Two types of lipidic particles

Abstract

The molecular arrangements in lipidic particles (LIP) seen on freeze fracture faces of liposomes have been a subject of much discussion. Miller and Hui proposed that these particles were intermembrane attachment sites, while Cullis and de Kruijff favored a model consisting of inverted micelles between bilayer leaflets.In our study of polymorphic phase transitions in mixtures of unsaturated phosphatidylethanolamine (PE) and phosphatidylcholine (PC), We have found evidence of two distinct types of LIP. In most PE/PC mixtures such as soybean PE and egg PC, the LIP have a dimple-like appearance and are definitely conical in cross fracture (fig. 1). Both concave and convex forms are commonly found on the same fracture face (fig. 2). In mixtures of dilinoleoyl-PE and palmitoyl-oleoyl-PC, the LIP are larger and have a round, bead-like appearance. These LIP often align in rows between entrapped water space (fig. 3), or stack in a cubic arrangement (fig. 4). In less ordered areas, conical and spherical LIP can be viewed as complementary lattice.