Liquid crystalline membranes

Electronic Conduction in Liquid Crystalline Membranes. Role of Unsaturated Lipids... 

The liquid crystalline membrane has a phase transition temperature, just like a thermotropic liquid crystalline compound (see Chapter 3). In this case the phase transition is called a gel point and the stmctural change at this phase transition is illustrated in Figure 7.8. 

For the comparison of NMR and X-ray diffraction measurements, the effect of membrane hydration can be relevant. A minimum of 11 to 16 water molecules per lipid molecule is needed to form a primary hydration shell for PC, PE, and PG. Additional water is in exchange with the primary hydration shell. With increasing hydration (10-70 wt% H2O) the —P -N+ dipole of the phosphocholine headgroup was shown to move with its cationic end away from the hydrocarbon layer. This explains why the —P -N+ dipoles in liquid crystalline membranes are generally slightly tilted away from the membrane surface up to an angle of about 30°, while they are oriented parallel to the surface in the crystal structure. 

Section II, A deals with the spectra of systems with very slow or very fast rotational rates. In the intermediate region (for diffusion coefficients 10 < / < 10 s and the spectral widths usually encountered with membranes), the 3 P-NMR spectra are sensitive to both type and rate of the motion. Although the lipids of liquid crystalline membranes usually have motional rates in the fast-limit region, this is often not so at lower temperatures (Fig. 9). In such cases, the spectra can be used as a source of information about the nature of the motion and orientation of the headgroup. 

Biological membranes provide the essential barrier between cells and the organelles of which cells are composed. Cellular membranes are complicated extensive biomolecular sheetlike structures, mostly fonned by lipid molecules held together by cooperative nonco-valent interactions. A membrane is not a static structure, but rather a complex dynamical two-dimensional liquid crystalline fluid mosaic of oriented proteins and lipids. A number of experimental approaches can be used to investigate and characterize biological membranes. However, the complexity of membranes is such that experimental data remain very difficult to interpret at the microscopic level. In recent years, computational studies of membranes based on detailed atomic models, as summarized in Chapter 21, have greatly increased the ability to interpret experimental data, yielding a much-improved picture of the structure and dynamics of lipid bilayers and the relationship of those properties to membrane function [21]. 

FIGURE 9.12 All illustration of the gel-to-liquid crystalline phase transition, which occurs when a membrane is warmed through the transition temperature, T. Notice that the surface area must increase and the thickness must decrease as the membrane goes through a phase transition. The mobility of the lipid chains increases dramatically. 

A solid emulsion is a suspension of a liquid or solid phase in a solid. For example, opals are solid emulsions formed when partly hydrated silica fills the interstices between close-packed microspheres of silica aggregates. Gelatin desserts are a type of solid emulsion called a gel, which is soft but holds its shape. Photographic emulsions are gels that also contain solid colloidal particles of light-sensitive materials such as silver bromide. Many liquid crystalline arrays can be considered colloids. Cell membranes form a two-dimensional colloidal structure (Fig. 8.44). 

Membranes are composed of phospholipids and proteins. The fatty acid composition of the phospholipids in a membrane influences how it is affected by the cold. In general, as the temperature of a cell is lowered the lipids in the membrane bilayer undergo a phase transition from a liquid crystalline (fluid) state to a gel (more solid) state. The temperature at which this transition takes place is very narrow for phospholipids composed of a simple mixture of fatty acids, but is quite broad for the phospholipids in cellular membranes. It is usually implied from various methods... 

A final example of the simulation of a complex system is a series of MD simulations of bilayer membranes. Membranes are crucial constituents of living organisms they are the scene for many important biological processes. Experimental data are known for model systems for example for the system sodium decanoate, decanol and water that forms smectic liquid crystalline structures at room temperature, with the lipids organized in bilayers. 

The fluidity of the cellular membrane presents a limiting factor at low temperatures. After a decrease in temperature, membranes are too rigid and must therefore be desaturated. Normally, membranes are in a liquid crystalline form... 

FIG. 7 Structures of various liquid-crystalline phases of membrane lipids. (A) Normal hexagonal phase (Hi) (B) lamellar phase (C) inverted hexagonal phase (Hu). Cubic phases consisting of (D) spherical, (E) rod-shaped, and (F) lamellar units. The hydrocarbon regions are shaded and the hydrophilic regions are white. (Reprinted by permission from Ref. 11, copyright 1984, Kluwer Academic Publishers.)... 

Major determinants of membrane fluidity may be grouped within two categories [53] (1) intrinsic determinants, i.e., those quantifying the membrane composition and phase behavior, and (2) extrinsic determinants, i.e., environmental factors (Table 1). In general, any manipulation that induces an increase in the molal volume of the lipids, e.g., increase in temperature or increase in the fraction of unsaturated acyl chains, will lead to an increase in membrane fluidity. In addition, several intrinsic and extrinsic factors presented in Table 1 determine the temperature at which the lipid molecules undergo a transition from the gel state to liquid crystalline state, a transition associated with a large increase in bilayer fluidity. 

The investigation of semi-solid objects such as lipid membranes [34], liquid crystalline dispersions [35, 36], drug delivery systems [37], food [38] and also... 

Phospholipids, which are one of the main structural components of the membrane, are present primarily as bilayers, as shown by molecular spectroscopy, electron microscopy and membrane transport studies (see Section 6.4.4). Phospholipid mobility in the membrane is limited. Rotational and vibrational motion is very rapid (the amplitude of the vibration of the alkyl chains increases with increasing distance from the polar head). Lateral diffusion is also fast (in the direction parallel to the membrane surface). In contrast, transport of the phospholipid from one side of the membrane to the other (flip-flop) is very slow. These properties are typical for the liquid-crystal type of membranes, characterized chiefly by ordering along a single coordinate. When decreasing the temperature (passing the transition or Kraft point, characteristic for various phospholipids), the liquid-crystalline bilayer is converted into the crystalline (gel) structure, where movement in the plane is impossible. 

---