Membrane lipid bilayers packing

Another factor affecting the lifetime of a membrane fluorophore probe is its proximity to the surface. The lifetimes of the DPH, DPH-phosphatidyl-choline (DPH-PC), and trimethylammonium-DPH (TMA-DPH) probes decrease in the order DPH > DPH-PC > TMA-DPH, as the probe locates nearer to the surface of the lipid bilayer.(7) The same is found for the anthroyl-stearate probes.(8) More recently, it has been shown that with TMA-DPH, the lifetime appears to be fairly sensitive to the differences in lipid bilayer packing induced by differing degrees of unsaturation in the phospholipid fatty acyl chains.(9) This aspect of the use of TMA-DPH and possibly other probes remains to be further exploited. 

Diffusion without the aid of a transport mediator, as depicted in Figure 19.4a, may occur to some extent under the influence of a very high transmembrane concentration difference and, in the case of charged species, a high transmembrane potential. Of course, such diffusion processes are more likely to occur where the lipid bilayer packing is perturbed, that is, where it is less dense. Such packing defects could be induced by incorporating certain compounds such as alcohols in the bilayer. Membranes may, furthermore, become leaky by the penetration of antibiotics, toxins, and so on. Because of their specihcity these biomolecules may induce selective permeability. 

A continuous lipidic cubic phase is obtained by mixing a long-chain lipid such as monoolein with a small amount of water. The result is a highly viscous state where the lipids are packed in curved continuous bilayers extending in three dimensions and which are interpenetrated by communicating aqueous channels. Crystallization of incorporated proteins starts inside the lipid phase and growth is achieved by lateral diffusion of the protein molecules to the nucleation sites. This system has recently been used to obtain three-dimensional crystals 20 x 20 x 8 pm in size of the membrane protein bacteriorhodopsin, which diffracted to 2 A resolution using a microfocus beam at the European Synchrotron Radiation Facility. 

It is well known that the permeabilities across biological membranes and model lipid bilayers strongly depend on both the degree of lipid chain packing in the membranes... 

As seen in Eq. (6), AGjw depends on the packing density, tim, of the lipid membrane which in turn depends on the lipid composition. According to the above results, Km is expected to increase with increasing packing density of the membrane, and this has indeed been demonstrated by functionally reconstituting P-gp in different lipid bilayers [62],... 

Unlike other Eukarya, animal cells lack cell walls, though they tend to be surrounded by a highly developed glycocalyx of up to 140 nm in thickness [108]. This diffuse layer of densely packed oligosaccharides has a heterogeneous composition and is connected to the membrane via lipids or integral proteins. The boundary of the cell usually extends beyond the mere lipid bilayer with its embedded proteins, and the extracellular structures provide initial sites of interaction or are themselves targets for MAPs such as antimicrobial peptides [115]. 

The membrane is a dynamic assembly and things are diffusing rapidly in the plane of the bilayer. The middle of the bilayer has been likened to olive oil. As with oil, cooling the lipid bilayer will cause the hydrocarbons to become more ordered (structured). The side chains pack closer to each other, and the fluidity of the membrane is lower. Things that disrupt the ability of the side chains to pack in a regular fashion make the membrane more fluid (Fig. 3-4). These include high temperature, lipids with shorter chains (double bonds. The shorter lipids and the m-double bonds cause the occurrence of holes (packing defects). 

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. 

The betulinic acid level in the E. florida leaves increased significantly in the May, June, Jully (autumn - winter) and, September, October and November (winter) which was mainly due to the accumulation of this compound in vegetal tissue. Some authors related with the pentacyclic triterpenes, just as betulinic, acid ursolic, acid, p-amyrine and lupeol, are supposed to be toxic to insects, due to their ability to inhibit acyl chain packing in the lipid bilayers of the insect membranes [Rodriguez et al, 1997 Prades et al., 2011]. 

We have not yet said much about the second major constituent of eukaryotic membrane lipids, cholesterol. Cholesterol broadens the melting transition of the phospholipid bilayer (see fig. 17.20). Below the Tm, cholesterol disorders the membrane because it is too bulky to fit well into the neatly packed arrangement of the fatty acid chains that is favored at low temperatures. Above the Tm, cholesterol restricts further disordering because it is too large and inflexible to join in the rapid fluctuations of the chains. If the amount of cholesterol in a phospholipid bilayer is increased to about 30%, roughly the amount in the plasma membranes of typical animal cells, the melting transition becomes so broad as to be almost undetectable. 

Deuterium NMR (2H-NMR) is a powerful technique to obtain information on both the degree of order and the molecular dynamics of liquid crystalline media. It has extensively been used on model as well as natural membranes. Deuterium NMR has been used as a probe to investigate chain packing in lipid bilayers, and the effects of hydrocarbons and alcohols and their location in the membrane have been determined [123]. 

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... 

Stretch-activated proteins in animal cell membranes that are candidates for osmosensing activity include mechanosensitive ion channels and the membrane-localized enzyme phospholipase A2 (PLA2). The former proteins remain to be conclusively linked to osmosensing. Activity of PLA2 is sensitive to packing of the lipid bilayer of the cell and is responsive to osmotic changes, two attributes that mark it as a prime candidate for a stretch-activated sensor (Lehtonen and Kinnunen, 1995). 

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