Freeze‐fracture of lipids and model membrane systems

Abstract

Model membrane systems are used extensively in all aspects of membrane research, and freeze-fracture is the preeminent procedure for directly visualizing local structure in these lipid dispersions. Here we describe in detail the formation of liposomes and how freeze-fracture is routinely employed as a complementary technique to biophysical and biochemical procedures in the characterization of multilamellar vesicles (most commonly known as liposomes) and unilamellar vesicles. Many preparative procedures exist for the formation of multi- and unilamellar vesicles. Examples of each system are given and their properties as well as freeze-fracture morphology are discussed. The detection of lipid-phase transitions is considered, in particular, with emphasis on the application of freeze-fracture to the study of lipid polymorphism. We briefly discuss the fracturing of apolar lipids which do not adopt bilayer structures but which can be stabilized into microemulsions by a phospholipid monolayer. Finally, a critical assessment is made of filipin as a morphological marker for cholesterol domains in the plane of the bilayer.

Sequestration Revisited: Integrating Traditional Electron Microscopy, De Novo Assembly and New Results

“…Fixation conditions here are apparently more favorable for taking up the hexagonal configuration which is characteristic of phosphatidylethanolamines. 26 Our results in general emphasize the importance of careful analysis when interpreting EM pictures, especially with regard to the formation of artifacts of fixation and embedding. For routine purposes, we still recommend the use of simple aldehyde fixation with only low concentrations of sucrose, if any.…”

Single Particle Fluorescence Burst Analysis of Epsin Induced Membrane Fission

“…1B , the observed size variation in liposomes appears to be between 35–50% CV, based on Monte Carlo simulations of particle distributions in which the particle brightness is assumed to be proportional to surface area. While the observed liposome variation is somewhat larger than expected, several factors likely broaden the observed intensity distribution, including a small fraction of multi-lamellar objects [ 23 ] and the discreet distribution of dye molecules between objects of the same absolute size. As a complementary measurement, we examined each liposome sample by FCS ( Fig.…”

Shape change and physical properties of giant phospholipid vesicles prepared in the presence of an AC electric field

“…finmed in the present study by freeze-fracture electron microscopy. Freeze fracture can be ambiguous in the case of small vesicles or even LUVs, where this technique can only be used qualitatively to assess lamellarity (Hope et al, 1989b). However, there is a high probability of crossfractures in giant liposomes, and thus the lamellarity can be ascertain by this method.…”