Liquid crystalline vesicles

Recent physical-chemical observations on native mammalian systems reveal that the proposed mixed micellar mechanism of lipid solubilization and transport in both bile and in upper small intestinal contents is incomplete [1,260-263]. Bile is predominantly a mixed micellar solution but, particularly when supersaturated with Ch, also contains small liquid-crystalline vesicles which, as suggested from model systems [239], are another vehicle for Ch and L transport. In dog bile which is markedly unsaturated with Ch [258], these vesicles exist in dilute concentrations and may be markers of the detergent properties of BS on the cells lining the biliary tree and/or related to the mode of bile formation at the level of the canaliculus. In human hepatic bile, which is generally dilute and markedly supersaturated with Ch, these vesicles may be the predominant form of Ch and L solubilization and transport [261]. If hepatic bile is extremely dilute, it is theoretically possible that no BS-L-Ch micelles may be present [268] all of the lipid content may be aggregated... 

Both micelles and unilamellar liquid-crystalline vesicles coexist during human fat digestion in the aqueous portions of upper intestinal content [262,263, Chapter 14]. In terms of composition and size, the micelles are not like biliary micelles, as has been traditionally believed, but are considerably larger 100 A) with relative lipid compositions which fall on the limits of the mixed lipids/Ch-BS phase boundary. The liquid-crystalline vesicle (or liposomal) phase of upper small intestinal contents is less well defined physico-chemically [262,263] its crucial importance in fat digestion and absorption is obvious in view of the fact that fat absorption proceeds efficiently in the total absence of BS micelles [262]. 

The use of ordered supramolecular assemblies, such as micelles, monolayers, vesicles, inverted micelles, and lyotropic liquid crystalline systems, allows for the controlled nucleation of inorganic materials on molecular templates with well-defined structure and surface chemistry. Poly(propyleneimine) dendrimers modified with long aliphatic chains are a new class of amphiphiles which display a variety of aggregation states due to their conformational flexibility [38]. In the presence of octadecylamine, poly(propyleneimine) dendrimers modified with long alkyl chains self-assemble to form remarkably rigid and well-defined aggregates. When the aggregate dispersion was injected into a supersaturated... 

The phase transition of bilayer lipids is related to the highly ordered arrangement of the lipids inside the vesicle. In the ordered gel state below a characteristic temperature, the lipid hydrocarbon chains are in an all-trans configuration. When the temperature is increased, an endothermic phase transition occurs, during which there is a trans-gauche rotational isomerization along the chains which results in a lateral expansion and decrease in thickness of the bilayer. This so-called gel to liquid-crystalline transition has been demonstrated in many different lipid systems and the relationship of the transition to molecular structure and environmental conditions has been studied extensively. 

As indicated by Puig et al. (35). surfactant retention and attendant pressure buildup in the rock can be greatly reduced if the surfactant dispersion is converted into the liquid crystalline state. Unilameller vesicles are preferred in the field work rather than the multilamellar... 

The mixing of nematogenic compounds with chiral solutes has been shown to lead to cholesteric phases without any chemical interactions.147 Milhaud and Michels describe the interactions of multilamellar vesicles formed from dilauryl-phosphotidylcholine (DLPC) with chiral polyene antibiotics amphotericin B (amB) and nystatin (Ny).148 Even at low concentrations of antibiotic (molar ratio of DLPC to antibiotic >130) twisted ribbons are seen to form just as the CD signals start to strengthen. The results support the concept that chiral solutes can induce chiral order in these lyotropic liquid crystalline systems and are consistent with the observations for thermotropic liquid crystal systems. Clearly the lipid membrane can be chirally influenced by the addition of appropriate solutes. 

Fig. 1 a-f. Various forms of surfactant aggregations in solution a Monolayer b bilayer c liquid crystalline phase (lamellar) d vesicle (liposome) e micelle f reverse micelle. (Reproduced from [39] with permission of PL Luisi)... 

Figure 5.2 Top-diagramatic representation of a detergent molecule, (a) Single tailed (b) double tailed (c) zwitterionic (d) bolamphiphilic. Bottom - different types of surfactant aggregates in solution (A) monolayer (B) bilayer (C) liquid-crystallin phase lamellar (D) normal micelles (E) cylindrical micelles (hexagonal) (F) vesicles (liposomes) (G) reversed micelles.

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... 

Several investigators have used radioactive tracer methods to determine diffusion rates. Bangham et al. (32) and Papahadjopoulos and Watkins (33) studied transport rates of radioactive Na+, K+, and Cl" from small particles or vesicles of lamellar liquid crystal to an aqueous solution in which the particles were dispersed. Liquid crystalline phases of several different phospholipids and phospholipid mixtures were used. Because of uncertainties regarding particle geometry and size distribution, diffusion coefficients could not be calculated. Information was obtained, however, showing that the transport rates of K+ and Cl" in a given liquid crystal could differ by as much as a factor of 100. Moreover, relative transport rates of K+ and Cl" were quite different for different phospholipids. The authors considered that ions had to diffuse across platelike micelles to reach the aqueous phase. 

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