Lipid bilayers mesophases

Compared to other biomolecular systems, lipid bilayer membranes and lyotropic lipid mesophases in general have been shown to respond most sensitively to hydrostatic pressure. The methods used in the high pressure studies have mainly included X-ray and neutron diffraction, fluorescence, IR and Raman spectroscopy, light transmission and volumetric measurements. Only a small amount of work has been performed using NMR techniques combined with high-pressure, a field which was pioneered by Jonas and co-workers " although the method is very powerful, non-invasive and allows the study of a series of structural and dynamic properties of the systems in detail and with atomic resolution. 

LC polymers can either be thermotropic, their LC phases or mesophases are formed by heating a solid or cooling a liquid or solid, or lyotropic, the mesophases are formed by association of molecules in solution. The photochemistry and photophysics of molecules in lyotropic phases such as detergent micelles and lipid bilayers has been a very active research area for about 30 years. This extensive literature [12] is not reviewed in this chapter which is necessarily restricted to thermotropic LC polymers. 

The phase transition of lipid bilayers which comprise phospholipid mixtures, or phospholipids with different lengths of acyl chains, are of great importance due to their similarities with biomembranes or with lipid drug carriers such as liposomes, it is important to study the phase transitions and to detennine the exact crystalline mesophases of the mixed system. Thermal analysis studies indicated that the mi.xed lipids provide higher melting temperatures compared to those of pure lipids. This phenomenon occurrs w hen the lengths of the acyl chains are substantially different. 

Fig. 4. Schematic drawing of lipid-water mesophases (Lc, lamellar crystalline Lps Pp., lamellar gel L , lamellar liquid-crystalline Qn, Qn°, Qn , inverse bicontinuous cubics Hu, inverse hexagonal). The cubic phases are represented by the G, D, and P minimal surfaces, which locate the midplanes of fluid hpid bilayers.

The amphiphilic molecules can occur in lyotropic and thermotropic mesomorph, and the organization of lipid bilayers such as liposomes can exist in a temperature range where all the mesophases from gel stale to liquid crystal state are favored. In the gel state the phospholipid acyl chains are closely packed and the molecular movements are deteriorized, while in the liquid crystalline state, the faity acid moieties are in a more fluid state and are able to move more freely. 

The inclusion of cholesterol disturbs the crystalline structure of the gel phase, and the phospholipid chains are more mobile than in its absence. This prevents the crystallization of the hydrocarbon chains into the rigid crystalline gel phase. In the more fluid liquid crystalline phase, the rigid cholesterol molecules restrict the movement of the hydrocarbon chains. In consequence, the addition of cholesterol to lipid bilayers or lamellar mesophases gradually diminishes the gel-liquid crystal transition temperature and the enthalpy and broadens the DSC transition peak [72,73]. No transition can be detected by DSC at 50% cholesterol [73,74] (curve/of Fig. 7), which is the maximum concentration of cholesterol that can be incorporated before phase separation. However, laser Raman spectroscopic studies show that a noncooperative transition occurs over a very wide temperature range [75]. 

The lamellar mesophase can be diluted with water and it has almost infinite swelling capacity, provided the lipid bilayers contain charged molecules and the... 

The structural features that determine the pharmacophoric segment of sartans have also been examined by combination of quantitative structure-activity relationship (QSAR) and conformational analysis. The results show that the site of action for sartans as ATI antagonists include amino acids Lysl99, VallOS and His256 located in the mesophase of membrane lipid bilayers [14],... 

When a liquid-crystalline mesophase of a surfactant-water system is cooled below JCrafft point, a gel is formed. In the gel state, the lipid bilayers are separated by alternating water layers as in the lamellar phase. The hydrocarbon bilayers are solidified... 

The authors of this work think that the location of a protein in the cubic phase is a function of its hydrophobic-hydrophilic balance and affinity for interaction with the lipid bilayer. It was previously demonstrated in the hterature that smaQ proteins of amphiphilic nature can adsorb or penetrate into the hpid bUayer, affecting the mean interfadal curvature and inducing phase transformations. The effect coufd be stabilizing, for instance with the protein cytochrome c embedded in monoolein cubic mesophases [32, 33], or could induce structural variations as in the case of protein transfer [34]. In addition, it was su ested that the steric parameters, the interfacial activity of soluble proteins and their effect on the hpid hydration are important factors in determining the partitioning of the proteins in the cubic phase. 

Comparson of the transitions observed by differential scanning calorimetry in membranes of M. laidlawii and in water dispersions of the lipids from the membranes support the concept that most of the lipids exist as a smectic mesophase in the membranes. The evidence for a bilayer structure is straightforward in this case. Lipid transition temperatures are a function of fatty acid composition and correlate well with biological properties. The calorimeter possesses advantages over high resolution NMR for M. laidlawii, and perhaps in many other systems, because the data can be interpreted less ambiguously. In M. laidlawii membranes the bilayer appears to be compatible with the same physical properties observed in other membranes—a red-shifted ORD, lack of ft structure in the infrared, reversible dissociation by detergents, and poorly... 

More recently, it has been emphasized that inhomogeneous distribution or phase separation in fluid multicomponent lipid membranes could cause structural transformations from lamellar to intermediate nonlamellar phases or to bicontinuous bilayer phases of nonuniform interfacial curvature [30]. An example of a lipid mesophase with nonperiodic organization is the sponge (L3) phase [31,32]. The sponge (L3) mesophase appears to be an isotropic fluid devoid of... 

Lamellar mesophases are the most commonly encountered mesophases, ubiquitous in double- and higher-chained amphiphiles (including virtually all concentrated lipid-water systems). Their ideal mesostructure consists of planar, parallel stacks of amphiphilic bilayers, forming a ID smectic lattice (Figure 16.9). 

Diffusivities of binary, ternary and multi-component liquid crystalline mixtures, e.g. of soap (potassium laurate (PL), water [25, 58], and lipid (dipalmitoylphosphatidylcho-line (DPPC) [25, 59] systems in lamellar, hexagonal, cubic, nematic and micellar mesophases [25,60,61] have been studied extensively by pulsed-field-gradient NMR [25] and optical techniques [62], partly because of their intimate relation to the structure and dynamical performance of biological membranes [18]. The main distinction from thermotropic phases is that for layered structures a noticeable diffusion occurs only within the layers (i.e. lateral, frequently written as Dl, but in our notation DjJ, whereas it is negligibly small and difficult to detect across the layers [60-62] (transverse migration, for bilayers denoted by flip-flop ) so the mobility is essentially two dimensional, and the anisotropy ratio is so great that it is seldom specified explicit-... 

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