Lipid packing, cholesterol effects

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. 

In summary, the dimeric lipids (29a-29h) with low m-value (3-4) and high m-value (20-22) showed exceptional thermal, lipid-packing and cholesterol-association properties. Obviously the introduction of a polymethylene spacer chain at the level of headgroup brought about dramatic effect on the aggregation behavior, membrane organization and lipid packing of 29. 

Effect of Cholesterol. Cholesterol inclusion into the lipid bilayers composed of DPPC or DSPC, eliminates apparent Tc and reduces permeability at and above the usual Tc. On the other hand, cholesterol inclusion increases packing of fluid bilayer composed of lipids with unsaturated fatty acyl chains. Since cholesterol rich liposomes are stable in plasma, cholesterol is commonly used as a liposomal component. 

However, there is a structure consistent with both the required space group and the optical properties. The gyroid surface, which occurs frequently in lipid-water systems, provides such a possibility. If we assume that cholesterol skeletons form rod-like infinite helices, this structure represents an effective three-dimensional packing of such helices. Thus, the rods form a body-centered arrangement as shown in Fig. 5.5. In this structure, there is a helical twist between the rods, in addition to the cholesteric twist within each rod. The h)rperbolic structure is a consequence of the chirality of the esters, which induces torsion into the packing arrangement. A racemic mixture does not exhibit this phase natural cholesteric esters contain a single enantiomer only. 

It has recently been shown that introduction of a small, hydrophobic molecule like isoprene may lead to a more ordered and better packed lipid membrane as revealed by MD simulations. The stabilized membrane is then protected against temperature-induced disordering of the tails. Similar thermoprotective effects are induced by cholesterol which also increases the molecular packing of the tails and, in contrast to isoprene, affects the dynamics of the lipids in the bilayer. ... 

Incorporation of cholesterol into model membranes increase the order parameter of the hydrocarbon chains but leaves the lipid lateral diffusion almost unaffected (12). Therefore it can be concluded that the effect of cholesterol on the packing properties of the bilayers is more important than its influence on lipid bilayer dynamics. 

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