Bilayer membranes, ionic

Molecules containing an ionic group and long alkyl chain(s) compose a molecular assembly such as a micell, a bilayer membrane, or a vesicle (liposome) (Fig. 4).19a) These assemblies bind reaction components by hydrophobic interaction, give a high... 

Fig. 4. Illustrative structure of micell, bilayer membrane, and liposome (vesicle). O Ionic group v— Long alkyl group...

Fig. 4 Stability and permeability of self-assembled amphiphilic structures. Amphiphilic molecules such as fatty acids having carbon chain lengths of 9 or more carbons form bilayer membranes when sufficiently concentrated, a Pure bilayers of ionized fatty acid are relatively unstable but become markedly more stable as long chain alcohols are added, b Dimensions of the amphiphile also play a role. Shorter chain amphiphiles (9-10 carbons) are less able to form bilayers, while those of intermediate chain length (12-14 carbons) produce stable bilayers that also are permeable to ionic and polar solutes. Longer chain lengths (16-18 carbons) produce bilayers that are increasingly less permeable to solutes [48]...

Figure 1-8. Membrane model with globular proteins interspersed within a lipid bilayer. The ionic head of phospholipids is represented by -O, and the fatty acid side chains leading to thenonpolar tail are indicated by the two wavy lines.

Evans et al. also showed that the 1 1 mixture of BAN and (3, y-distearoyl-phos-photidylcholine (DSPC) gives a smectic A texture in the temperature range of 57.3 to 100°C [21]. This is the first notice of lyotropic lamellar liquid crystals formed in the ionic medium. Additionally, Seddon et al. [28] and Neve et al. [29] have described the long-chained A-alkylpyridinium or l-methyl-3-alkylimidazolium ions to display smectic liquid-crystalline phases above their melting points, when Cl or tetrachloro-metal anions like CoCl " and CuCl " are used as the counter ions. Lin et al. have also noted the liquid crystal behavior of 1-alkylimidazolium salts and the effect on the stereoselectivity of Diels-Alder reactions [30]. However, liquid crystals are classified as ionic liquid crystals (ILCs), and they are distinguished from liquid crystals that are dispersed in ionic liquids. Although the formation of micelles and liquid crystal phases in ionic liquids have been thus reported, there has been no mention of the self-assembly of developed nano-assemblies that are stably dispersed in ionic liquids. In the next section the formation of bilayer membranes and vesicles in ionic liquids is discussed. 

SUGAR-PHILIC IONIC LIQUIDS DISSOLUTION OF CARBOHYDRATES AND FORMATION OF GLYCOLIPID BILAYER MEMBRANES, lONOGELS... 

Intrinsic to ionic surfactants and amphiphiles are solvophihc (or ionophilic) groups. We were therefore interested in learning whether the simple dialkylammonium bromides (2C,jN, n = 12, 14 Figure. d) that belong to the original family of synthetic bilayer membranes [10] form a bilayer in ionic liquids. Amphiphiles 2C N" were dispersed in three ionic hquids (Schemes 11.1-11.2) by ultrasonication (concentrations, 10 mM). While 2C12N+ as homogeneously dispersed in the conventional... 

As discussed in Section 3.7, when these phospholipids are mixed with water, they assemble in an arrangement called a lipid bilayer. The ionic heads of the phospholipid are oriented on the outside and the nonpolar tails on the inside. The identity of the fatty acids in the phospholipid determines the rigidity of this bilayer. When the fatty acids are saturated, they pack well in the interior of the lipid bilayer. and the membrane is quite rigid. When there are many unsaturated fatty acids, the nonpolar tails cannot pack as well and the bilayer is more fluid. Thus, important characteristics of this lipid bilayer are determined by the three-dimensional stmcture of the molecules that comprise it. 

The process of self-assembly is a dynamic one. With some single-chained ionic surfactants, e.g. sodium dodecyl sulphate), the residence time of a monomer in a micelle is of the order of 10 sec, and the lifetime of the micelle as a whole is of the order of a millisecond. At the other extreme, for highly insoluble phospholipids that form the bulk of bilayer membranes, the... 

In principle, all the suprastructures shown in a still highly simplified manner in Fig. 11.13 are in equilibria that depend on many parameters such as pH (Section 11.3.1), concentration (Section 11.3.2), ionic strength (Section 11.3.1), nature of the bilayer (Sections 11.3.4 and 11.3.7), and so on. This dynamic supramolecular polymorphism restricts meaningful structural studies to conditions that are relevant for function but often incompatible with routine analytical methods (e.g. nanomolar to low micromolar concentrations in lipid bilayer membrane. The fact that the active conformers or supramolecules are often not the thermodynamically dominant ones [21] (Section 11.3.2) calls for additional caution as well as selective methods of detection). As a general rule, the complexity of the supramolecular polymorphism of synthetic ion channels and pores decreases with increasing complexity (size) of the monomer (in other words, synthetic efforts are often worthwhile [2] compare Fig. 11.2). 

Kimizuka, N. and Nakashima, T. (2001) Spontaneous self-assembly of glycolipid bilayer membranes in sugar-philic ionic liquids and formation of ionogels. Langmuir, 17, 6759-6761. 

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