Lipid bilayers thickness

The lamellar spacing of a monoglyceride gel phase as a function of water content is plotted in Figure 14. The gel phase of the neutral monoglyceride has a lipid bilayer thickness of 49.5 A, and it swells to a unit layer thickness of 64 A (20). If an ionic amphiphilic substance (e.g. a soap) is solubilized in the lipid bilayer, it is possible to obtain a gel phase with high water content. As with the gel phases with infinite swelling that were discussed above, there is, however, a minimum water layer thickness which in this monoglyceride gel is about 40 A. 

Lewis, B. A. and Engelman, D. M. (1983). Lipid bilayer thickness varies linearly with acyl chain length in fluid phosphatidylcholine vesicles. J. Mol. Biol. 766 211. 

In the case of monostearin, with a lipid bilayer thickness of 50.5 A, the gel swells to a water layer thickness of 14 A. When 1-2% (w/w) of a charged lipid is also present, however (for example sodium stearate) it is possible to achieve swelling to water layer thickness values near the wavelength of light. Such gel phases can exhibit the colours of the visible spectrum. 

Fig. 2 An ultrathin membrane (e.g. BLM) separating two aqueous solutions. Biface is defined as two interconnecting interfaces where material and energy transport are possible. f = A F/tm, P = electrical potential, Arf = potential difference across the membrane = Em, m = lipid bilayer thickness (estimate varies from 5 to 5 nm) (see Fig. 1 and text for details).

N. Mobashery, C. Nielsen, O. S. Andersen, The conformational preference of gramicidin channels is a function of lipid bilayer thickness, FEBS Letters 412 (1997) 15. 

Phospholipids e.g. form spontaneously multilamellar concentric bilayer vesicles73 > if they are suspended e.g. by a mixer in an excess of aqueous solution. In the multilamellar vesicles lipid bilayers are separated by layers of the aqueous medium 74-78) which are involved in stabilizing the liposomes. By sonification they are dispersed to unilamellar liposomes with an outer diameter of 250-300 A and an internal one of 150-200 A. Therefore the aqueous phase within the liposome is separated by a bimolecular lipid layer with a thickness of 50 A. Liposomes are used as models for biological membranes and as drug carriers. 

Phospholipids are found widely in both plant and animal tissues and make up approximately 50% to 60% of cell membranes. Because they are like soaps in having a long, nonpolar hydrocarbon tail bound to a polar ionic head, phospholipids in the cell membrane organize into a lipid bilayer about 5.0 nm (50 A) thick. As shown in Figure 27.2, the nonpolar tails aggregate in the center of the bilayer in much the same way that soap tails aggregate in the center of a micelle. This bilayer serves as an effective barrier to the passage of water, ions, and other components into and out of cells. 

Cell membranes consist of two layers of oriented lipid molecules (lipid bilayer membranes). The molecules of these two layers have their hydrocarbon tails toward each other, while the hydrophilic heads are outside (Fig. 30.1a). The mean distance between lipid heads is 5 to 6mn. Various protein molecules having a size commensurate with layer thickness float in the lipid layer. Part of the protein molecules are located on the surface of the lipid layer others thread through the layer (Fig. 30.1fc). Thus, the membrane as a whole is heterogeneous and has a mosaic structure. 

The thickness of the membrane phase can be either macroscopic ( thick )—membranes with a thickness greater than micrometres—or microscopic ( thin ), i.e. with thicknesses comparable to molecular dimensions (biological membranes and their models, bilayer lipid films). Thick membranes are crystalline, glassy or liquid, while thin membranes possess the properties of liquid crystals (fluid) or gels (crystalline). 

Figure 6 Intestinal cell membrane model with integral membrane proteins embedded in lipid bilayer. The phospholipid bilayer is 30-45 A thick, and membrane proteins can span up to 100 A through the bilayer. The structure of a typical phospholipid membrane constituent, lecithin is illustrated. (From Ref. 76.)...

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]. 

Let us first consider the lipid molecular structures required. First is the hydrophobic matching. The length of the hydrophobic chain determines the thickness of the hydrophobic part of the lipid bilayer, this should correspond closely to the dimension of the native membrane. As most biological membranes contain diacylglycerol lipids with hydrophobic chain lengths of 16 18 carbon atoms. Thus, synthetic lipids should possess relatively long hydrocarbon chain length, e.g., 16-18 carbon atoms. 

In order to verify that the adsorbed lipid membrane indeed forms a bilayer film, another experiment is conducted with an aim to detect the formation of a monolayer lipid. It starts with a piranha-cleaned micro-tube treated with silane to render its inner surface hydrophobic. POPC liposome is then injected into the microtube. It is known that POPC lipid will form a monolayer to such a surface by orienting their hydrophobic tails toward the hydrophobic wall. The experimental results using a mode with similar sensitivity as the previous experiment are shown in Fig. 8.39. The resonance shift in this case is 22 pm, which is about half of that observed for the adsorption of a lipid bilayer. These two experiments suggest that the microtube resonator is capable of accurately determining an adsorbed biomolecular layer down to a few nm thicknesses. 

Figure 2. This figure gives a schematic illustration of various fluctuations that exist in lipid bilayers. From top to bottom (1) the increase in area and concomitant reduction in membrane thickness is strongly damped. (2) Up and down movements of the lipids are restricted to small amplitudes, i.e. much less than the tail length. (3) Interpenetration of lipids into the opposite monolayer is, in first approximation, forbidden. (4) Conformations of the lipid tails have only few gauche defects, so that the tail is only slightly curved. Reproduced from (58) with permission from the Biophysical Society...

Lindahl, E. and Edholm, O. (2000). Mesoscopic undulations and thickness fluctuations in lipid bilayers from molecular dynamics simulations, Biophys. J.,19,426-433. 

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