotropic phase behavior of cationic lipids: calorimetric, infrared spectroscopic and X-ray diffraction studies of lipid bilayer membranes composed of 1,2-di-O-myristoyl-3-N,N,N-trimethylaminopropane (DM-TAP).
abstract: The thermotropic phase behavior of lipid bilayer model membranes composed of the cationic lipid 1,2-di-O-myristoyl-3-N,N,N-trimethylaminopropane (DM-TAP) was examined by differential scanning calorimetry, infrared spectroscopy and X-ray diffraction. Aqueous dispersions of this lipid exhibit a highly energetic endothermic transition at 38.4 degrees C upon heating and two exothermic transitions between 20 and 30 degrees C upon cooling. These transitions are accompanied by enthalpy changes that are considerably greater than normally observed with typical gel/liquid--crystalline phase transitions and have been assigned to interconversions between lamellar crystalline and lamellar liquid--crystalline forms of this lipid. Both infrared spectroscopy and X-ray diffraction indicate that the lamellar crystalline phase is a highly ordered, substantially dehydrated structure in which the hydrocarbon chains are essentially immobilized in a distorted orthorhombic subcell. Upon heating to temperatures near 38.4 degrees C, this structure converts to a liquid-crystalline phase in which there is excessive swelling of the aqueous interlamellar spaces owing to charge repulsion between, and undulations of, the positively charged lipid surfaces. The polar/apolar interfaces of liquid--crystalline DM-TAP bilayers are not as well hydrated as those formed by other 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.
authors.affiliation: Department of Biochemistry, University of Alberta, Edmonton, Canada.
authors.forename: R N
authors.initials: RN
authors.lastname: Lewis
authors.forename: S
authors.initials: S
authors.lastname: Tristram-Nagle
authors.forename: J F
authors.initials: JF
authors.lastname: Nagle
authors.forename: R N
authors.initials: RN
authors.lastname: McElhaney
mesh_terms: D002152:Calorimetry, Differential Scanning; D002412:Cations; D008051:Lipid Bilayers / Q000737:chemistry*; D008055:Lipids / Q000737:chemistry*; D000644:Quaternary Ammonium Compounds / Q000737:chemistry*; D017550:Spectroscopy, Fourier Transform Infrared; D013499:Surface Properties; D013696:Temperature; D014961:X-Ray Diffraction
chemical_list: D002412:Cations; D008051:Lipid Bilayers; D008055:Lipids; D000644:Quaternary Ammonium Compounds