Bilayer alternate

Cationic lipids interact electrostatically and form stable complexes (lipoplexes) with the polyanionic nucleic acids. The structure of most lipoplexes is a multi-lamellar sandwich in which lipid bilayers alternate with layers of DNA strands [16, 62-64] (Fig. 20). Although infrequent, nonlamellar structures have also been found. The free energy gain upon lipoplex formation was shown to be essentially of entropic nature resulting from the counterion release and macromolecule dehydration [65, 66]. 

Liposomes are single- or multilayered phospholipids vesicles. They are roughly spherical in shape and consist of lipid bilayers alternating with aqueous regions. 

The gel phase consists of crystalline lipid bilayers alternating with water layers. When the L -phase is cooled through the hydrocarbon chain crystallization temperature, a gel phase can be formed which is usually metastable. There are even amphiphile-water systems which exhibit thermodynamically stable gel phases as the only type of lipid-water phase. One example is the tetradecylamine-water system (Larsson and Al-Mamun, 1973) shown in Fig. 8.17. Other lipids which only give gel phases in their aqueous systems are cholesterol sulphate and cholesterol phosphate (Abrahamsson et al., 1977). The gel phase of tetradecylamine consists of bilayers with vertical chains in the orthorhombic chain packing. At low water content this structure swells to a water layer thickness of 14 A. At very high water concentrations, however, another gel phase with the same lipid bilayers but with a water layer several hundred A thick is formed. The reason for this seems... 

When water is added to the lamellar liquid-crystalline phase above its limit of swelling, so that there is a two-phase mixture of water and the liquid-crystalline phase, liposomes can be formed by minor shearing effects, such as stirring. A liposome consists of spherically concentric lipid bilayers alternating... 

A-S-D Triad Monolayer and Second Donor Bilayer Alternate Systems To increase the charge separation efficiency, the increase in the lifetime of the charge separation perpendicular to the film by adding a second door D or an acceptor A to form A-S-D-D or A -A-S-D quadruplet should be examined. In fact, in the reaction center of natural photosynthesis, A -A-S-D structure is used for its efficient charge separation [68]. Because of laboriousness of quadruplet syntheses. 

Fig. 12. Comparison of packing in a- and y-forms of isotactic polypropylene The upper panel is an idealization of the pattern in Fig. 11, each molecule is represented by a triangle with the helix direction indicated. Every two rows can be considered a bilayer. In the y-Frxm, lower panel, the bilayers alternate chain dnectkm. Hie ptnns represent chain directions neariy orthogonal ( 80°) to the triangles [30]...

Electrochemical deposition (or electropolymerisation) is performed by using an electrochemical cell, whose liquid electrolyte contains the monomer under polymerisation. The procedure consists of a growth of poljuner layers typically via monomer oxidation. In particular, the polymer is deposited on the electrode where oxidation takes place (anode) [248,249]. This method can be used for direct fabrication of electrode/pol3mier bilayers. Alternatively, the active polymeric layer can be successively peeled from the deposition electrode, so that to be coupled to another type of passive substrate. 

The notion of developing capsules that could insert into bilayers and have rather sophisticated functions clearly required some control studies. The obvious first step was to determine if pyrogallol[4]arenes themselves would insert into bilayer membranes. Pyrogallol[4]arenes are obviously amphiphiles, but not all amphiphiles immediately insert into a membrane. Depending on the properties of the amphiphile, the membrane, and the bulk medium, the amphiphile may prefer to aggregate rather than insert into the bilayer. Alternately, it may form a layer on the surface of the membrane without actually penetrating it. 

Fig. 3. (a) Chemical stmcture of a synthetic cycHc peptide composed of an alternating sequence of D- and L-amino acids. The side chains of the amino acids have been chosen such that the peripheral functional groups of the dat rings are hydrophobic and allow insertion into Hpid bilayers, (b) Proposed stmcture of a self-assembled transmembrane pore comprised of hydrogen bonded cycHc peptides. The channel is stabilized by hydrogen bonds between the peptide backbones of the individual molecules. These synthetic pores have been demonstrated to form ion channels in Hpid bilayers (71). 

Interdiffusion of bilayered thin films also can be measured with XRD. The diffraction pattern initially consists of two peaks from the pure layers and after annealing, the diffracted intensity between these peaks grows because of interdiffusion of the layers. An analysis of this intensity yields the concentration profile, which enables a calculation of diffusion coefficients, and diffusion coefficients cm /s are readily measured. With the use of multilayered specimens, extremely small diffusion coefficients (-10 cm /s) can be measured with XRD. Alternative methods of measuring concentration profiles and diffusion coefficients include depth profiling (which suffers from artifacts), RBS (which can not resolve adjacent elements in the periodic table), and radiotracer methods (which are difficult). For XRD (except for multilayered specimens), there must be a unique relationship between composition and the d-spacings in the initial films and any solid solutions or compounds that form this permits calculation of the compo-... 

In the first version with a mobile phase of constant composition and with single developments of the bilayer in both dimensions, a 2-D TLC separation might be achieved which is the opposite of classical 2-D TLC on the same monolayer stationary phase with two mobile phases of different composition. Unfortunately, the use of RP-18 and silica as the bilayer is rather complicated, because the solvent used in the first development modifies the stationary phase, and unless it can be easily and quantitatively removed during the intermediate drying step or, alternatively, the modification can be performed reproducibly, this can result in inadequate reproducibility of the separation system from sample to sample. It is therefore suggested instead that two single plates be used. After the reversed-phase (RP) separation and drying of the plate, the second, normal-phase, plate can be coupled to the first (see Section 8.10 below). 

The possible alternatives for a solute molecule to interact with a bilayer of solvent molecules is depicted in figure 5. It is seen that such a surface offers a wide range of sorption and displacement processes that can take place between the solute and the stationary phase surface. There are, in fact, three different surfaces on which a molecule can... 

---