Phospholipids crystallization

Therraodjmamics of phospholipids depend on their molecular structure the conformation and the geometry of the phospholipids polar head, the orientation of the molecular backbone (glycerol in the case of glycerophospholipids) and the acyl-chaiiis orientation. NMR spectroscopy has been used extensively in order to reveal the lipid orientation in membrane structures [20]. NMR experiments have shown that except for phospatidic acid, in all phospholipid crystal structures a part of the sn-2 fatty acyl chain is parallel to the bilaycr surface. In egg phosphatidylcholines for example the averege size of the sn-2 acyl chain is 18 carbons while the sn- fatty acid is 16 carbons long. The final configuration of the two fatty acyl chains extends to the same depth from the bilayer surface and a thermodynamicaly stable conformation of the acyl lipid chains is formed [21]. 

McConnell, H. M., Keller, D., Gaub, H. (1986). Thermodynamic models for the shapes of monolayer phospholipid crystals, J. Phys. Chem., 90 1717. 

A. Miller and H. Mohwald, Collecting Two-Dimensional Phospholipid Crystals in Inhomogeneous Electric Fields, Europhys. Lett. 2 67 (1986). 

H.M. McConnell, D. Keller, and H. Gaub, Thermodynamic Models for the Shapes of Monolayer Phospholipid Crystals, I. Phys. Chem. 90 1717 (1986). 

The classical X-ray studies on soaps and neutral fats have been reviewed by Chapman (1965) and Luzzatti (1968) among others. Here we shall concentrate on the results concerning polar lipids, and especially the recent studies on phospholipid crystals. Lipid crystals contain only a single species of lipid (there can be no heterogeneity of hydrocarbon chain length) and have a low water content. 

Relatively few single crystal structures have been obtained of phospholipids, but those which are known reveal the same major structural features. The results are of direct relevance to biomembrane structure since phospholipids crystallize in bilayers and the characteristic features of the molecular structure are preserved in fully hydrated, fluid phospholipid bilayers. 

Hauser, H., Pascher, I., and Sundell, S., 1980, Conformation of phospholipids. Crystal structure of lysophosphatidylcholine analogue, J. Mol. Biol., 137 249. 

The variety of molecules used to prepare LB films is enonnous. and only a small selection of examples can be presented here. Liquid crystals and biomolecules such as phospholipids, for example, can also be used to prepare LB films. The reader is referred to tire literature for infonnation about individual species. 

Fig. 13. Lyotropic liquid crystal structures (a) micelle formed by a typical soap (b) vesicle formed by a typical phospholipid.

Phospholipids. For the removal of ionic contaminants from raw zwitterionic phospholipids, most lipids were purified twice by mixed-bed ionic exchange (Amberlite AB-2) of methanolic solutions. (About Ig of lipid in lOmL of MeOH). With both runs the first ImL of the eluate was discarded. The main fraction of the solution was evaporated at 40°C under dry N2 and recryst three times from n-pentane. The resulting white powder was dried for about 4h at 50° under reduced pressure and stored at 3°. Some samples were purified by mixed-bed ion exchange of aqueous suspensions of the crystal/liquid crystal phase. [Kaatze et al. J Phys Chem 89 2565 7955.]... 

Beamer LJ, Carroll SF, Eisenberg D (1997) Crystal structure of human BPI and two bound phospholipids at 2.4 angstrom resolution. Science 276 1861—1864... 

The increase in Ca is initiated rapidly and begins to recover after 1 min. The order of potency correlates fairly well with the solubilities of these compounds in organic solvents (37) and their abilities to accumulate in phospholipid vesicles (38), i.e., 6>y>a>p, but not with their insecticidal activity (y 6>a p 39). At these concentrations, crystals of p-, a-, and y-HCH were evident in the cell suspensions when we made simultaneous measurements of the right-angle light scatter, indicating that the order of aqueous solubilities is 6>y>a>p. However, stimulation by 6-HCH at concentrations below its aqueous solubility limit shows a typical dose dependency of the response (Figure 10). 

Papahadjopoulos, D., and Watkins, J. C. (1967). Phospholipid model membranes. II. Permeability properties of hydrated liquid crystals, Biochim. Biophys. Acta. 135. 639-652. 

FIG. 14 Schematic illustration of an archaeal cell envelope structure (a) composed of the cytoplasmic membrane with associated and integral membrane proteins and an S-layer lattice, integrated into the cytoplasmic membrane, (b) Using this supramolecular construction principle, biomimetic membranes can be generated. The cytoplasmic membrane is replaced by a phospholipid or tetraether hpid monolayer, and bacterial S-layer proteins are crystallized to form a coherent lattice on the lipid film. Subsequently, integral model membrane proteins can be reconstituted in the composite S-layer-supported lipid membrane. (Modified from Ref. 124.)... 

Phospholipids or similar water-insoluble amphiphilic natural substances aggregate in water to form bilayer liquid crystals which rearrange when exposed to ultrasonic waves to give spherical vesicles. Natural product vesicles are also called liposomes. Liposomes, as well as synthetic bilayer vesicles, can entrap substances in the inner aqueous phase, retain them for extended periods, and release them by physical process. 

Phospholipids, which are one of the main structural components of the membrane, are present primarily as bilayers, as shown by molecular spectroscopy, electron microscopy and membrane transport studies (see Section 6.4.4). Phospholipid mobility in the membrane is limited. Rotational and vibrational motion is very rapid (the amplitude of the vibration of the alkyl chains increases with increasing distance from the polar head). Lateral diffusion is also fast (in the direction parallel to the membrane surface). In contrast, transport of the phospholipid from one side of the membrane to the other (flip-flop) is very slow. These properties are typical for the liquid-crystal type of membranes, characterized chiefly by ordering along a single coordinate. When decreasing the temperature (passing the transition or Kraft point, characteristic for various phospholipids), the liquid-crystalline bilayer is converted into the crystalline (gel) structure, where movement in the plane is impossible. 

The tissue sections must be washed with organic solvents when the detection targets include peptides and proteins. Washing with organic solvents promotes the ionization of peptides and proteins mainly by removing phospholipids from the sections.14 Washing also flushes out salts that could interfere with the crystallization of the matrix. 

Both the N- (a-methylbenzy 1) stearamide and phospholipid systems as detailed above proved to be difficult systems with which to work. The inability of N- a-methylbenzy 1)stearamide to form stable monolayers or even to spread from the crystal on anything but very acidic subphases presents a significant technical challenge despite the presence of a chiral headgroup that is unobstructed by other molecular features. On the other hand, the phospholipid surfactants that spread to form stable films both from solution and from their bulk crystals on pure water subphases at ambient temperatures displayed no discernible enantiomeric discrimination in any film property. The chiral functionality on these biomolecules is apparently shielded from intermolecular interactions with other chiral centers to the extent... 

Cholesterol-phospholipid-lipoprotein liquid crystal phase, 15 112 Cholestyramine, 14 420... 

The 31P n.m.r. of phospholipids has been the subject of a number of papers.62-89 These have been primarily aimed at investigating the conformation and motion of phospholipids in bilayers, but information has also been obtained on gel-to-liquid crystal transformations of phospholipids.65-67 A P31 1H nuclear Overhauser effect indicates that there is little tendency for mixed phosphatidylcholine/phosphatidyl-ethanolamine vesicles to segregate in separate domains.68 69 A phosphonium analogue (23) of choline chloride has been prepared and converted chemically into... 

P—S bond is elongated to 196.5 pm and the P—0(H) bond shortened to 148 pm.164 The structures of the m-vinyl carbonate (136)165 and the cyclic phosphonic anhydride (96)123 have been determined, and for the latter compound the ring was found to be almost planar. A -Ray diffraction data of deoxyuridine phosphate,166 phospholipid multilayers,167 nucleotides,168 and skeletal muscle169 have been analysed. The crystal structure of the phosphazene (137) has been established.170... 

A further possibility is the formation of liquid crystals on contact with body fluids at the site of application. The initially applied drug solution interacts with body fluids such as plasma, tears, or skin lipids and undergoes a phase transition into a mono-or multiphasic system of liquid crystals (Fig. 15). For example, oily solutions of reverse micellar solutions of phospholipids, which solubilize additional drug, trans-... 

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