Hydrated bilayers

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... 

Upon the spontaneous rearrangement of anhydrous phospholipids in the presence of water into a hydrated bilayer structure, a portion of the aqueous phase is entrapped within a continuous, closed bilayer structure. By this process water-soluble compounds are passively entrapped in liposomes. The efficiency of encapsulation varies and depends, for example, on the method of preparation of liposomes and the phospholipid concentration during preparation. Different parameters can be used to describe the encapsulation efficiency ... 

The In situ spectra, b, d, and f, of the hydrated bilayer show noticeable changes to the methyl asymmetric stretching bands. 

A DSC heating scan of a fully hydrated aqueous dispersion of dipalmitoylphosphatidylcholine (DPPC), which has been annealed at 0°C for 3.5 days, is displayed in Fig. 2. The sample exhibits three endothermic transitions, termed (in order of increasing temperature) the subtransition, pretransition, and main phase transition. The thermodynamic parameters associated with each of these lipid phase transitions are presented in Table 1. The presence of three discrete thermotropic phase transitions indicates that four different phases can exist in aimealed, fully hydrated bilayers of this phospholipid, depending on temperature and thermal history. All of these phases are lamellar or bilayer phases differing only in their degree of organization. 

Minami H, Inoue T. Dehydration of hydrated bilayer of dipalmitoylphospha-tidylcholine caused by beryllium ion evidence from a differential scanning calorimetry of bilayer phase transition. J Colloid Interface Sci 1998 206 338-341. 

Local anesthetics. Anesthetics interact with membranes and increase the gel to liquid-crystalline transition of fully hydrated bilayers. They induce a volume expansion which has the opposite effect of HHP and so they antagonize the effect of HHP on membranes fluidity and volume, making membranes more fluid and expanded. The application of HHP to membrane-anesthetic systems may even result in the expulsion to the aqueous environment. The local anesthetic tetracaine (TTC) can be viewed as a model system for a large group of amphiphilic molecules. From volumetric measiuements on a sample containing e.g. 3 mol% TTC, it has been found that the main tansition at ambient pressure shifts to a lower temperature. The expansion coefficient a drastically increases relative to that of the pure lipid system in the gel phase, and the incorporation of the anesthetic into the DMPC bilayer causes an about 15 % decrease of relative to that of the pure lipid system. The addition of 3 mol% TTC shifts the pressure-induced liquid-crystalline to gel phase transition towards somewhat higher pressures. Larger values for the compressibilities are found for both lipid phases by addition of 3 mol% TTC, and there is no apparent difference in the coefficient of compressibility between the gel and liquid-crystalline phases. Comparison of the IR spectra of DMPC and DMPC/TTC mixtures at pH 5.5 as a function of pressure shows an abrupt... 

Other low molecular weight compounds. With the aid of solid state NMR, Tian et al studied the interaction of cannabinoid receptor agonists, CP55940 and WIN55212-2, with hydrated bilayers of dipalmitoylphosphatidylcholine (DPPC) deuterated at the 2 and 16 positions of both acyl chains with deuterium atoms serving as probes for the... 

Fully hydrated bilayers composed of a single phospholipid species undergo a well-defined thermotropic phase transition (see Fig. 2.1) in which the lipid chains change from an ordered, pseudocrystalline or gel state to a fluid or liquid crystalline state which is similar to that found in biological membranes. The lipid chain configuration in the high-temperature, fluid phase is that described in the final section of the previous chapter. In the low temperature, ordered phase, the chains are essentially in the parallel, all-trans state,and may be tilted relative to the bilayer normal, the state as in phosphatidylcholines, or untilted the Lq state as in phosphatidylethanolamines. Additionally, an inter-... 

The bilayers reach a limiting hydration at approximately 20% water in the gel phase. Beyond this the transition temperature remains constant and further water is not incorporated between the bilayers, but is present as a free water phase in which the fully hydrated bilayers are dispersed. Above the phase transition the bilayers can accommodate more water and for phosphatidylcholine a free water phase does not appear until a/40% water content. 

Figure 1 Infi aFed spectra (1800 to 16(X) cm ) of 9-heptadecanone (9HP) incorporated in hydrated bilayers of DHPC containing 0 mol % (A), 8 mol % (B), 29 mol % (C) and 45 mol % cholesterol (D) at the indicated temperatures.

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