Biochimica et Biophysica Acta, 732 (1983) 289-299 289 Elsevier BBA 71729 PHYSICOCHEMICAL CHARACTERIZATION OF LARGE UNILAMELLAR PHOSPHOLIPID VESICLES PREPARED BY REVERSE-PHASE EVAPORATION NEJAT DLJZGONE$, a,b,f., JAN WILSCHUT s, KEELUNG HONG ", ROBERT FRALEY "'**, CAROLE PERRY c, DANIEL S. FRIEND d, THOMAS L. JAMES c and DEMETRIOS PAPAHADJOPOULOS a.e " Cancer Research Institute and Departments of b Anesthesia, " Pharmaceutical Chemistry, d Pathology and e Pharmacology, University of California Medical Center, San Francisco, CA 94143; / Bruce Lyon Memorial Research Laboratory, Children's Hospital Medical Center, Oakland, CA 94609 (U.S.A.) and ~ Department of Physiological Chemistry, University of Groningen, Bloemsinge110, 9712 KZ Groningen (The Netherlands) (Received April 15th, 1983) Key words: Phospholipid vesicle," Reverse-phase evaporation; Phase transition," Dipalmitoylphosphatidylcholine Properties of large unilamellar vesicles (LUV), composed of phosphatidylcholine and prepared by reverse-phase evaporation and subsequent extrusion through Unipore polycarbonate membranes, have been investigated and compared with those of small unilamellar vesicles (SUV) and of multilamellar vesicles (MLV). The unilameilar nature of the LUV is shown by IH-NMR using Pr 3+ as a shift reagent. The gel to liquid-crystalline phase transition of LUV composed of dipalmitoylphosphatidylcholine (DPPC) monitored by differential scanning calorimetry, fluorescence polarization of diphenyihexatriene and 90 ° light scattering, occurs at a slightly lower temperature (40.80C) than that of MLV (42°C) and is broadened by about 50%. The phase transition of SUV is shifted to considerably lower temperatures (mid-point, 38°C) and extends over a wide temperature range. In LUV a well-defined pretransition is not observed. The permeability of LUV (DPPC) monitored by leakage of carboxyfluorescein, increases sharply at the phase transition temperature, and the extent of release is greater than that from MLV. Leakage from SUV occurs in a wide temperature range. Freeze-fracture electron microscopy of LUV (DPPC) reveals vesicles of 0.1-0.2 ~m diameter with mostly smooth fracture faces. At temperatures below the phase transition, the larger vesicles in the population have angled faces, as do extruded MLV. A banded pattern, seen in MLV at temperatures between the pretransition and the main transition, is not observed in the smaller LUV, although the larger vesicles reveal a dimpled appearance. Introduction Multilamellar vesicles (MLV) prepared by mechanical shaking of fully hydrated phospholipid * Address for correspondence: Cancer Research Institute, M- t 282, University of California, San Francisco, CA 94143, U.S.A. ** Present address: Monsanto Company, St. Louis, MO 63166 and Department of Biology, Washington University, St. Louis, MO 63130, U.S.A. bilayers, and small unilamellar vesicles (SUV), produced from the former by ultrasonic irradia- tion have been used as models for biological mem- branes. The structure, permeability and phase transition characteristics of such vesicles made of a large variety of phospholipids and their mixtures with cholesterol have been investigated extensively (reviewed in Refs. 1 and 2). These studies have been very useful for our understanding of how the lipid bilayer component may influence the struc- ture and function of biological membranes. Never- 0005-2736/83/$03.00 © 1983 Elsevier Science Publishers B.V.