Bilayer phases

Hydrated bilayers containing one or more lipid components are commonly employed as models for biological membranes. These model systems exhibit a multiplicity of structural phases that are not observed in biological membranes. In the state that is analogous to fluid biological membranes, the liquid crystal or La bilayer phase present above the main bilayer phase transition temperature, Ta, the lipid hydrocarbon chains are conforma-tionally disordered and fluid ( melted ), and the lipids diffuse in the plane of the bilayer. At temperatures well below Ta, hydrated bilayers exist in the gel, or Lp, state in which the mostly all-trans chains are collectively tilted and pack in a regular two-dimensional... 

Phase transitions have been characterized in a number of different pure and mixed lipid systems. Table 9.1 shows a comparison of the transition temperatures observed for several different phosphatidylcholines with different fatty acyl chain compositions. General characteristics of bilayer phase transitions include the following ... 

Bilayer phase transitions are sensitive to the presence of solutes that interact with lipids, including multivalent cations, lipid-soluble agents, peptides, and proteins. 

In bilayer phase the dipole arrangement is antiferroelectric-like. However there are... 

Korlach, J., Schwille, P., Webb, W. W. and Feigenson, G. W. (1999) Characterization of lipid bilayer phases by confocal microscopy and fluorescence correlation spectroscopy. Proc. Natl. Acad. Sci. USA, 96, 8461-8466. 

Benzyl alcohol. We have demonstrated that all benzyl aleohol (BzOH) moleeules dissolved in water are also immediately transported and trapped in the lipid bilayer phase, when it is mixed with EPC bilayer [46]. The trapped site of BzOH, however, differs from that of PrBe. As shown in Fig. 8, the ring proton signal of BzOH in water is at first broadened and shifted to a higher field [from (a) to (b)j then the signal is split into two (e). 

The DD site can be ensured by the chemical shift changes of the BPA signals. The chemical shift differences of the ring proton signals of BPA on the delivery from water to bilayer phases are —0.04 and —0.11 ppm for the ortho and meta sites, respectively. Negative values mean upfield shifts recall the HCS rule. It is concluded that both benzene rings of BPA are trapped in the bilayer from the water phase and the meta site penetrates more deeply into the hydrophobic interior. 

In the preceding section, we explored the relationship between log P ct and log mem- now focus on the partitioning of the charged species into phospholipid bilayer phases. More surprises are in store. 

The exact mechanism of leaking induced by heating the dry product above Tm is unclear, but the authors exclude a bilayer phase transition during rehydratation or a fusion between the liposomes as a cause of the leakage. 

Nagle, J. F. (1980). Theory of the main lipid bilayer phase transition, Ann. Rev. Phys. Chem., 31, 157-195. 

The vast majority of biological membranes are in the liquid-crystalline phase. There are many experimental studies on model bilayer phase behavior [3]. Briefly, at low temperatures lipid bilayers form a gel phase, characterized by high order and rigidity and slow lateral diffusion. There is a main phase transition, as the temperature is increased, to the liquid-crystalline phase. The liquid-crystalline phase has more fluidity and fast lateral diffusion. 

Xiang, T., Xu, Y. and Anderson, B.D. (1998) The barrier domain for solute permeation varies with lipid bilayer phase structure. Journal of Membrane Biology, 165, 77-90. 

Waszczuk et al. [329] have carried out radiometric studies of UPD of thallium on single-crystal Ag electrode from perchloric acid solutions. Deposition of Tl on Ag(lOO) to obtain monolayer, bilayer, and bulk crystallites has been studied by Wang et al. [330]. These studies have shown that apart from the substrate geometry, the nature of the substrate-adatom interactions also influence the structure of the UPD metal adlayers. This is because of the fact that, contrary to Au and Pt electrodes, Tl forms a well-ordered bilayer phase before bulk deposition on Ag(lOO) surface occurs. 

Figure 8-12 (A) 31P NMR spectra of different phospholipid phases. Hydrated soya phosphatidylethanolamine adopts the hexagonal Hn phase at 30°C. In the presence of 50 mol% of egg phosphatidylcholine only the bilayer phase is observed. At intermediate (30%) phosphatidylcholine concentrations an isotropic component appears in the spectrum. (B) Inverted micelles proposed to explain "lipidic particles" seen in freeze fracture micrographs of bilayer mixture of phospholipids, e.g., of phosphatidylethanolanine + phosphatidylcholine + cholesterol. From de Kruijft et al.m Courtesy of B. de Kruijft.

Fig. 6.36 Phase diagram calculated using SCFT for a blend of a symmetric diblock with a homopolymer with fl = 1 (see Fig. 6.32 for a blend with a diblock with / = 0.45) as a function of the copolymer volume fraction

Fig. 6.40 A phase diagram calculated using SCFT for a mixture containing equal amounts of two homopolymers and a symmetric diblock, all with equal chain length (Janert and Schick 1997a). A-rich and B-rich swollen lamellar bilayer phases are denoted LA and LH respectively whilst the corresponding disordered phases are denoted A and B. The con-solute line of asymmetric bilayer phases LA and Lu, shown dotted, is schematic.The dashed line is the unbinding line. The arrows indicate the locations of the unbinding transition X jN and multicritical Lifshitz point, cMiV " 6.0.

The surfactant properties of polymeric silicone surfactants are markedly different from those of hydrocarbon polymeric surfactants such as the ethylene oxide/propylene oxide (EO/PO) block copolymers. Comparable silicone surfactants often give lower surface tension and silicone surfactants often self-assemble in aqueous solution to form bilayer phases and vesicles rather than micelles and gel phases. The skin feel and lubricity properties of silicone surfactants do not appear to have any parallel amongst hydrocarbon polymeric surfactants. 

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