Self-assembled membranes

In Sec. 3.5 we noted that lipids are defined as water-insoluble compomids that can be extracted from living organisms by weakly polar or nonpolar solvents. Lipids are a stmcturally diverse group of compoimds, and the fact that they are not water-soluble means that they can serve as a barrier between two aqueous compartments (as self-assembling membranes) or as compact energy reserves (as droplets). 

Self-assembling membrane (SAM) is a new type of organic ultrathin membrane emerging in recent years. It consists of specific organic molecules synthesized by... 

Chow LW, Bitton R, Webber MJ, Carvajal D, Shull KR, Sharma AK, et al. A bioactive self-assembled membrane to promote angiogenesis. Biomaterials 2011 32 1574-82. 

Li, J.H., Xu, Y.Y., Zhu, L.P., Wang, J.H. and Du, C.H. 2009b. Fabrication and characterization of a novel TiOj nanoparticle self-assembly membrane with improved fouling resistance. 

Goetz R, Gompper G, Lipowsky R (1999) Mobility and elasticity of self-assembled membranes. Phys Rev Lett 82 221-224... 

The central role of biomembranes in cellular function underlines the importance of membrane research, an area that is by its very nature highly interdisciplinary and ranges from molecular biology to physical chemistry. At its core, however, is an essentially supramolecular interaction, the hydrophobic effect, which causes phospholipid amphiphiles to self-assemble into bilayers and then into complex closed compartments. The elegant self-assembly process that forms these remarkably large structures is an area of keen interest in its own right, but much recent study into self-assembled membranes aims to replicate the functions of biomembranes. Indeed, for cells, phospholipid bilayers are more than just delimiting boundaries they are... 

Van Rijn P, Tutus M, Kathrein C, Zhu L, Wessling M, Schwanehtng U, Boker A (2013) Challenges and advances in the field of self-assembled membranes. Chem Soc Rev 42 6578-6592... 

Self-assembled membranes constructed from phospholipids and other surfactants have been extensively investigated to understand their formation, encapsulation and release, and templating properties (7-25). Lipids and surfactants are amphiphilic molecules with hydrophilic, polar headgroups and nonpolar tails. As a result of the hydrogen bonding and electrostatic interactions of the hydrophilic headgroups and the van der Waals interactions between the hydrophobic tails, amphiphiles form organized membrane microstructures when dispersed in water or oil. When... 

Membrane and microfiuidic devices have also been adopted for the precision manufacture of solids from double-emulsion templates. To date, several different types of particles have been successfully produced by incorporating use of various membrane and microfiuidic devices in processes of polymerization, gel formation, crystallization, and molecular or particle self-assembly. Membrane emulsification is more suited to the fabrication of less sophisticated particulates, such as solid lipid micro-Znanoparticles, gel microbeads, coherent polymeric microspheres, and inorganic particles such as silica microparticles. Microfiuidic devices allow more sophisticated particle designs to be created, such as colloidosomes, polymerosomes, 3D colloidal assemblies, asymmetric vesicles, core-shell polymer particles, and bichromal particles. 

For structures with a high curvature (e.g., small micelles) or situations where orientational interactions become important (e.g., the gel phase of a membrane) lattice-based models might be inappropriate. Off-lattice models for amphiphiles, which are quite similar to their counterparts in polymeric systems, have been used to study the self-assembly into micelles [ ], or to explore the phase behaviour of Langmuir monolayers [ ] and bilayers. In those systems, various phases with a nematic ordering of the hydrophobic tails occur. 

Langmuir-Blodgett films (LB) and self assembled monolayers (SAM) deposited on metal surfaces have been studied by SERS spectroscopy in several investigations. For example, mono- and bilayers of phospholipids and cholesterol deposited on a rutile prism with a silver coating have been analyzed in contact with water. The study showed that in these models of biological membranes the second layer modified the fluidity of the first monolayer, and revealed the conformation of the polar head close to the silver [4.300]. 

It is interesting to compare the thermal-treatment effect on the secondary structure of two proteins, namely, bacteriorhodopsin (BR) and photosynthetic reaction centers from Rhodopseudomonas viridis (RC). The investigation was done for three types of samples for each object-solution, LB film, and self-assembled film. Both proteins are membrane ones and are objects of numerous studies, for they play a key role in photosynthesis, providing a light-induced charge transfer through membranes—electrons in the case of RC and protons in the case of BR. 

The fact that CD spectra of BR in LB and self-assembled films show similar behavior with respect to temperamre is also not strange. Because the basic block of the fihn in both cases is the membrane fragment, which is already closely packed, there is no principal difference between these samples with regard to packing. The difference in the distribution of these fragments cannot be critical for thermal stability. 

Therefore, the following method was suggested and realized (the scheme is shown in Fig. 17). A 1.5 M solution of KCl or NaCl (the effect of preventing BR solubility of these salts is practically the same) was used as a subphase. A platinum electrode was placed in the subphase. A flat metal electrode, with an area of about 70% of the open barriered area, was placed about 1.5-2 mm above the subphase surface. A positive potential of +50 -60 V was applied to this electrode with respect to the platinum one. Then BR solution was injected with a syringe into the water subphase in dark conditions. The system was left in the same conditions for electric field-induced self-assembly of the membrane fragments for 1 hour. After this, the monolayer was compressed to 25 mN/m surface pressure and transferred onto the substrate (porous membrane). The residual salt was washed with water. The water was removed with a nitrogen jet. 

In the first step, lipid model membranes have been generated (Fig. 15) on the air/liquid interface, on a glass micropipette (see Section VIII.A.1), and on an aperture that separates two cells filled with subphase (see Section VIII.A.2). Further, amphiphilic lipid molecules have been self-assembled in an aqueous medium surrounding unilamellar vesicles (see Section VIII.A.3). Subsequently, the S-layer protein of B. coagulans E38/vl, B. stearother-mophilus PV72/p2, or B. sphaericus CCM 2177 have been injected into the aqueous subphase (Fig. 15). As on solid supports, crystal growth of S-layer lattices on planar or vesicular lipid films is initiated simultaneously at many randomly distributed nucleation... 

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