Cholesterol liquid crystalline bilayers

Natural biological membranes consist of lipid bilayers, which typically comprise a complex mixture of phospholipids and sterol, along with embedded or surface associated proteins. The sterol cholesterol is an important component of animal cell membranes, which may consist of up to 50 mol% cholesterol. As cholesterol can significantly modify the bilayer physical properties, such as acyl-chain orientational order, model membranes containing cholesterol have been studied extensively. Spectroscopic and diffraction experiments reveal that cholesterol in a lipid-crystalline bilayer increases the orientational order of the lipid acyl-chains without substantially restricting the mobility of the lipid molecules. Cholesterol thickens a liquid-crystalline bilayer and increases the packing density of lipid acyl-chains in the plane of the bilayer in a way that has been referred to as a condensing effect. 

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... 

The interactions obviously differed between the lipid bilayers and the natural membranes. Furthermore, cholesterol slightly hinders the drug partitioning into the liquid-crystalline bilayers, in agreement with several previous reports, and the drug molecules interact electrostatically with membrane proteins at the hydrophilic interface adjacent to the polar headgroups of the phospholipid molecules (7). 

J. Forbes, C. Husted and E. Oldfield, High-field, high-resolution magic-angle samplespinning nuclear magnetic resonance spectroscopic studies of gel and liquid crystalline lipid bilayers and the effects of cholesterol, J. Am. Chem. Soc., 1988, 110, 1059-1065. 

In a subsequent study [30], cholesteryl-substituted 18C6 derivative 11 and diaza [18]crown-6 12 (Scheme 6) were used to create solid-supported bilayer lipids. The liquid crystalline crown derivatives 11,12 were dissolved in chloroform and mixed with squalene or squalene saturated with cholesterol. The solid-supported bilayers were prepared in freshly cut stainless steel wires. A 10-4 to 10-1 mol L-1 solution of MCI (M = Li, Na, K, Rb, Cs) or MgCl2 was used as aqueous phase. Measurement of the membrane potential revealed a Nemst response to the concentration of M+ in solution. It was possible to differentiate between the different cations which might be used for the preparation of new ion sensors. For the detection of K+ and Rb+, aza crown derivative 12 proved to be the most selective. A problem was the presence of traces of Fe2+/3+ that made the measurements difficult. It was also not... 

The third class of lipids found in stratum corneum extracts is represented by cholesterol and cholesteryl esters. The actual role of cholesterol remains enigmatic, and no clear reason for its role in the barrier function has been proposed so far. However, it is possible that contrary to what is the role in cell membranes where cholesterol increases close packing of phospholipids, it can act as kind of a detergent in lipid bilayers of long-chain, saturated lipids.30,31 This would allow some fraction of the barrier to be in a liquid crystalline state, hence water permeable in spite of the fact that not only ceramides, but also fatty acids found in the barrier are saturated, long-chain species.28,32... 

From these requirements we may infer a structure where the bulk of the intercellular lipids exist in the crystalline, close-packed state in stacked bilayer structures (Figure 2.5) due to the large amounts of long-chain saturated species. However, circumstantial evidence, for example, TEWL, indicates that a fraction of the lipid compartment should be in the liquid crystalline state, but as yet we do not know the composition of this fraction. Again the role of cholesterol may be crucial, as mentioned earlier. 

In the SC lipids form two crystalline lamellar phases.27 The mixture of both phases produces the optimal barrier to water loss from SC. The balance between the liquid crystalline and the solid crystal phases is determined by the degree of fatty acid unsaturation, the amount of water, and probably by other yet undiscovered factors. A pure liquid crystal system, produced by an all-unsaturated fatty acid mixture, allows a rapid water loss through the bilayers with a moderate barrier action. The solid system produced with an all-saturated fatty acid mixture causes an extreme water loss due to breaks in the solid crystal phase.6,23 Studies with mixtures prepared with isolated ceramides revealed that cholesterol and ceramides are very important for the formation of the lamellar phases, and the presence of ceramide 1 is crucial for the formation of the long-periodicity phase.27 The occurrence of dry skin associated with cold, dry weather for example, may result from an extensive, elevated level of skin lipids in the solid state. Therefore, a material that maintains a higher proportion of lipid in the liquid crystalline state may be an effective moisturizer.6... 

Physically, the membrane may exist in two states the "solid" gel crystalline and the "liquid" fluid crystalline states. For each type of membrane, there is a specific temperature at which one changes into the other. This is the transition temperature (Tc). The Tc is relatively high for membranes containing saturated fatty acids and low for those with unsaturated fatty acids. Thus, bilayers of phosphatidylcholine with two palmitate residues have a Tc = 41°C but that with two oleic acid residues has a Tc = -20°C. The hybrid has a Tc = -5°C. Sphingomyelin bilayer, on the other hand, may have a Tc of close to body temperature. In the gel crystalline state, the hydrophobic tails of phospholipids are ordered, whereas in the fluid crystalline state they are disordered. At body temperature, all eukaryotic membranes appear to be in the liquid crystalline state, and this is caused, in part, by the presence of unsaturated fatty acids and in part by cholesterol. The latter maintains the fatty acid side chains in the disordered state, even below the normal Tc. There is thus no evidence that membranes regulate cellular metabolic activity by changing their physical status from the gel to the fluid state,... 

Vibrational spectroscopy shows that inclusion of cholesterol in phospholipid bilayers tends to decrease the fluidity of the hydrophobic region above the main transition point Tm and to increase it below Tm. The presence of cholesterol in DPPC or DMPC muti-layered vesicles does not affect the transition point but simply broadens the transition by decreasing the CH2-stretching wavenumber in the liquid crystalline phase and by increasing it in the gel-like phase (Lippert and Peticolas, 1971 Spiker and Levin, 1976 Casal and Mantsch, 1984). There is also evidence that lipid-cholesterol interaction increases the amount of bound water in the headgroups (Levin et al., 1985). 

An obvious hypothesis is that this unusual membrane lipid composition is related directly to membrane function in some way. Within the restricted area of lipid bilayers, lipid composition is known to be an important determinant of physical properties. There are several prominent examples. First, the temperature at which the hydrocarbon chains melt when assembled in bilayers (the gel-to-liquid-crystalline transition temperature, marks an abrupt change in many of the physical properties of such bilayer systems for example, water permeability through such bilayers increases by several orders of magnitude above the transition. Second, the presence of cholesterol within bilayers composed of amphipathic lipids has a profound effect on lipid motion, mechanical properties (such as resistance to shear), and permeability to water. 

The occurrence of cholesterol and related sterols in the membranes of eukaryotic cells has prompted many investigations of the effect of cholesterol on the thermotropic phase behavior of phospholipids (see References 23-25). Studies using calorimetric and other physical techniques have established that cholesterol can have profound effects on the physical properties of phospholipid bilayers and plays an important role in controlling the fluidity of biological membranes. Cholesterol induces an intermediate state in phospholipid molecules with which it interacts and, thus, increases the fluidity of the hydrocarbon chains below and decreases the fluidity above the gel-to-liquid-crystalline phase transition temperature. The reader should consult some recent reviews for a more detailed treatment of cholesterol incorporation on the structure and organization of lipid bilayers (23-25). 

The cholesterol activity on lipid bilayers has been subjected to extensive studies. As described above, cholesterol belongs to the class of neutral (non-polar) lipids and can be characterized as an amphiphilic molecule having a hydroxyl group at the C-3 position and a non-polar region. Frequently, cholesterol is incorporated into lipid membranes and studies concerning the influence on the phase transition between gel and liquid crystalline phases of the lipid bilayers hav been published [26, 27]. 

Membrane lipids, and particularly cholesterol, are instrumental not only in the control of diffusion across biological membranes but also in the determination of the activity of membrane-bound enzymes, their modulation by hormones and other agents, and the determination of membrane fluidity (for original references, see [4,6]). It is generally accepted that incorporation of cholesterol in a lipid bilayer membrane tends to decrease significantly the permeability of these membranes to water. Movement of water across these membranes occurs primarily by dissolution in the membrane matrix. The decrease in the rate of water transport as a result of cholesterol incorporation is due mainly to a decrease in membrane fluidity. As a general rule, it is found that the presence of cholesterol in membranes or the incorporation of cholesterol into dispersions composed of phosphatidylserine or ganglioside lead to a decrease in the fluidity of the hydrocarbon chains of lipid membranes which are in the liquid-crystalline state [4,20]. 

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