Phase separation in membranes

Lipids in model systems are often found in asymmetric clusters (see Figure 9.8). Such behavior is referred to as a phase separation, which arises either spontaneously or as the result of some extraneous influence. Phase separations can be induced in model membranes by divalent cations, which interact with negatively charged moieties on the surface of the bilayer. For example, Ca induces phase separations in membranes formed from phosphatidylserine (PS)... 

Shimshick, E. J. et al. Lateral phase separations in membranes. Journal of Supramolecular Structure 2 285-295,1973. 

This paper covers several aspects of the membrane manufacturing process, membrane structural characteristics, and current applications. In addition, this paper briefly discusses applications of the manufacturing process to two polymers and a variety of solvents. Phase Separation in Membrane Formation... 

A. Platt-Aloia and W. W. Thomson, Freeze fracture evidence for lateral phase separations in membranes of chilling-injured avocado fruit. Protoplasma (in press). 

FIGURE 9.8 An illustration of the concept of lateral phase separations in a membrane. Phase separations of phosphatidylserine (green circles) can be indnced by divalent cations snch as Ca-+. 

The mixed liposomal solutions were prepared by the ethanol-injection method(13) in order to obtain completely transparent solutions. It is interesting to note that miscibility of the photochromic amphiphiles with DPPC depend on the position of bulky azobenzene. If azobenzene is incorporated close to the end of long alkyl chain, a stable mixed bilayer state cannot be formed. On the other hand, when the azobenzene moiety is located near the head group or at the center of the hydrocarbon tail, the azobenzene amphiphiles are successfully incorporated into the bilayer membrane. No individual micelle formation nor phase separation in the bilayer was observed at 25 °C by absorption spectroscopy. However, the microstructure of the mixed liposomes depends on the type of azobenzene amphiphiles. 

The use of DPH lifetimes for the analysis of phase separations and membrane properties has been described for mode) systems.n fl) In the case of both parinaric acids and DPH, one of the motivations for examining phase separation in a model lipid bilayer is the possibility that phase separations might be detectable in natural membranes. However, this technique has not been able to satisfactorily resolve lateral phase separations in natural membranes, either because they do not exist or because they are much more complex and even possibly transient in nature. Alternatively, it could be argued that if a probe could be found with the characteristics of trans-parinaric acid but perhaps with an even greater phase partitioning ability, then this approach might be reevaluated. 

This condition has been recently used in a vaporization-exchange model for water sorption and flux in phase-separated ionomer membranes. The model allows determining interfacial water exchange rates and water permeabilities from measurements involving membranes in contact with flowing gases. It affords a definition of an effective resistance to water flux through the membrane that is proportional to... 

Ringsdorf, Sackmann, and coworkers characterized the behavior of mixtures of the polymerizable bis-dienoylammonium lipid 14 and DMPC [42]. Evidence for phase separation in these mixtures was obtained from electron microscopy and light scattering. Since the intensity of scattered light is dependent on the physical state of the membrane, plots of scattering intensity versus temperature exhibit inflections at phase transitions. This technique was used in conjunction... 

Fig. 2. Phase diagram describing lateral phase separations in the plane of bilayer membranes for binary mixtures of dielaidoylphosphatidylcholine (DEPC) and dipalmitoyl-phosphatidylcholine (DPPC). The two-phase region (F+S) represents an equilibrium between a homogeneous fluid solution F (La phase) and a solid solution phase S presumably having monoclinic symmetry (P(J. phase) in multilayers. This phase diagram is discussed in Refs. 19, 18, 4. The phase diagram was derived from studies of spin-label binding to the membranes.

Since cholesterol is an important component of many biological membranes mixtures of polymerizable lipids with this sterol are of great interest. In mixed monolayers of natural lipids with cholesterol a pronounced condensation effect , i.e. a reduction of the mean area per molecule of phospholipid is observed68. This influence of cholesterol on diacetylenic lecithin (18, n = 12), however, is not very significant (Fig. 32). Photopolymerization indicates phase separation in this system. Apparently due to the large hydrophobic interactions between the long hydrocarbon chains of... 

Figure 3.15 Polypropylene structures, (a) Type I open cell structure formed at low cooling rates, (b) Type II fine structure formed at high cooling rates [37]. Reprinted with permission from W.C. Hiatt, G.H. Vitzthum, K.B. Wagener, K. Gerlach and C. Josefiak, Microporous Membranes via Upper Critical Temperature Phase Separation, in Materials Science of Synthetic Membranes, D.R. Lloyd (ed.), ACS Symposium Series Number 269, Washington, DC. Copyright 1985, American Chemical Society and American Pharmaceutical Association...

Izatt, Reed, M., Ion Separations in Membrane and Solid Phase Extraction Systems, 4, 225. 

Ion Separation in Membrane and Solid Phase Extraction Systems... 

Connell SD, Snuth DA. The atonuc force microscope as a tool for studying phase separation in lipid membranes. Mol. Membr. Biol. 2006 23 17-28. 

Shimshick EJ, McConnel HM. Lateral phase separation in phospholipid membranes. Biochemistry 1973 12 2351-2360. 

Wu SHW, McConnell HM. Phase separations in phospholipid membranes. Biochemistry 1975 14 847-854. 

Phase separation in model lipid membranes is established clearly (120), but the significance of lipid rafts in cell membranes has been controversial (121) largely because of their isolation by harsh conditions of detergent resistance. More recently, methods have been developed to visualize lipid rafts in living cells (122, 123) and to identify proteins within them by less invasive methods (124). 

Arnold T, Linke D. Phase separation in the isolation and purification of membrane proteins. BioTechniques 2007 43 427 30. 

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