Liquid-ordered phase

Recently, due to increased interest in membrane raft domains, extensive attention has been paid to the cholesterol-dependent liquid-ordered phase in the membrane (Subczynski and Kusumi 2003). The pulse EPR spin-labeling DOT method detected two coexisting phases in the DMPC/cholesterol membranes the liquid-ordered and the liquid-disordered domains above the phase-transition temperature (Subczynski et al. 2007b). However, using the same method for DMPC/lutein (zeaxanthin) membranes, only the liquid-ordered-like phase was detected above the phase-transition temperature (Widomska, Wisniewska, and Subczynski, unpublished data). No significant differences were found in the effects of lutein and zeaxanthin on the lateral organization of lipid bilayer membranes. We can conclude that lutein and zeaxanthin—macular xanthophylls that parallel cholesterol in its function as a regulator of both membrane fluidity and hydrophobicity—cannot parallel the ability of cholesterol to induce liquid-ordered-disordered phase separation. 

Pseudo-glycolipids. The interest for glycolipids is connected with their occurrence in biological systems, as well as their physicochemical properties, the two viewpoints being sometimes correlated. For example in membranes, lipid rafts are sub-domains which contain liquid-ordered phases.73... 

There is a large cholesterol concentration gradient in cells from 0-5 mol% in the ER membrane to 25-40 mol% in the plasma membrane [12], Cholesterol has a condensing effect on liquid-crystalline bilayers, causing increased rigidity and thickness [13]. At high concentrations, cholesterol induces an intermediate liquid-ordered phase between the gel and liquid-disordered phases [13]. A number of... 

McMullen, T.P.W., Lewis, R.N.A.H., McElhaney, R.N. Cholesterol-phospholipid interactions, the liquid-ordered phase and lipid rafts in model and biological membranes. Curr. Opin. Colloid Interface Sci. 2004, 8, 459-68. 

Quantitative investigation of recognition of this pair of liposomes was performed with isothermal titration microcalorimetry (ITC). It has been found that one-to-one binding between adenine and barbituric acid in the lipid/water/lipid interface occurs. However at T= 58°C, above the main lipid phase transition, the situation is different and no liposomal binding is detected. This is mainly due to the molecular disorder within the bilayer (liquid-disordered/liquid ordered phase coexistence) that limits the capacity of complementary moieties to bind, due to the weakening of the hydrogen bonds at these high temperatures. 

La liquid-ordered phase bilayers prepared from binary mixtures of the same phosphatidylcholine and cholesterol but is much slower ( 10 s ) in Lx liquid-ordered phase membranes prepared from sphingomyelin and cholesterol (55). The activation free energy for the process, which corresponds to the energy necessary to put the translocating lipid molecule at the bilayer mid-plane, is 100kJmol In contrast, the rate constant for transmembrane translocation of cholesterol may be 1 s (56). 

Even though some plasma membranes, such as nerve myelin membranes, contain a high concentration of lipids that form gel phase bilayers, the presence of cholesterol keeps these membranes in a fluid phase. However, interaction with the rigid cholesterol ring affects hydrocarbon chains of lipids in the liquid crystal phase (L ) and leads to formation of a new phase, the liquid ordered (Lq) phase (27). The phase is well characterized by a variety of physical methods and does not exist in pure lipids or their mixtures. In the liquid ordered phase, the long axis rotation and lateral diffusion rates are similar to the La phase, but the acyl chains are predominantly in an all-trans conformation and, hence, the order parameters are similar to the Lp phase (see Table 1). Recently, the cholesterol-rich Lq phase has been strongly associated with microdomains in live cells—the so-called lipid rafts. ... 

Inaoka, Y. and Yamazaki, M. (2007) Vesicle fission of giant unilamellar vesicles of liquid-ordered-phase membranes induced by amphiphiles with a single long hydrocarbon chain. Langmuir, 23 (2), 720-728. 

Liquid ordered phase In lipids, a meso-phase state with properties between the liquid disordered state and the gel state, in which lipid molecules diffuse rapidly but have somewhat restricted conformational freedom. 

The lamellar phase is particularly important because, as mentioned above, it represents more or less the structure of cell membranes under equilibrium conditions. It should be added now that the name lamellar phase is not fully accurate because there are at least five lamellar phases, namely the subgel or crystalline phase, the gel or solid phase, the rippled phase, the fluid or liquid-crystalline phase, and the liquid-ordered phase. Lipids in the subgel and in the gel lamellar phase (sometimes denoted... 

Pott, T. and Dufourc E.J. (1995). Action of melittin on the DPPC-Cholesterol liquid-ordered phase a solid state and P-NMR study. Biophys. J. 68 965-977. 

The simple theory put forward by Meinhardt et al. accounts in a unified manner for both ripple phases and raft states in membranes. The prerequisites for the formation of such modulated phases is local phase separation (e.g., in the ripple case, between a liquid and a gel phase, or in the raft case, between a liquid disordered and a liquid ordered phase) and curvature stress in at least one of the two phases (typically the ordered one), resulting, e.g., from a size mismatch between head group and tails. In order to reproduce rippled states or rafts, coarsegrained simulation models must meet these criteria. This is often not the case. For example, the standard version of the popular MARTINI model does not have a ripple phase, because the low-temperature gel phase of saturated phospholipids is until ted. 

Liquid ordered-phase bilayers were formed when mixing a cationic surfactant (cetylpyridinium chloride, CPQ and sterol in a 30/70 molar ratio. 

Figure 6.2 Lipid raft structure. The central raft domain contains lipids in the liquid ordered phase, in which the chains are extended. This domain is also enriched in cholesterol, and many types of protein are bound to such domains...

The compositional phase diagram for lipid mixtures can be complicated, with regions of two-phase and three-phase coexistence. Most notably, a new fluid-like phase, the / (liquid ordered) phase has been defined in these mixtures, with increased chain ordering in comparison to the... 

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