Bilayer-type aggregates

Several amphiphilic butadienes have been synthesized and studied concerning their polymerization properties in monomolecular layers, LB-multilayers, bilayer type aggregates (vesicles), and in the crystalline state. 

We then investigated the formation of hybrid molecular assemblies in combinations of anionic peptide lipid 9 with cage-type hosts 7 and 8 after a previous method [44], Lamella-type aggregates are observed for a mixture of host 7 and lipid 9 at a 1 20 molar ratio in the dispersion state by negative staining electron microscopy. Phase transition parameters (temperature at peak maximum, T enthalpy change, AH entropy change, dS half-width of an endothermic peak, and hydrodynamic diameters (d,y) for the bilayer... 

There are a great many methods by which small molecules can be assembled to form nanotubes and other similar columnar aggregates [50]. Unlike the stacked disc methodology employed by Fenniri s group, Russell and co-workers took a completely different approach, which consisted of nanotubes made by way of rolled-up bilayers [48], Their study of biocidal nanotubes from a self-assembled diacetylene salt employed a hydrophobically driven bilayer type of self-assembly step. This bilayer formation was based on the alignment of the hydrophobic chains, including aligned Ji-systems of the acetylenes on the interior of the bilayer and the polar hydrobromide salts of an amine on the exterior of the bilayer (Fig. 7.6). 

Figure 6.11 shows the activity of an artificial enzyme can be controlled based on the phase behavior of a lipid bilayer. The catalytic site for hydrolysis was supplied by a monoalkyl azobenzene compound with a histidine residue which was buried in the hydrophobic environment of a hpid bilayer matrix formed using a dialkyl ammonium salt. Azobenzene compound association depended on the state of the matrix bilayer. The azobenzene catalyst aggregated into clusters when the bilayer matrix was in a gel state. In contrast, the azobenzene derivative can be dispersed into the liquid crystalhne phase of the bilayer matrix above its phase transition temperature. This bilayer-type artificial enzyme catalyzed the hydrolysis of a Z-phenylalanine p-nitrophenyl ester. The activation energy for this reaction in the gel state is twice as large as that observed in the hquid crystalline state. The clustering of the catalysts upon phase separation suppress their catalytic activity, probably due to the disadvantageous electrostatic environment around the catalysts and the suppressed substrate diffusion. This activity control is unique to such molecular assembhes. 

Figure 43 (Top) Schematic model of the helical nanoribbon, (a) Molecular structure and backbone of cholate illustrating the facial amphiphiUcity and molecular size (b) the molecular aggregate that is the minimum constitution unit of a bilayer type of cholate host framework (the blue dotted lines denoted H-bond) (c) scheme of twisted nanoribbon composed of parallel, longitudinal stripes and (d) top view and (e) cross-section view of the molecular model. (Bottom) (f and g) TEM and (h) SEM images of ZnS semiconductor helical nanotubes. (Reproduced with permission from Ref. 77. American Chemical Society, 2009.)...

Importantly, it has been found that snrfactants often self-assemble at hydrophilic surfaces to form quasi-two-dimensional analogs of the aggregate stmctures observed in a bulk solution, that is, spherical or cylindrical snrface micelles or bilayer-type stmctnres, whereas at hydrophobic surfaces they tend to form monolayers or hemimicellar aggregates. 

A multitude of different variants of this model has been investigated using Monte Carlo simulations (see, for example [M])- The studies aim at correlating the phase behaviour with the molecular architecture and revealing the local structure of the aggregates. This type of model has also proven useful for studying rather complex structures (e.g., vesicles or pores in bilayers). 

In biological systems, one often observes membrane structures with nonzero spontaneous curvatures, e.g. in mitochondria. This type of bilayer structure is also essential in various transport related processes such as endo- and exocy-tosis (see Chapter 8 of this volume). These curved membrane systems may be stabilised by protein aggregation in the bilayer, or may be the result of the fact that biological membranes are constantly kept off-equilibrium by lipid transport and/or by (active) transport processes across the bilayer. These interesting... 

Another important area of progress is the enlargement of the scope of the structure of aqueous aggregates. The bilayer membrane formed from dialkyl amphiphiles belongs to a new class of the aqueous aggregate, totally different from the conventional surfactant micelle. A trialkylammonium compound gives yet another type of aggregation. 

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.

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