Amphiphiles, bilayer-forming

Lamellar mesophases are the most commonly encountered mesophases, ubiquitous in double- and higher-chained amphiphiles (including virtually all concentrated lipid-water systems). Their ideal mesostructure consists of planar, parallel stacks of amphiphilic bilayers, forming a ID smectic lattice (Figure 16.9). 

Phospholipid molecules form bilayer films or membranes about 5 nm in thickness as illustrated in Fig. XV-10. Vesicles or liposomes are closed bilayer shells in the 100-1000-nm size range formed on sonication of bilayer forming amphiphiles. Vesicles find use as controlled release and delivery vehicles in cosmetic lotions, agrochemicals, and, potentially, drugs. The advances in cryoelec-tron microscopy (see Section VIII-2A) in recent years have aided their characterization [70-72]. Additional light and x-ray scattering measurements reveal bilayer thickness and phase transitions [70, 71]. Differential thermal analysis... 

A series of aggregation structures of bilayer forming azobenzene amphiphiles, CnAzoCmN+Br, both in single crystals and cast films was determined by the X-ray diffraction method and uv-visible absorption spectroscopy. From the relationship between chemical structures and their two-dimensional supramolecular structure, factors determining the molecular orientation in bilayer structure were discussed. Some unique properties based on two-dimensional molecular ordering were also discussed. 

In this paper, UV-visible absorption spectra and X-ray diffraction experiments of single crystals and solvent cast films of the azobenzene amphiphiles, CnAzoCmN+Br, were systematically investigated. Structural characterization of the cast bilayer films are discussed in comparison with aqueous solutions and single crystals. Some novel functional properties of the cast films are described, too. We also emphasize that the two-dimensional molecular assemblies, cast films and crystals of bilayer-forming amphiphiles, are suitable candidates for "crystal engineering" because of their simple structures compared with usual three-dimensional molecular crystals. 

Porous polymers, e.g. a membrane filter or a nylon capsule [35], are soaked in organic solvents containing bilayer forming amphiphiles and then dried. 

Bilayer-forming amphiphiles are cast with hydrophobic polymer(e.g. poly(vinyl chloride)) from organic solvents. A large cluster of the bilayer membrane are formed as phase separated micro domains in the polymer matrix [36]. 

Figure 4.1. Schematic view of bilayer forming one- and two-tail one-headed amphiphiles and two-headed bolaamphiphile that can form a monolayer.

Fig. 35. Structural elements (modules) of bilayer-forming amphiphiles [69]...

The lipid part of the membrane is essentially a two-dimensional liquid in which the other materials are immersed and to which the cytoskeleton is anchored. This last statement is not totally correct, as some membrane bound enzymes require the proximity of particular lipids to function properly and are thus closely bound to them. Simple bilayers formed from lipids in which both hydrocarbon chains are fully saturated can have a highly ordered structure, but for this reason tend to be rigid rather than fluid at physiological temperatures. Natural selection has produced membranes which consist of a mixture of different lipids together with other amphiphilic molecules such as cholesterol and some carboxylic acids. Furthermore, in many naturally occurring lipids, one hydrocarbon chain contains a double bond and is thus kinked. Membranes formed from a mixture of such materials can retain a fluid structure. The temperature at which such membranes operate determines a suitable mixture of lipids so that a fluid but stable structure results at this temperature. It will be seen that the lipid part of a membrane must, apart from its two-dimensional character, be disordered to do its job. However, the membrane bound proteins have a degree of order, as will be discussed below. 

Crowns have also been used in concert with other amphiphiles to form mixed bilayers. An example is the mixture of amphiphilic crown and amphiphilic ammonium salt (16) blended to form a mixed bilayer. Circular dichroism (CD) was used to detect the... 

Fig. 7.6 Bilayer formed by self-assembly of amphiphilic hydrobromide diacetylene salts.

It is commonly assumed that transfer processes can be modeled by bulk phase thermodynamics and that surface or interfacial effects are negligible. These assumptions may, in the case of partitioning into amphiphilic structures formed by micelles or bilayer membranes, not always be appropriate. These interfacial solvents have a large surface to volume ratio, similar to interfacial solvents used in reversed-phase liquid chromatography. The partitioning into such phases is the basis of the chromatographic separation. 

What physical properties are required if a molecule is to become incorporated into a stable bilayer As discussed earlier, all bilayer-forming molecules are amphiphiles, with a hydrophilic head and a hydrophobic tail on the same molecule. If amphiphilic molecules were present in the mixture of organic compounds available on the early Earth, it is not difficult to imagine that their self-assembly into molecular aggregates was a common process. 

It should be now be clear that the molecular assembly in ionic liquids is governed by (1) the balance of ionophilicity and ionophobicity of the constituent molecules and (2) the chemical structure of the ionic liquids. It may well be that an increase in intermolecular interactions in the bilayer-forming amphiphiles makes their... 

Sophisticated amphiphile designs allow us to develop bilayer-based organic-inorganic hybrids. In the example shown in Fig. 4.29, amphiphiles with alkoxysi-lane heads are used as bilayer-forming components. At the surface of the vesicle formed, cross-linked silanol groups form an inorganic silica-like structure. 

Physical chemists established a process called self-organization in which water-insoluble amphiphiles firstly form a molecular brush on the water surface and then assemble to spherical droplets or bladders in bulk water if a threshold concentration (cmc, critical micellar concentration) is surpassed. It was also shown that the self-organization of molecular mono- and bilayers is commonly not followed by crystal growth which would normally be favoured as it diminishes surface energies. Repulsive hydration and undulation effects were held responsible for preventing the growth of the delicate bilayer structures to 3D crystals. 

Certain pathological cells produce excessive amounts of a particular enzyme, e.g. neuroblastoma/acetylcholinesterase. If such an enzyme is capable of breaking open specially designed vesicles which encapsulate a cytotoxic drug, then cell selective therapeutic activity can be achieved. The double-chain amphiphile 23 forms vesicles and incorporates an acetylcholine-like head group which was readily hydrolyzed by acetylcholinesterase (AcE). The primary alcohol then ejected one of the two tails and the remaining single chain amphiphile destroyed the bilayer membrane-bound fluorescent dyes were set free within seconds. ... 

In the above expression, e and a (with <7 being the tail-tail collision diameter) are the energy and the length units respectively, rc is the cut-off length, while the decay range is determined by the parameter wc. The later was shown to be of key importance for the model and can be used to tune the amphiphile phase diagram and the material properties such as area per amphiphile, bilayer structure, elastic constants, and diffusivity. The functional form of the pair potential between the lipid tails given by the combination of the WCA potential with the attractive interaction of (9) is reproduced by the solid line in the inset on the left of Fig. 8, while the main panel shows the phase behavior of the amphiphiles. 

Many amphiphiles exist naturally and play a major role in biological processes. Phospholipids (Scheme 2) are of especial importance because their twin lipophilic groups allow them to form bilayers, so that they are the building blocks of membranes and they, and similar amphiphiles, will form vesicles in water [14,16,17]. 

Dioctadecyldimethylammonium chloride (DODAC) is a popular synthetic amphiphile that forms vesicles. The quenching efficiency of pyrene derivatives by several quenchers has been studied [98,105]. When iodide is the quencher for the emission of 1-pyrenesulfonate, the k A g(eff) ratios were respectively 0.016 and 0.023 at 20°C and 50°C. These values were smaller than observed for CTAC, suggesting that iodide was efficiently bound to the vesicle. It is known that water penetrates deep into the bilayer [106] and thus, I" may be located in these small water regions. It is also worth noting that an increase to a temperature above the transition temperature (33-37°C) [107] did not change... 

We like to emphasize here, that the efficiency of a helper lipid like DOPE is, however, influenced by the packing of the alkyl tails in the membrane formed by the cationic carrier amphiphile. This was demonstrated by a comparative study of two structurally related bilayer-forming surfactants, SAINT-2 (with two C18 l tails) and SAINT-5 (with two C18 0 tails) (Figure 16.10). Both carriers display transfection activity, but DOPE exerts a positive effect on SAINT-2-mediated transfection, but has a negligible effect on transfection mediated by SAINT-5. Interestingly, DOPE effectively enhances DNA dissociation from the lipoplexes formed from both carriers. Most likely, membrane stiffness plays an important role here. Since the bilayer composed of SAINT-5 is more rigid than that formed from SAINT-2, because of the absence of unsaturation in the tails, the plasmid DNA becomes less effectively condensed, and the lipoplex is structurally deformed. This has no effect on cellular uptake but reduces the efficiency of translocation of the plasmid across the membranes of the endosome and/or of the cell nucleus. 

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