Phosphatidylcholine lamellar phase

Monoolein will also form the cubic phase together with lecithin (e.g. dioleoyl phosphatidylcholine, see Figure 1), but above about 50% (w/w) lecithin the cubic phase is transformed into the lamellar phase (2). Moreover, water may be replaced by glycerol, completely or partly, in the cubic phase. Vegetable oils, e.g. sesame oil, can be incorporated to some extent (a few percent) in the cubic phase, and the same holds for bile salts. 

Lamellar phases of phospholipids often exhibit myehnic figures when contacted with water. Electron micrographs [24,26] showed that each tubular myehnic figure in the egg-yolk phosphatidylcholine/water system consisted of a water core surrounded by many concentric bilayers. More recently Raman spectroscopy techniques have confirmed the concentric bilayer arrangement [1,18]. Myelinic figures are not equilibrium structures, however, and eventually break up to form vesicles or other lamellar structures. Indeed, adding water to a vessel whose inner walls are coated with a thin layer of a lamellar phase of low water content is a well-known way of forming vesicles. 

Figure 8-10 2H NMR spectra of dimyristoyl phosphatidyl-cholme-d27/water in lamellar phases at 40°C. One chain of the phosphatidylcholine is fully deuterated, containing 27 atoms of 2H. The mole ratios of water to lipid were 5.0 in (A) and 25.0 in (B). The average interfacial areas per alkyl chain as measured hy X-ray diffraction were 0.252 nm2 for (A) and 0.313 nm2 for (B). 2H NMR spectra are presented as "powder patterns" because the lipid molecules are randomly oriented in the magnetic field of the spectrometer as if in a powder. This gives rise to pairs of peaks symmetrically located on both sides of the origin. The separation distances are a measure of the quadrupole splitting of the NMR absorption line caused by the 2H nucleus. The various splittings of the resonances of the 13 -CH2- and one -CH3 groups reflect differences in mobility.109 The peaks have been assigned tentatively as indicated. From Boden, Jones, and Sixl.115 Courtesy of N. Boden.

Devaux and McConnell (39) measured a lateral diffusion coefficient of about 2 X 10"8 cm2/sec at 25°C for phosphatidylcholine (PC) diffusing along bimolecular leaflets in an oriented water-PC lamellar phase. Spin-labeled PC was used in this work. 

Studies of the interaction of monovalent cations with the dipalmytoyl phosphatidylcholine/water system have shown that monovalent cations have practically no effect on the lamellar phase repeat distance [293-295]. Direct measurements of the force versus distance relationship between zwitterionic phospholipid bilayers deposited on mica surfaces have also demonstrated that double layer forces arising from ion binding are... 

Recent studies by Lin and co-workers [63] have shown that not only the fatty acid composition influences the kind and ratio of carbonyls formed during heating of lipids, but also the structure of the reaction medium. The type of self-assembly stmcture had a significant influence on reaction yields. Egg phosphatidylcholin, which forms a lamellar phase together with water, generated significantly more 41 than phosphatidylethanolamine which adopts a different nanostructure, namely a reversed hexagonal structure. 

Figure 1.89 Structure of dipalmitoyl L-a-phosphatidylcholine (DPPC) to illustrate the main molecular parameters and structural features that dictate the formation of crystalline lamellar phases of macromolecular lipid assemblies. Schematic representation of a phospholipid molecule is also show (inset).

Marsh, D. (1991). Analysis of the chainlength dependence of lipid phase transition temperatures main and pretransitions of phosphatidylcholines main and non-lamellar transitions of phosphatidylethanolamines, Biochem. Biophys. Acta-Biomembranes, 1062, 1-6. 

Liquid crystals, liposomes, and artificial membranes. Phospholipids dissolve in water to form true solutions only at very low concentrations ( 10-10 M for distearoyl phosphatidylcholine). At higher concentrations they exist in liquid crystalline phases in which the molecules are partially oriented. Phosphatidylcholines (lecithins) exist almost exclusively in a lamellar (smectic) phase in which the molecules form bilayers. In a warm phosphatidylcholine-water mixture containing at least 30% water by weight the phospholipid forms multilamellar vesicles, one lipid bilayer surrounding another in an "onion skin" structure. When such vesicles are subjected to ultrasonic vibration they break up, forming some very small vesicles of diameter down to 25 nm which are surrounded by a single bilayer. These unilamellar vesicles are often used for study of the properties of bilayers. Vesicles of both types are often called liposomes.75-77... 

At temperatures below the main transition, a basic equilibrium stracture is the subgel (crystalline) Lc phase. Its formation usually requires prolonged low-temperature incubation. In addition to the Lc phase, many intermediate stable, metastable, and transient lamellar gel structures are adopted by different lipid classes—with perpendicular or tilted chains with respect to the bilayer plane, with fully interdigitated, partially interdigitated, or noninterdigitated chains, rippled bilayers with various ripple periods, and so forth. (Fig. 1). Several polymorphic phase transitions between these structures have been reported. Well-known examples of polymorphic transitions are the subtransition (Lc- L ) and the pretransition (Lp/- Fp/) in phosphatidylcholines (33). Recently, a polymorphic transition that included rapid, reversible transformation of the usual gel phase into a metastable, more ordered gel phase with orthorhombic hydrocarbon chain-packing (so-called Y-transition) was reported to represent a common pathway of the bilayer transformation into a subgel (crystalline) Lc phase (62). 

Stable oil-in-water emulsions can also be obtained by dispersing polar lipids such as phospholipids into triglycerides and then emulsifying the oil in water. The presence of charged phosphatidylcholine components of phosphohpids improves the stabilization of the emulsions. In most of these systems, the polar phospholipids form a separate phase at the interface where they form lamellar bilayers and a monolayer separated by triglyceride oil, between the outer water phase and the iimer triglyceride oil phase (Figure 10.3). 

Figure 7.7 Lamellar lattice constant of DEPC (l,2-dielaidoyl-glycero-3-phosphatidylcholine) in excess water after p-jumps inducing the L Lp transition (left) using different p-jump amplitudes and the Lp transition (top right). The series of diffraction patterns shown bottom right, taken at various time intervals after the p-jump, correspond to the same conditions as in the top-right figure. The emergence of the intermediate L, phase is clearly seen. Reproduced from Reference 33 with permission of the Royal Society of Chemistry.

Many mixtures of phospholipids, especially phosphatidylcholines, do not demonstrate these subtransitions. The pretransition is thought to be associated with a structural change from a one dimensional lamellar to a two dimensional system [131]. The temperature of this gel to liquid-crystalline phase transition increases with molecular weight (chain length) of the fatty acid. Because naturally occurring phospholipids have a broader distribution of the fatty acids, the transition also tends to be broader. 

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