Lipid bilayer self-assembly

With time and improved synthetic protocols, larger templates (fuUerenes, dendrimers, nanoparticles, colloids, micelles, lipid bilayers, self-assembled block copolymers, oligonucleotides, DNA and proteins) have been imprinted [14] and the choice of matrices has expanded to liquid crystal polysiloxanes, carbon networks, zeolites, layered aluminophosphates and colloidal crystals, though organic polymer networks remain the dominant imprint casting medium [14]. 

Khalid, S., Bond, P. J., Holyoake, J., Hawtin, R. W., 8c Sansom, M. S. P. (2008). DNA and lipid bilayers Self-assembly and insertion. Journal of the Royal Society Interface, 5, S241. 

The emission of Trp 19 in melittin shifts to the red side peaking at 341 nm (Fig. 18), and the probe location slightly moves away from the lipid interface toward the channel center. Consistently, we observed a larger fraction of the ultrafast solvation component (35%) and a smaller contribution of slow ordered-water motion (38%). Melittin consists of 26 amino acid residues (Fig. 9), and the first 20 residues are predominantly hydrophobic, whereas the other 6 near the carboxyl terminus are hydrophilic under physiological conditions. This amphipathic property makes melittin easily bound to membranes, and extensive studies from both experiments [156-161] and MD simulations [162-166] have shown the formation of an 7-helix at the lipid interface. Self-assembly of 7-helical melittin monomers is believed to be important in its lytic activity of membranes [167-169]. Our observed hydration dynamics are consistent with previous studies, which support the view that melittin forms an 7-helix and inserts into the lipid bilayers and leaves the hydrophilic C-terminus protruding into the water channel. The orientational relaxation shows a completely restricted motion of Trp 19, and the anisotropy is constant in 1.5 ns (Fig. 20b), which is consistent with Trp 19 located close to the interface around the headgroups and rigid well-ordered water molecules. 

Explain why lipids will self-assemble into a bilayer structure. In a nonpolar solvent such as chloroform, do you expect to see bilayer formation How will different solvents affect the assembly ... 

FIG. 1 Self-assembled structures in amphiphilic systems micellar structures (a) and (b) exist in aqueous solution as well as in ternary oil/water/amphiphile mixtures. In the latter case, they are swollen by the oil on the hydrophobic (tail) side. Monolayers (c) separate water from oil domains in ternary systems. Lipids in water tend to form bilayers (d) rather than micelles, since their hydrophobic block (two chains) is so compact and bulky, compared to the head group, that they cannot easily pack into a sphere [4]. At small concentrations, bilayers often close up to form vesicles (e). Some surfactants also form cyhndrical (wormlike) micelles (not shown). 

In reconstitution experiments, the self-assembly of the pore-forming protein a-hemolysin of Staphylococcus aureus (aHL) [181-183] was examined in plain and S-layer-supported lipid bilayers. Staphylococcal aHL formed lytic pores when added to the lipid-exposed side of the DPhPC bilayer with or without an attached S-layer from B coagulans E38/vl. The assembly of aHL pores was slower at S-layer-supported compared to unsupported folded membranes. No assembly could be detected upon adding aHL monomers to the S-layer face of the composite membrane. Therefore, the intrinsic molecular sieving properties of the S-layer lattice did not allow passage of aHL monomers through the S-layer pores to the lipid bilayer [142]. 

Lipid bilayers are formed by self-assembly, driven by the hydrophobic effect. When lipid molecules come together in a bilayer, the entropy of the surrounding solvent molecules increases. 

The lipid molecule is the main constituent of biological cell membranes. In aqueous solutions amphiphilic lipid molecules form self-assembled structures such as bilayer vesicles, inverse hexagonal and multi-lamellar patterns, and so on. Among these lipid assemblies, construction of the lipid bilayer on a solid substrate has long attracted much attention due to the many possibilities it presents for scientific and practical applications [4]. Use of an artificial lipid bilayer often gives insight into important aspects ofbiological cell membranes [5-7]. The wealth of functionality of this artificial structure is the result of its own chemical and physical properties, for example, two-dimensional fluidity, bio-compatibility, elasticity, and rich chemical composition. 

Cazacu, A., Tong, C., van der Lee, A., Fyles, T.M. and Barboiu, M. (2006) Columnar self-assembled ureidocrown-efhers — an example of ion-channel organization in lipid bilayers. Journal of the American Chemical Society, 128 (29), 9541-9548. 

The most versatile method to prepare such hollow capsules is self-assembly [203-205, 214, 215]. Owing to their amphiphilic nature and molecular geometry, lipid-based amphiphiles can aggregate into spherical closed bilayer structures in water so-called liposomes. It is quite reasonable that the hollow sphere structure of liposomes makes them suitable as precursors for the preparation of more functional capsules via modification of the surfaces with polymers and ligand molecules [205, 216, 217]. Indeed, numerous studies based on liposomes in this context have been performed [205, 209, 213]. 

Other surface-active compounds self-assemble into bilayer structures (schematically illustrated in Fig. 10b), which normally spherilize into structures termed vesicles. When vesicles are formed from phospholipids, the term liposome is used to identify the structures, which also provide useful drug delivery systems [71]. Solutes may be dispersed into the lipid bilayer or into the aqueous interior, to be subsequently delivered through a variety of mechanisms. Liposomes have shown particular promise in their ability to act as modifiers for sustained or controlled release. 

Of course there are many phenomena that equilibrate on the nanosecond timescale. However, the majority of relevant events take much more time. For example, the ns timescale is much too short to allow for the self-assembly of a set of lipids from a homogeneously distributed state to a lamellar topology. This is the reason why it is necessary to start a simulation as close as possible to the expected equilibrated state. Of course, this is a tricky practice and should be considered as one of the inherent problems of MD. Only recently, this issue was addressed by Marrink [56]. Here the homogeneous state of the lipids was used as the start configuration, and at the end of the simulation an intact bilayer was found. Permeation, transport across a bilayer, and partitioning of molecules from the water to the membrane phase typically take also more time than can be dealt with by MD. We will return to this point below. 

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