Molecular Assemblies of Amphiphiles

The syntheses given are also useful for connecting porphyrins with other chromophores and reactive groups, e.g., quinones. If the reported yields are reproducible, large electron donor-acceptor supramolecules should become accessible on a large scale. 

In biological systems molecular assemblies connected by non-covalent interactions are as common as biopolymers. Examples are protein and DNA helices, enzyme-substrate and multienzyme complexes, bilayer lipid membranes (BLMs), and aggregates of biopolymers forming various aqueous gels, e.g., the eye lens. About 50% of the organic substances in humans are accounted for by the membrane structures of cells, which constitute the medium for the vast majority of biochemical reactions. Evidently organic synthesis should also develop tools to mimic the structure and properties of biopolymer, biomembrane, and gel structures in aqueous media. 

An exhaustive review by H. Ringsdorf (M. Ahlers, 1990) on biomembrane models and a recent book by F. Voegtle (1991) on supramoiecular chemistry are recommended for further studies in this area. 

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This paper is not a review covering the entire field of carbohydrate-recognition in any organized system. Many excellent papers have already been devoted to supramolecular systems such as cyclodextrins, podands, coronands or cryptants able to entrap carbohydrate molecules [1]. This article only deals with the molecular recognition of mono and oligosaccharides in organized self-assemblies of amphiphilic carbohydrates (possibly blended with other lipids) in aqueous medium i.e. in assemblies mimicking the cell membrane. 

The size of molecular assembly of six synthetic dialkyl amphiphiles as determined by a quasi-elastic light scattering is varied in the presence of nonionic MEGA-n surfactants (N-D-gluco-N-methylalkanamide C = 7-9). 

Wang W, Tetley L, Uchegbu I F (2001). The level of hydrophobic substitution and the molecular weight of amphiphilic poly-L-lysine-based polymers strongly affects their assembly into polymeric bilayer vesicles. /. Colloid Interface Sci. 237 200-207. 

In Chapter 4, Massignani, Lomas and Battaglia review the fabrication processes used to form polymersomes, membrane-enclosed structures that are formed through self-assembly of amphiphilic copolymers. The resulting molecular properties, methods to control their size, loading strategies and applications of polymersomes are also detailed. 

Noncovalent spherical assemblies of amphiphilic lipids, on the other hand, are either short-lived already in aqueous solution (micelles) or collapse immediately upon drying (vesicles).This behavior is due to the character of the forces which form them. Curvature is retained by repulsive hydration forces. If the hydration sphere is removed from the head groups, the amphiphiles will pack together and form crystalline sheets and 3-D crystals. Neither crystallites nor micelles and vesicles can be considered as noncovalent polymers, because they change their molecular arrangement drastically when going from the dissolved to the dry state. They do not have material properties. 

A class of self-assembled structures that deserves special attention is the bilayer. This is a lamellar structure composed of two molecular layers of amphiphilic molecules. Amphiphiles having a J/v close to unity usually assemble into bilayers in which (in aqueous media) the apolar parts of the molecules are directed toward each other. Free-floating bilayers do not exist it is too unfavorable to expose the hydro-phobic edges to water. The bilayer closes into a spherical geometry, the so-called vesicle, or its edges are embedded in a nonaqueous environment. See Figure 11.13. 

We have used a simple molecular model employed in simulations of oil/water/amphiphiles to investigate the self-assembly of amphiphiles in systems displaying liquid/vapor equilibrium. Studies of oil/water/amphiphile systems rarely concentrate on the role of the oil phase. Although it is rarely emphasized, many (if not most)... 

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