Transitions and molecular packing in highly purified 1,2-dipalmitoyl- phosphatidylcholine–water phases. I. Transitions, improved phase diagrams, method of packing analysis, molecular structures of diglyceride, and polar regions

Abstract

Results of extensive studies of phases prepared from highly purified 1,2-dipalmitoyl-sn-phosphatidylcholine and 1,2-dipalmitoyl-sn -glycerol are presented. The methods used included x-ray diffraction and thermal analysis and both temperature and water content were varied. Details of the many well-defined phases obtained, all with bilayer structures, are given in tables. An improved phase diagram was obtained from thermal data which showed well-defined transitions, and from discontinuities in a plot of bilayer repeat spacings with water content. For analysis of molecular packing the bilayer was divided into diglyceride and phosphorylcholine regions. The properties of the diglyceride regions of both compounds are discussed. Estimates of the size and shape of the polar groups are made and the general principles involved in their packing and the influence of water are discussed. Among new features reported, are the existence of the 0⊥ close chain packing for the crystalline dipalmitoyl glycerol but not in any lecithin phase. Three single crystal lecithin phases with different water contents were prepared which are more stable than the hexagonal chain phases usually reported in the same regions. These hexagonal phases slowly recrystallize to more stable forms, but are usually obtained on cooling melts or higher temperature phases. However, the single crystal phases only exist over restricted composition ranges while phases with hexagonal, square, and disordered chain packing can accommodate a variation in water content by a tilt of the chain axes and changes in bilayer thickness. Transition widths vary and are extremely sensitive to impurities.