Vesicles phase structure/transition

Rheological measurements on various vesicle phases have shown that the Cox-Merz rule - the complex viscosity of a sample at any frequency is equal to its apparent viscosity at the frequency-equivalent shear rate - is not fulfilled for these systems (10, 19-23) two examples of this are shown in Figure 10.25. At low frequencies or shear rates, respectively, both viscosities are equal, but beyond a certain shear rate the apparent viscosity is above the complex viscosity, and this difference increases with the shear rate. This behaviour can be qualitatively understood by a structural transition under the influence of shear. The oscillatory measurements do not lead to structural changes because in these experiments the deformation is normally kept below the limiting value for such a change. 

Many authors have described structural transitions in dilute lamellar phases under the influence of shear. For example, Roux and co-workers (19) studied different dilute lamellar phases which were stabilized by undulation forces and contained flat bilayers with defects, at rest. With increasing shear rates, these bilayers undergo a transition into relatively monodisperse multilamellar vesicles above a characteristic shear rate. The size of the formed vesicles is indirectly proportional to the shear rate. Beyond a second characteristic shear rate, the vesicles are again transformed into flat oriented bilayers. These results were explained in terms of a balance between shear stress and elastic forces which come from the bending and the Gaussian moduli of the bilayers. The same authors observed a similar sequence with increasing shear rate for other lamellar phases. It was found... 

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. 

Another concern with freeze-drying LEH is the instability of liposome structure upon lyophilization. Vesicle formation occurs in the presence of bulk water and when water is removed, loss of structural integrity is inevitable. Fusion, crystal formation, and phase transition are observed, resulting... 

This review describes experimental techniques, then gives some selected results of H, and NMR studies of pressure effects on the structure, dynamics and phase transitions of phospholipid bilayers. Other examples deal with 2D-NOESY experiments on lipid vesicles and pressure effects on the interaction of anaesthetics with phospholipid bilayers. Furthermore, we discuss... 

PRESSURE EFFECTS ON THE STRUCTURE, DYNAMICS AND PHASE TRANSITIONS OF PHOSPHOLIPID VESICLES... 

Polymeric phospholipids based on dioctadecyldimethylammonium methacrylate were formed by photopolymerization to give polymer-encased vesicles which retained phase behavior. The polymerized vesicles were more stable than non-polymerized vesicles, and permeability experiments showed that vesicles polymerized above the phase transition temperature have lower permeability than the nonpolymerized ones [447-449]. Kono et al. [450,451] employed a polypeptide based on lysine, 2 aminoisobutyric acid and leucine as the sensitive polymer. In the latter reference the polypeptide adhered to the vesicular lipid bilayer membrane at high pH by assuming an amphiphilic helical conformation, while at low pH the structure was disturbed resulting in release of the encapsulated substances. 

Fig. 37a, b. Model proposed according to 31P-NMR signal shape of phase transition range. Membrane structure of mixed vesicles prepared from oppositely charged vesicles a inverse micelle model b bulge model of clusters with different spontaneous membrane bending [310]... 

The chemical compositions and isomeric structures of the fatty acid chains of phospholipids is well known to have large effects on the physical properties of lipid bilayers, such as the temperatures of endothermic chain melting phase transitions. Lipid vesicles sensitized with lipid haptens can be agglutinated with specific antibodies directed against the haptens (see Fig. 1). 

Lipid A was investigated by FTIR, X-ray diffraction, and fluorescence techniques (Naumann et al., 1987). Analysis of the CH2 scissoring vibrations showed that lamellar structures exist at temperatures below 30 °C, but that no stable closed vesicles are formed. Orientation measurements of lipopolysaccharides prepared from E. coli and Salmonella minnesota by ATR-FTIR also showed a highly ordered gel phase. Besides, the transition from the gel pha.se a to the liquid crystalline phase was also studied (Brandenburg and Seydel, 1988). 

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