Chemistry of Self-Assembling Bilayers and Related Molecular Layers
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ear polyions with water-soluble proteins or inorganic nanoparticles.3 Biological membranes comprise a supramolecular system that is among the most important building blocks of biological cells. Lipid bilayers are composed of regularly aligned amphiphiles in two molecular thicknesses and constitute the central structural unit of biomembranes. They can be spontaneously produced by self-assembly of lipid molecules dissolved in water. This self-assembling property (i.e., solid supports or templating surfaces are not required) combined
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with their regular molecular organization is a most pronounced characteristic, compared with other ultrathin films. Synthetic analogues of lipid bilayers— synthetic bilayer membranes—possess physicochemical characteristics identical to those of biolipid bilayers.4 Therefore, generalization of the bilayer concept greatly expands possibilities of selfassembling molecular systems and twodimensional molecular organization. Synthetic Bilayers Are Characterized by the Ease of Molecular Design5 The molecular design of bilayer-forming molecules is readily made on the basis of the modular concept shown in Figure 2. The four molecular modules required for double-chain amphiphiles are tail, connector, spacer, and head. In the series of simple dialkylammonium salts, the tail may consist of normal alkyl chains of C]0 to C2o- The dispersions.may contain vesicles with the inner aqueous phase or multilayer lamellae. The polar head group is either cation ic, anionic, nonionic, or zwitterionic. The connector portion that links the hydrophobic alkyl chains and the hydrophilic head group helps promote alignment of the alkyl chains. A spacer unit that intervenes between the head group and the connector unit exerts significant influence on the molecular orientation within the bilayer. Stable bilayer membranes would be produced even from single-chain amphiphiles whose conformations are restricted by incorporation of rigid segments or by intermolecular interactions. The head group in the single-chain amphiphiles can be cationic, anionic, non-
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