Structures lipid bilayers

III. LIPID BILAYER STRUCTURE A. Overall BIlayer Structure... 

The artificial lipid bilayer is often prepared via the vesicle-fusion method [8]. In the vesicle fusion process, immersing a solid substrate in a vesicle dispersion solution induces adsorption and rupture of the vesicles on the substrate, which yields a planar and continuous lipid bilayer structure (Figure 13.1) [9]. The Langmuir-Blodgett transfer process is also a useful method [10]. These artificial lipid bilayers can support various biomolecules [11-16]. However, we have to take care because some transmembrane proteins incorporated in these artificial lipid bilayers interact directly with the substrate surface due to a lack of sufficient space between the bilayer and the substrate. This alters the native properties of the proteins and prohibits free diffusion in the lipid bilayer [17[. To avoid this undesirable situation, polymer-supported bilayers [7, 18, 19] or tethered bilayers [20, 21] are used. 

Before going to the discussion of DD, we first summarize the general features of lipid bilayer structures and dynamics. 

Sretcher, M.S., 1972a, Asymmetrical lipid bilayer structure for biological membranes. Nature New Biol, 236 11-12. 

The low lipid-protein ratio of 0.5 together with the size of the sarcoplasmic vesicles implies that only approximately 30% of their membranes can be occupied by a regular lipid bilayer structure. Consequently, a large fraction of the membrane protein must interrupt the lipid bilayer and reach throughlt. The fact that only one polypeptide chain constitutes the structural unit of the calcium transport protein strong-... 

Although the lipid bilayer structure is quite stable, its individual phospholipid and sterol molecules have some freedom of motion (Fig. 11-15). The structure and flexibility of the lipid bilayer depend on temperature and on the kinds of lipids present. At relatively low temperatures, the lipids in a bilayer form a semisolid gel phase, in which all types of motion of individual lipid molecules are strongly constrained the bilayer is paracrystalline (Fig. ll-15a). At relatively high temperatures, individual hydrocarbon chains of fatty acids are in constant motion produced by rotation about the carbon-carbon bonds of the long acyl side chains. In this liquid-disordered state, or fluid state (Fig. 11—15b), the interior of the bilayer is more fluid than solid and the bilayer is like a sea of constantly moving lipid. At intermediate temperatures, the lipids exist in a liquid-ordered state there is less thermal motion in the acyl chains of the lipid bilayer, but lateral movement in the plane of the bilayer still takes place. These differences in bilayer state are easily observed in liposomes composed of a single lipid,... 

Figure 8-4 Bimolecular lipid layers and membranes. (Top) A molecule of phosphatidylcholine. (Center) Lipid bilayer structure. (Bottom) Bilayer structure as seen by the electron microscope with osmium tetroxide staining.

Figure 8. Molecular arrangements in aqueous precipitates of insulin and lecithin-car diolipin (13). Left the lipid bilayer structure used in the preparations center the precipitate formed by a 9/1 molar ratio of lecithin-cardiolipin and right the precipitate formed by a 6/4 molar ratio.

The existence of a forbidden water layer thickness range, which seems to be a general phenomenon with these gel phases, might be relevant to cell adhesion and equilibrium distances at cell contact. The gel represents one type of lipid bilayer structure that occurs in membranes (see below), and, because of the dominance of neutral lipid molecules, the... 

The plasma membrane of fungi has the typical lipid bilayer structure, with phosphatidylcholine, phosphatidylethanolamine, and ergosterol as major lipid con-... 

Notably, all these characteristic alterations are conferred by cell membranes particularly the cytoplasmic membrane that is basically a lipid-bilayer structure imbedded with certain proteins. The lipid-bilayer forms a barrier to surround and protect the cell contents and the transmembrane proteins are responsible for the cell communications with the environment. For example, receptor proteins mediate the growth signals produced by growth factors and mitogens and any other stimuli. Ion channel proteins control the flux of ions across the cytoplasmic membrane to regulate membrane potential, osmolar-ity (or cell volume), etc. 

On the other hand, PAMPA is a purely artificial method and PAMPA membranes do not reassemble real lipid bilayer structures as barriers for permeation but much thicker barriers. The thickness and material of the supporting PVDF filters also influences artificially the permeation of compounds depending on the lipophilicity of the compounds more than the thin polycarbonate filter does in CACo2 experiments. Also the best choice of membrane constituents for PAMPA experiments is still under investigation and it seems that it will depend a lot on the goal of the PAMPA experiment which membrane is used (e.g. blood brain barrier permeation or intestinal absorption). One has to take into account that PAMPA today is a summary term on a lot of different methods applied in different laboratories using different membrane constituents, sink conditions, permeation times etc., which makes inter laboratory comparison difficult. 

Katsaras J, Gutberlet T, eds. Lipid Bilayers Structure and Interactions. 2001. Springer-Verlag, Berlin. 

J.F. Nagle and S. Tristram-Nagle. 2000. Lipid bilayer structure Curr. Opin. Struct Biol. 10 474-480. (PubMed)... 

Chemical reactions occurring in geometries resembling those found in a cell can be investigated with the help of nanotube-vesicle networks (NVNs, Fig. 23.3 [fO]). NVNs are highly flexible lipid bilayer structures in which the main building blocks are surface-immobilized vesicles connected by nanotubes. 

The membranes of living cells have a lipid bilayer structure. 

Liposomes are characterized by a lipid bilayer structure with clearly separated hydrophilic and hydrophobic regions. Hydrophilic portions of bilayer lipids are directed towards the internal and external aqueous phases, whereas hydrophobic portions of both lipid layers are directed towards each another, forming the internal core of the membrane. A useful feature of liposomes used for drug delivery is that they allow for localization and encapsulation both water-soluble and water-insoluble substances, either together or separately. Water-soluble materials are entrapped in... 

Slater SJ, Kelley MB, Taddeo FJ, Ho C, Rubin E, Stubbs CD. The modulation of protein kinase C activity by membrane lipid bilayer structure. J Biol Chem 1994 269 4866 871. 

Stranded -helix in a lipid bilayer structure [95] to be that given in Figure 34A, curve a, and the absorption curve for the system is given in Figure 34B. Again, this is a unique CD pattern that is quite distinct from the CD patterns of the above described... 

Another most interesting aspect concerns the mechanical coupling of the polymer cushion with the membranes and their incorporated proteins. This could lead to interfacial architectures that show interesting features of structure formation by the coupling of the specific entropy driven properties of polymers in general with the self-organization capability of lipid bilayer structures. Experiments along these lines are under way. 

It was found that the presence of cholesterol markedly influences the hydrogenation of mixed phospholipid dispersions [12] because it restricts partition of the catalyst into the lipid bilayer structure. It was also found that no dihydrocholesterol forms during hydrogenation of the phospholipid, showing that cholesterol is not a substrate for reaction under the conditions employed. 

In summary, a lipid molecule on its route from the luminal bulk phase into the intracellular compartment of an enterocyte has to overcome two unstirred water layers and one plasma membrane of lipid bilayer structure. The unstirred water layer on the luminal side partly coincides with the mucus gel and the glycocalyx relatively little is known of the importance of these diffusional barriers. 

Of what practical value is the process of diffusion to the cell Certainly, diffusion is able to distribute metabolites effectively throughout the interior of the cell. But what about the movement of molecules through the membrane Because of the lipid bilayer structure of the membrane, only a few molecules are able to diffuse freely across a membrane. These include small molecules such as O2 and CO2. Any large or highly charged molecules or ions are not able to pass through the lipid bilayer directly. Such molecules require an assist from cell membrane proteins. Any membrane that allows the diffusion of some molecules but not others is said to be selectively ipermeahle. 

---