Lipid polar group

Figure 2 Snapshot from an MD simulation of a multilamellar liquid crystalline phase DPPC bilayer. Water molecules are colored white, lipid polar groups gray, and lipid hydrocarbon chains black. The central simulation cell containing 64 DPPC and 1792 water molecules, outlined m the upper left portion of the figure, is shown along with seven replicas generated by the periodic boundary conditions. (From Ref. 55.)...

This chapter has given an overview of the structure and dynamics of lipid and water molecules in membrane systems, viewed with atomic resolution by molecular dynamics simulations of fully hydrated phospholipid bilayers. The calculations have permitted a detailed picture of the solvation of the lipid polar groups to be developed, and this picture has been used to elucidate the molecular origins of the dipole potential. The solvation structure has been discussed in terms of a somewhat arbitrary, but useful, definition of bound and bulk water molecules. 

From the data presented here several conclusions may be reached regarding the effect of cholesterol on lipid bilayers. It is shown that, even if the presence of cholesterol in bilayers serves to moderate temperature-induced changes, its ability to affect the location of solubilized molecules is highly temperature dependent We have also shown, in accord with previous work (11), that the presence of cholesterol in the gel phase results in a larger separation between the lipid polar groups and this in turn allows water to penetrate into the lipid hydrophobic core. 

Recent experiments have shown that the non-specific, physical chemical interactions between small hydrophobic, water-insoluble molecules and the hydrocarbon chains of lipid membranes are important determinants of the rate at which these molecules enter cells and are metabolized (3.34). Cholesterol has the capability of modifying these interactions and also increases the affinity of vesicle surfaces for amphiphillic molecules (4) separating the lipid polar groups (35). 

Cholesterol can modify both the hydrophobic attraction between lipid hydrocarbon chains and electrostatic interactions between lipid polar groups. The influence it has on the location of 9HP reflects this dual effect At low temperature, the "spacer" effect of cholesterol allows the ketone to gain access directly to the lipid-water interface. At high temperatures, a more disordered hydrocarbon core favors the solubilization of the guest molecule. 

La ", by virtue of an ionic radius similar to and a valence higher than Ca., was predicted by Lettvin et al. (1964) to bind at superficially located Ca " adsorption sites in a less reversible manner than does Ca " itself. This work was closely followed by a report on the displacement of Ca " from membranes in the presence of La " (Doggenweiler and Frenk 1965). A differential binding of La by the constituents making up the outer strata of the unit membrane - the lipid polar groups and the nonpolar components - was demonstrated. Lanthanum ions were found to displace Ca from a monolayer of phosphatidyl serine and phosphatidyl ethanolamine but not from phosphatidyl choline. 

The structure of cholic acid helps us understand how bile salts such as sodium tauro cholate promote the transport of lipids through a water rich environment The bot tom face of the molecule bears all of the polar groups and the top face is exclusively hydrocarbon like Bile salts emulsify fats by forming micelles m which the fats are on the inside and the bile salts are on the outside The hydrophobic face of the bile salt associates with the fat that is inside the micelle the hydrophilic face is m contact with water on the outside... 

Generally, the recrystaUization of S-layer protein into coherent monolayer on phospholipid films was demonstrated to depend on (1) the phase state of the hpid film, (2) the nature of the lipid head group (size, polarity, and charge), and (3) the ionic content and pH of the subphase [122,138] (Table 6). 

Lipids have the common property of being relatively insoluble in water (hydrophobic) but soluble in nonpolar solvents. Amphipathic lipids also contain one or more polar groups, making them suitable as constituents of membranes at lipidiwater interfaces. 

The nonpolar lipid core consists of mainly triacylglycerol and cholesteryl ester and is surrounded by a single surface layer of amphipathic phospholipid and cholesterol molecules (Figure 25-1). These are oriented so that their polar groups face outward to the aqueous medium, as in the cell membrane (Chapter 14). The protein moiety of a lipoprotein is known as an apo-lipoprotein or apoprotein, constituting nearly 70% of some HDL and as litde as 1% of chylomicrons. Some apolipoproteins are integral and cannot be removed, whereas others are free to transfer to other hpoproteins. 

Interaction with a lipid bilayer driven by a potential difference and by polar and/or hydrophobic forces between the amino acid side chains of the pardaxin tetramers and the polar membrane lipid head group triggers insertion from a "raft" like structure. 

This is the most polar group of lipids in natural lipid samples. When developed in a nonpolar solvent system, phospholipids remain at the origin and more polar solvent system should be used to elute and separate individual phospholipids. The most popular system is the Wagner system, which consists of chloroform metha-nohwater (65 25 4) [51] for the separation of common phospholipid species in natural tissue samples. 

The cells of all contemporary living organisms are surrounded by cell membranes, which normally consist of a phospholipid bilayer, consisting of two layers of lipid molecules, into which various amounts of proteins are incorporated. The basis for the formation of mono- or bilayers is the physicochemical character of the molecules involved these are amphipathic (bifunctional) molecules, i.e., molecules which have both a polar and also a non-polar group of atoms. Examples are the amino acid phenylalanine (a) or the phospholipid phosphatidylcholine (b), which is important in membrane formation. In each case, the polar group leads to hydrophilic, and the non-polar group to hydrophobic character. 

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