Swelling of lecithin

Figure 5.1 The swelling of lecithin crystallites in water produces myelin figures composed of many hydrated molecular bilayers. The process depicted in the four pictures (kindly provided by Prof. I. Sakurai) takes a few hours.

These fibres are solid-like and should not be confused with the fluid myelin figures and their helical precursors obtained upon the swelling of lecithin crystals (see Figure 5.1). The fluid structures flow and change their shape and width constantly, whereas the solid types simply widen after addition of more material. Once a crystalline fibre is formed it adds material to the highly curved edges, much less to the more planar bilayer surfaces (Figure 5.7). 

The swelling of phospholipids in water, with the formation of myelin tubes, is also related to this transition temperature. With highly unsaturated phospholipids, such as egg yolk lecithin, myelin tube formation occurs readily at room temperature, but with the fully saturated phospholipids it occurs at higher temperatures. 

El-Nokaly and co-workers (1981) have shown that lecithin forms a lamellar liquid-crystalline phase with solvents other than water, such as ethylene glycol. The swelling of this phase is rather similar to the corresponding aqueous phase with a somewhat higher limit of swelling. An analogue to the hydration force , discussed above, also exists in lamellar non-aqueous phases (Bergenstahl and Persson, 1986). 

Fig. 52. X-ray diffraction data concerning the hexagonal liquid crystalline phase formed by NaC and lecithin in water at maximum swelling of the liquid crystalline phase. Data taken from (3). Vertical axis in A horizontal axis lecithin-bile salt (NaC) molar ratio. Dt = distance from the center of one cylinder to another (X-ray diffraction). Dc = diameter of the lecithin-bile salt cylinder. /> , = thickness of water layer between the cylinders. Insert on the left gives two-dimensional packing of lecithin-bile salt cylinders in a compact hexagonal array.

The results for lecithin dispersions are shown in Fig. 6 when water was replaced by glycerol. Even for this system, a swelling of toe L -phase is observed, and for con-... 

Lecithin (Lee) is a swelling insoluble amphiphile having truncated-cone shape complementary to Ch in water it forms liquid crystals which can dissolve high proportions of cholesterol. A bilayer of lecithin is destroyed by bile salt (BS) simple micelles. 

Sodium cholate is insoluble in chloroform and in nonpolar solvents in general, but it is very soluble in alcohol and in water. Lecithin, on the contrary, is soluble in chloroform and only swells in water without dissolving in it. These differences in solubility are evidently related to the molecular structure and to the position of the hydrophilic groups in each of these molecules. The lecithin molecule has two important paraffinic chains and a group of hydrophilic functions (choline phosphate) localized at one end. In the presence of water, the lecithin molecules are oriented with their hydrophilic groups toward the water, and they hide their paraffinic chains inside a structure formed of two superposed layers of molecules. Conversely, in a nonpolar solvent the paraffinic chains are turned toward the solvent, while the polar groups are hidden inside the micelle. 

From a commercial point of view, soybean is the most important source for lecithin however, lecithin production from sunflower (180) and rapeseed (181-183) wet gum has also been successful. Wet gum is removed during the first refining step of crude oil that is the degumming process. Traditionally, crude oil obtained from solvent-extraction process is agitated with 1-3% water at elevated temperatures (70-80°C). Under these conditions, phosphohpids and glycohpids start to swell and become insoluble in the oil. The hydrated mass is removed via centrifugation, and the dehydration is carried out under vacuum until the residual moisture is below 1%. 

In an earlier review [3], mixed micelles formed by bile salts were classified into those with (i) non-polar lipids (e.g., linear or cyclic hydrocarbons) (ii) insoluble amphiphiles (e.g., cholesterol, protonated fatty acids, etc.) (iii) insoluble swelling amphiphiles (e.g., phospholipids, monoglycerides, acid soaps ) and (iv) soluble amphiphiles (e.g., mixtures of bile salts with themselves, with soaps and with detergents) and the literature up to that date (1970) was critically summarized. Much recent work has appeared in all of these areas, but the most significant is the dramatic advances that have taken place in our understanding of the structure, size, shape, equilibria, and thermodynamics of bile salt-lecithin [16,18,28,29,99-102,127, 144,218,223,231-238] and bile salt-lecithin-cholesterol [238,239] micelles which are of crucial importance to the solubihty of cholesterol in bile [1]. This section briefly surveys recent results on the above subclasses. Information on solubilization, solubilization capacities or phase equilibria of binary, ternary or quaternary systems or structures of liquid crystalline phases can be found in several excellent reviews [5,85,207,208,210,211,213,216,217] and, where relevant, have been referred to earlier. 

Fig. 17. Longitudinal and cross-sectional views of the proposed molecular models for the structure of bile salt-lecithin (BS-L) mixed micelles. All recent experimental data for BS-swelling amphiphile micelles are consistent with model B. In this model, reverse BS aggregates are present in high concentrations within the hydrophobic domains of L or other swelling amphiphile bilayers. BS also coat the perimeter of the disks as a bilayered ribbon . (From ref. 102 with permission.)...

As obtained from brain-tissue lecithin is a colorless or faintly yellowish, imperfectly crystaUine solid, or sometimes of a waxy consistency. It is very hygroscopic. It does not dissolve in H,0, ia wbicb, however, it swells up and forms a mass like starch-paste. It dissolves in alcohol or ether, very sparingly in the cold, but readily under the influence of heat. It dissolves in chloroform and in benzol Lecithin is very prone to decomposition, particularly at slightly elevated temperatures its chloride combines with PtCl to form aa insoluble yellowish chloroplatinate. 

Not only do complex lipids form vesicles, but lecithin-type bilayer crystallites also swell in water to form helical multilayers (Fig. 2.5.16a). The resulting fibers change their shapes continuously and are also of fluid character. 

Adhesion induced by lateral tension has been studied extensively for egg lecithin ( = egg phosphatidylcholine) membranes in pure water and salt solutions.The material parameters of the bilayers are H = (0.8 - 2) 10 2 gj.g u - (j,3 - 5). 10" erg. A first series of experiments utilized the rare adhesive contacts seen after the "disorderly swelling in water of very small quantities of the material. The tension was calculated from a rounding of the adhering membrane next to the contact area. Contact rounding is governed by the orientational correlation length (22), thus being optically resolvable only at ultralow tensions. 

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