Stable bilayers

Whereas the main challenge for the first bilayer simulations has been to obtain stable bilayers with properties (e.g., densities) which compare well with experiments, more and more complex problems can be tackled nowadays. For example, lipid bilayers were set up and compared in different phases (the fluid, the gel, the ripple phase) [67,68,76,81]. The formation of large pores and the structure of water in these water channels have been studied [80,81], and the forces acting on lipids which are pulled out of a membrane have been measured [82]. The bilayer systems themselves are also becoming more complex. Bilayers made of complicated amphiphiles such as unsaturated lipids have been considered [83,84]. The effect of adding cholesterol has been investigated [85,86]. An increasing number of studies are concerned with the important complex of hpid/protein interactions [87-89] and, in particular, with the structure of ion channels [90-92]. 

By comparing the structures of DM PC and DMTAP, it is clear that the structure of the head group is different and that DMTAP has a smaller head group. Thus, addition of DMTAP disturbs the formation of a thermodynamically stable bilayer structure. This energy cost reduces the self-spreading driving energy, which could be one of the reasons why the addition of DMTAP led to a decrease in p. 

It is believed that the Gaussian bending modulus k controls the membrane topology. In particular, a negative value of this constant is needed for stable bilayers. A positive value will induce nonlamellar topologies, such as bicontinuous cubic phases. Therefore, it is believed that k is negative for membranes. 

Qualitatively, the bilayer structures that result from DPD simulations are reasonable [65], In the simulation box, it is possible to find a stable bilayer in which the head groups shield the apolar core from the water phase. This means that the model effectively features a start-and-stop mechanism for... 

More recently, Kunitake and Okahata (1978b) discovered that a stable bilayer structure could be formed from a series of single-chain amphiphiles which possess rigid segments and flexible hydrocarbon tails as in [6]. 

These results unambiguously establish that the formation of stable bilayer structures is a fairly general physicochemical phenomenon which is in no way limited to the biolipid molecules. 

Fig. 4 Stability and permeability of self-assembled amphiphilic structures. Amphiphilic molecules such as fatty acids having carbon chain lengths of 9 or more carbons form bilayer membranes when sufficiently concentrated, a Pure bilayers of ionized fatty acid are relatively unstable but become markedly more stable as long chain alcohols are added, b Dimensions of the amphiphile also play a role. Shorter chain amphiphiles (9-10 carbons) are less able to form bilayers, while those of intermediate chain length (12-14 carbons) produce stable bilayers that also are permeable to ionic and polar solutes. Longer chain lengths (16-18 carbons) produce bilayers that are increasingly less permeable to solutes [48]...

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. 

One of the most important theoretical predictions is the existence of truly (i.e. infinitely) stable bilayers for C > Ce provided Ce < CMC. By fitting theoretical to experimental rfC) dependences it is possible to determine the equilibrium amphiphile concentration Ce and thus to judge whether in a given C range a bilayer, and in some cases, the corresponding disperse system, can be infinitely stable. BLMs, for example, are known to live for months and years. Thermodynamically, there is no difference between foam bilayers and BLMs so that the long lifetime of BLMs is apparently due to their existence in contact with amphiphile solutions of concentrations C either slightly bellow or above Cr. 

As an intermediate between solid supported layers and the inherent dynamic and nanostructured properties of phospholipid vesicle supports, silica and especially mesoporous silica nanoparticles may provide interesting platforms for dynamic bilayers. Previous studies have shown that stable bilayers can form on both amorphous [102] or functional silica [103, 104] and mesoporous nanoparticles [105] or membranes [106]. This type of biomimetic carrier has great potential as a type of trackable stabilized membrane capable of displaying cellular targeting elements in a close to natural configuration. 

T. Kunitake, Y. Okahata, M. Shimomura, S. Yasunami, K. Takarabe, Formation of Stable Bilayer Assemblies in Water from Single-Chain Amphiphiles Relationship between the... 

The synkinesis of isolable organized monolayers by the LB technique is not restricted to amphiphiles. They can also be obtained from stiff polymers with multiple hydrocarbon side-chains ( hairy rods ) . Regular and stable bilayers were obtained consisting of stiff rods made of cellulose or silicon phthalo-cyanate chains which were separated by fluid alkane regions. Furthermore the... 

Finally, we note that a growing body of evidence shows that the stability of a planar membrane can be enhanced by spreading it across a small aperture [97], For example, a DiPhyPC bilayer suspended across a 150nm radius orifice in a glass pipet remains intact when removed from buffer [150], This suggests that it may be possible to form arrays in which fluid, stable bilayer patches are surrounded by a patterned substrate that anchors the membrane. Air stability can also be achieved by coating a PSLB with a hydrophilic polymer film (e.g., a biospecifically adsorbed protein layer [23,149]). Both of these approaches maintain some degree of lateral lipid mobility in the membrane. 

Li et al. [37] described a series of A iV-dialkyl chitosans (alkyl = octyl, decyl, and dodecyl) (Fig. 7d), which in neutral water formed stable bilayered vesicles having hydrodynamic diameters in the range of 100-200 nm (DLS). It was found that the size of vesicles increased with increasing molecular weight of the hydrophilic chitosan backbone and/or increasing length of the hydrophobic alkyl side chains, which was attributed to a more compact structure of the membrane. 

To parameterize the polymersome model, the identification of the virial coefficients, vaij and wapv is driven by the requirement that the amphiphiles described by (10) and (12) should create a stable bilayer with given material properties. Assuming that the hydrophobic interior should be in a melt state, the coefficients vaa and Waaa are determined such that (12) enforces the A-blocks to create a melt in equilibrium with its vapor which, in a solvent free model, represents the surrounding water. It can be shown, from (12), that the equation of state of such a homogeneous melt, within mean-field approximation, is [138]... 

Within the last decade, scientists have recognized that roughly 25-50% of the lipids in a typical biomembrane do not form stable bilayer phases when they are purified and hydrated under normal cellular conditions (8). The prevalence of nonlamellar-prone lipids in biomembranes has stimulated research on the phase behavior of lipids and has led to suggestions that nonlamellar-prone lipids participate in essential cellular functions (2, 8). This theory has been supported by experiments that suggest that at least some cells have metabolic mechanisms that act to regulate the nonlamellar-prone lipids in the bilayers (7, 9, 10). 

Kimizuka, N., Wakiyama, T., Miyauchi, H., Yoshimi, T., Tokuhiro, M., Kunikate, T. (1996). Formation of stable bilayer membranes in binary aqueous-organic media from a dialkyl amphiphile with a highly dipolar head group, J. Am. Chem. Soc., 118 5808. 

H. Wang, X. Dong, Y. S. Lin, Highly stable bilayer MFI zeolite membranes for high temperature hydrogen separation, J. Membr. Sci. 450 (2014) 425 32. 

Kunitake, T. Okahata, Y. Shimomura, S. Yasunami, S. Takarabe, K. Formation of stable bilayer assemblies in water from single-chain amphiphiles Relationship between the amphiphile structure and the aggregate morphology. J. Am. Chem. Soc. 1981,103, 5401. 

In summary, we see that a surprisingly limited number of amphiphiles is required to form a stable bilayer. And although more amphiphiles are readily accepted into the microstructure, an upper limit is eventually reached. The evolution of bilayer structure from one limit to the other can be further appreciated by examining the density profiles of tail particles and free water particles within the bilayer these are shown in Figs. 5 and 6, respectively. As is... 

Fig. 7 The number of amphiphiles in primary bilayer vs. number of tail segments in amphiphile. ( ) first appearance of stable bilayer ( ) packing limit of bilayer...

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