Mixed bilayers

Figure 8 (Plate 4). Typical snapshot of DPD simulation results [64]. The hydrophobic part of mixed bilayers of DPPC-like lipids and up to 0.8 mole-fraction of the non-ionic surfactant Ci2E6 (left) and 0.9 (right). The surfactant C]2 chains are represented by grey curves, and the lipid C15 chains are black. The hole in the left conformation is transient on the right they are stable. Reproduced by permission of the Biophysical Society...

Monte Carlo may be used to study the lateral distribution of lipid molecules in mixed bilayers. This of course is a very challenging problem, and, to date, the only way to obtain relevant information for this is to reduce the problem to a very simplistic two-dimensional lattice model. In this case, the lipid molecules occupy a given site and can be in one of the predefined number of different states. These pre-assigned states (usually about 10 are taken), are representative conformations of lipids in the gel or in the liquid state. Each state interacts in its own way with the neighbouring molecules (sitting on neighbouring sites). Typically, one is interested in the lateral phase behaviour near the gel-to-liquid phase transition of the bilayer [69,70]. For some recent simulations of mixtures of DMPC and DSPC, see the work of Sugar [71]. 

The mixed liposomal solutions were prepared by the ethanol-injection method(13) in order to obtain completely transparent solutions. It is interesting to note that miscibility of the photochromic amphiphiles with DPPC depend on the position of bulky azobenzene. If azobenzene is incorporated close to the end of long alkyl chain, a stable mixed bilayer state cannot be formed. On the other hand, when the azobenzene moiety is located near the head group or at the center of the hydrocarbon tail, the azobenzene amphiphiles are successfully incorporated into the bilayer membrane. No individual micelle formation nor phase separation in the bilayer was observed at 25 °C by absorption spectroscopy. However, the microstructure of the mixed liposomes depends on the type of azobenzene amphiphiles. 

Fig. 25 Proposed packing arrangements in LDHs intercalated with a monolayer of elaidate anions (on the /e/f), a mixed bilayer of elaidate anions and neutral elaidic acid molecules (center) and oleate anions (on the right). Reprinted with permission from [216]. Copyright American Chemical Society...

Faller, R., Marrink, S.J. Simulation of domain formation in DLPC-DSPC mixed bilayers. Langmuir... 

A similar study by O Brien and coworkers utilized bilayers composed of a shorter chain diacetylenicPC (9) and DSPC or DOPC [37]. Phase separation was demonstrated in bilayers by calorimetry and photopolymerization behavior. DSC of the 9/DSPC (1 1) bilayers exhibited transitions at 40 °C and 55 °C, which were attributed to domains of the individual lipids. Polymerization at 20 °C proceeded at similar rates in the mixed bilayers and pure 9 bilayers. A dramatic hysteresis effect was observed for this system, if the bilayers were first incubated at T > 55 °C then cooled back to 20 °C, the DSC peak for the diacetylenicPC at 40 °C disappeared and the bilayers could no longer be photopolymerized. The phase transition and polymerizability of the vesicles could be restored simply by cooling to ca. 10 °C. A similar hysteretic behavior was also observed for pure diacetylenicPC bilayers. Mixtures of 9 and DOPC exhibited phase transitions for both lipids (T = — 18 °C and 39 °C) plus a small peak at intermediate temperatures. Photopolymerization at 20 °C initially proceeded at a similar rate as observed for pure 9 but slowed after 10% conversion. These results were attributed to the presence of mixed lipid domains... 

The extensive studies of the behavior of mixed monolayers or bilayers of di-acetylenic lipids and other amphiphiles parallel to some degree the studies of dienoyl-substituted amphiphiles. Since the dienoyl lipids do not contain a rigid diacetylenic group in the middle of the hydrophobic chains, they tend to be miscible with other lipids over a wide range of temperatures and compositions. In order to decrease the lipid miscibility of certain dienoyl amphiphiles, Ringsdorf and coworkers utilized the well-known insolubility of hydrocarbons and fluorocarbons. Thus two amphiphiles were prepared, one with hydrocarbon chains and the other with fluorocarbon chains, in order to reduce their ability to mix with one another in the bilayer. Of course it is necessary to demonstrate that the lipids form a mixed lipid bilayer rather than independent structures. Elbert et al. used freeze fracture electron microscopy to demonstrate that a molar mixture of 95% DM PC and 5% of a fluorinated amphiphile formed phase-separated mixed bilayers [39]. Electron micrographs showed extensive regions of the ripple phase (Pb phase) of the DM PC and occasional smooth patches that were attributed to the fluorinated lipid. In some instances it is possible to... 

In mixed bilayer vesicles diacetylenic and natural lipids exhibit the same miscibility behavior as in monomolecular films. This can be demonstrated using differential scanning calorimetry (DSC). The neutral lipid (23) is immiscible with DSPC or DOPC as indicated by the two phase transitions of the mixed liposomes which occur at the same temperatures as those of the pure components (Fig. 33 a). 

Fig. 36. Electron micrograph of ripple structure and patch formation in 1 1 mixed bilayers of (18, n = 12) and dimyristoylphosphatidylcholine. Bar represents 250 nm...

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... 

Several studies have been performed to analyze the effect of membrane composition on accumulation and transport of antineoplastic drugs. In particular, the role of anionic lipids in transport and passive diffusion, as well as on the binding of doxorubicin and its effect on the degree of order of lipid acyl chains was investigated. Using 2H-NMR spectroscopy the effect of doxorubicin on mixed bilayers of DOPS, DOPA, dioleoylphosphatidylcholine (DOPC), and DOPE was studied. It was found that doxorubicin does not affect acyl chain order of pure zwitterionic phospholipids but dramatically influences that of anionic lipids [112]. At 25 °C, in bilayers consisting of 67... 

Allen and Bevan (80) have applied the SMD technique to the study of reversible inhibitors of monoamine oxidase B, and this paper will be used as an example for discussion of the constant velocity SMD pulling method. They used the Gromacs suite of biomolecular simulation programs (18) with the united-atom Gromos 43al force field to parameterize the lipid bilayer, protein, and small-molecule inhibitors. The protein was inserted into their mixed bilayer composed of phosphatidyl choline (POPC) and phosphatidyl ethanolamine (POPE) lipids in a ratio known to be consistent for a mitochondrial membrane. Each inhibitor-bound system studied was preequilibrated in a periodic box of SPC water (20) with the simulations run using the NPT ensemble at 300 K and 1 atm pressure for 20 ns. Full atomic coordinates and velocities were saved in 200-ps increments giving five replicates for each inhibitor-bound system. A dummy atom was attached to an atom (the SMD atom shown in Fig. 7) of the inhibitor nearest to the... 

Preparation of Fast-Tumbling Bicelles (Mixed Bilayered Micelles)... 

Pandit, S.A., Khelashvili, G., Jakobsson, E., Grama, A., Scott, H.L. Lateral organization in lipid-cholesterol mixed bilayers. Biophys. J. 2007, 92, 440-7. 

The interaction between an acidic phospholipid, the natural (wheat) phosphatidylinositolmonophosphate PI and a linear cationic polysoap the poly(2-methyl-5-vinyl-hexylpyridinium bromide) PVPC6 has been studied with mixed spread monolayers and with hydrated (40%, w/w) mixed bilayers. The "electrostatic" interaction between PI and PVPC6 involves monolayer condensation and affects the bilayers hydration. In addition, the free energy of the bilayers structural water is modulated by this interaction. 

PI-PVPC6 association in mixed bilayers increases the free energy resp. enthalpy of water molecules. These are strongly perturbed by the phospholipid multibilayers and possibly also by the spread phospholipid monolayers. 

An interesting application of the membrane characteristics is the use of phase separation phenomena. The absorption maximum of the azobenzene unit shifts to shorter wavelengths when stacking of the chromophore occurs. In a mixed bilayer of an azobenzene-containing amphiphileCC QAzo) and 2C N 2C, (13), the phase separation was functions of the phase transition o both of 2C qN 2C] and C qAzo. 

The author and coworkers applied FM-AFM to the investigation of the DPPC-cholesterol (50-50 mol%) mixed bilayer. Figure 18.10 shows an FM-AFM image of a DPPC-cholesterol mixed bilayer formed on mica in PBS. 

H. Asakawa and T. Fukuma, The molecular-scale arrangement and mechanical strength of phospholipid/cholesterol mixed bilayers investigated by frequency modulation atomic force microscopy in liquid. 

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