Bilayer thickness

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... 

Structural characterizations of the immobilized bilayer assemblies are essential for the molecular design of the functional materials. On the bases of the systematic crystallographic investigation of single crystals of double-chain ammonium amphiphiles [9], Okuyama wrote a computer simulation program for the calculation of bilayer structures in cast bilayer films and bilayer thicknesses estimated from the repeating period in the X-ray diffraction data have been exclusively used for structural discussions [10,11]. 

X-ray diffraction from cast films provide useful information of bilayer structure. Periodic peaks in small and middle-angle diffraction from cast films on glass plates are attributed to the reflections from (h, 0,0) planes of the multiple lamella structure. The spacing of higher order reflections (h > 1) satisfies with numerical relation of 1 / h of the long period calculated from the first order reflection =1), which is equivalent to the bilayer thickness. Every cast film measured in this experiment showed more than 6 reflection peaks. 

Recently, in the theoretical studies on the simulation for N2 adsorption in micropore, some researchers102-104 reported that the monolayer adsorption occurs even in the micropore whose pore width is greater than the bilayer thickness of N2 (about 0.7 nm). In addition, Kaneko et al. showed the presence of the orientational phase transition of N2 on the graphitic micropore wall, which is the same as the phase transition of the monolayer on the flat graphite surface,105 and gave an effective method for the surface area determination in the microporous system.106 Therefore, even for micropores whose width is greater than 0.7 nm, dV MS can be... 

Pressure was applied in this study to fine tune the lipid chain-lengths and conformation and to select specific lamellar phases. For example, the phospholipid bilayer thickness increases by 1 A/kbar in the liquid-crystalline phase, and up to six gel phases have been found in fully hydrated DPPC dispersions in the pressure-temperature phase space up to 15 kbar and 80 °C, respectively. NMR spectral parameters were used to detect structural and dynamic changes upon incorporation of the polypeptide into the lipid bilayers. 

Even closer to cell membranes than monolayers and bilayers are organized surfactant structures called black lipid membranes (BLMs). Their formation is very much like that of an ordinary soap bubble, except that different phases are involved. In a bubble, a thin film of water — stabilized by surfactants — separates two air masses. In BLMs an organic solution of lipid forms a thin film between two portions of aqueous solution. As the film drains and thins, it first shows interference colors but eventually appears black when it reaches bilayer thickness. The actual thickness of the BLM can be monitored optically as a function of experimental conditions. Since these films are relatively unstable, they are generally small in area and may be formed by simply brushing the lipid solution across a pinhole in a partition separating two portions of aqueous solution. 

The lamellar spacing of a monoglyceride gel phase as a function of water content is plotted in Figure 14. The gel phase of the neutral monoglyceride has a lipid bilayer thickness of 49.5 A, and it swells to a unit layer thickness of 64 A (20). If an ionic amphiphilic substance (e.g. a soap) is solubilized in the lipid bilayer, it is possible to obtain a gel phase with high water content. As with the gel phases with infinite swelling that were discussed above, there is, however, a minimum water layer thickness which in this monoglyceride gel is about 40 A. 

M of PS vesicles using a 70-A2 surface area per PS head group and 50 A for the bilayer thickness (2.6 X 1013 vesicle/cm3). This concentration of PS would correspond to 0.20 mg/mL of PS using 790g/mol for PS based upon dioleyl chains. In Refs. 13 and 17 the value 710 g/mol for PS was used, based upon dipalmitoyl chains. ° In these cases, Equation IT was used instead of Equation 11. 

FIGURE 14.5 Entrapped volume and number of phospholipid molecules per liposome as a function of diameter of unilamellar egg PC vesicles. Theoretical curves were calculated assuming a bilayer thickness of 3.7 nm and a phospholipid sped c volume of 1.25 rtnolecule. (.) Entrapped volum jiL/ixmol phospholipid (----) entrapped volume/vesicle, iaL x 1015 (-.)/phospholipid molecules per vesicle, I0-5. 

Fig. 17 (a-d) Cryo-TEM images of diblock (sphere-rod) liposomes comprised of liquid-phase lipid nanorods (white arrows) connected to spherical vesicles. The lipid nanorods are stiff cylindrical micelles with an aspect ratio RilOOO. Their diameter equals the thickness of a lipid bilayer ( 4 nm) and their length reaches up to several micrometers, with a persistence length on the order of millimeters, (c) An inset of B, demonstrating the thickness of the nanorod white arrow heads point out a thickness of rj4 nm (approximate bilayer thickness, identical for the spherical vesicle and the nanorods), (d) Schematic of a MVLBisG2/DOPC sphere-rod diblock liposome. Reprinted with permission from [58]. Copyright 2008 American Chemical Society... 

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