Lateral phase separation

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-+. 

FIG. 9 A scheme representing lateral phase separation for anionic lipids from zwitterionic lipids in a mixed lipid bilayer induced by the peptide antibiotic, polymyxin-B. (Reprinted by permission from Ref. 41, copyright 1998, Elsevier Science.)... 

The measurement of fluorescence lifetimes is an integral part of the anisotropy, energy transfer, and quenching experiment. Also, the fluorescence lifetime provides potentially useful information on the fluorophore environment and therefore provides useful information on membrane properties. An example is the investigation of lateral phase separations. Recently, interest in the fluorescence lifetime itself has increased due to the introduction of the lifetime distribution model as an alternative to the discrete multiexponential approach which has been prevalent in the past. 

The fluorescence lifetime is sensitive to the environment of the fluorophore, and in membranes this usually means the surrounding fatty acyl chains or the membrane protein interfacial region (see summary in Table 5.3). Generally, the lifetime of membrane-bound fluorophores is rather less sensitive to the types of subtle alterations which are encountered in membranes as compared to the fluorescence anisotropy parameters. The gel-to-liquid crystalline phase transition is a notable exception where most fluorophores show an alteration in lifetime properties. Although, again, the anisotropy (see below) is the most sensitive parameter in this regard, the fluorescence lifetime has been used with considerable success in the study of phase transitions and lateral phase separations. Fluorophores used to yield information on the... 

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. 

Membrane conformational changes are observed on exposure to anesthetics, further supporting the importance of physical interactions that lead to perturbation of membrane macromolecules. For example, exposure of membranes to clinically relevant concentrations of anesthetics causes membranes to expand beyond a critical volume (critical volume hypothesis) associated with normal cellular function. Additionally, membrane structure becomes disorganized, so that the insertion of anesthetic molecules into the lipid membrane causes an increase in the mobility of the fatty acid chains in the phospholipid bilayer (membrane fluidization theory) or prevent the interconversion of membrane lipids from a gel to a liquid form, a process that is assumed necessary for normal neuronal function (lateral phase separation hypothesis). 

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.

Fig. 5. The 13C nuclear magnetic resonance line widths of the (enriched) choline methyl resonances in dipalmitoylphosphatidylcholine (A) and in dielaidoylphosphatidylcholine (O), as a function of temperature. Spectra taken at 90.5 MHz similar results were also obtained at 25.2 MHz. Note that the higher-melting lipid, dipalmitoylphosphatidylcholine, shows a readily observable enhanced line broadening at temperatures TU 32°C, corresponding to the onset of the lateral phase separation. (Data from Ref. 4.) [Reprinted with permission from P. Brulet and H. M. McConnell, J. Am. Chem. Soc., 98, 1314 (1977). Copyright by American Chemical Society.]...

Between the pretransition temperature and Tm solid and liquid regions may coexist within a bilayer.101 The term lateral phase separation has been applied to this phenomenon.105 106 Since changes in the equilibrium between solid and liquid can be induced readily, e.g., by changes in the ionic environment surrounding the bilayer, lateral phase separation may be of significance in such phenomena as nerve conduction.107... 

Langmuir-Blodgett layers 392,394 Large calorie, numerical value of 283 Lateral phase separation 395 Lathyrism 438 Lavoisier 281 Lead 31... 

The bilayer morphology of thin asymmetric films of may be unstable. A regularly corrugated surface structure of the films was ascribed to spinodal transition into a laterally phase separated structure, where the surface morphology depended on the polymer incompatibility and the interfacial interactions [347, 348]. Recently, the phase separation and dewetting of thin films of a weakly incompatible blend of deuterated PS and poly(p-methylstyrene) have been monitored by SFM [349, 350]. Starting from a bilayer structure, after 454 h at T= 154 °C the film came to the final dewetting state where mesoscopic drops of... 

Direct or indirect effects on the defect structures at the phase boundaries of the lateral phase-separated bilayer domains, often the binding locus of enzymes. 

While the above system is an example where two-dimensional phase separation in the sense of Fig. la,b (or Fig. 5) occurs, there exist also good examples where no lateral phase separation exists in equilibrium, and the system forms a single interface parallel to the surfaces (Fig. Id). However, if one chooses the initial state such that the phase preferred by air is close to the substrate and the phase preferred by the substrate is next to the air surface [77], the system is unstable and surface phase inversion takes place. A laterally inhomogeneous state then occurs only as a transient phenomenon necessary to trigger the inversion kinetics, but not as an equilibrium state [77]. 

Wu, S. H. W. and McConell, H. M. Lateral phase separations and perpendicular transport in membranes. Biochemical and Biophysical Research Communications 55 4S4, 1973. 

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

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

Findblom G, Oradd G, Filippov A. Lipid lateral diffusion in bilayers with phosphatidylcholine, sphingomyelin and cholesterol. An NMR study of dynamics and lateral phase separation. Chem. Phys. Lipids 2006 141 179-184. 

The simple model does allow an entry point into the study of self-assembly of multicomponent lipid systems, lateral phase separation (clustering), membrane asymmetry, and in particular how these relate to curvature through packing. These form a central class of problems in membrane biology. 

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