Membrane models biological membranes

The study of mixed films has become of considerable interest. From the theoretical side, there are pleasing extensions of the various models for single-component films and from the more empirical side, one moves closer to modeling biological membranes. Following Gershfeld [200], we categorize systems as follows ... 

Seelig, J., and Seelig, A., 1981. Lipid conformadon in model membranes and biological membranes. Quarterly Review of Biophysics 13 19-61. 

L Herbette, AM Katz, JM Sturtevant. Comparisons of the interaction of propranolol and timolol with model biological membrane systems. Molec Pharmacol 24 259-269, 1983. 

Berkowitz, M.L. Detailed molecular dynamics simulations of model biological membranes containing cholesterol. Biochim. Biophys. Acta 2009, 1788, 86-96. 

The spread mixed lipid monolayer studies provide information about the packing and orientation of such molecules at the water interface. These interfacial characteristics affect many other systems. For instance, mixed surfactants are used in froth flotation. The monolayer surface pressure of a pure surfactant is measured after the injection of the second surfactant. From the change in n, the interaction mechanism can be measured. The monolayer method has also been used as a model biological membrane system. In the latter BLM, lipids are found to be mixed with other lipidlike molecules (such as cholesterol). Hence, mixed monolayers of lipids + cholesterol have been found to provide much useful information on BLM. The most important BLM and temperature melting phenomena is the human body temperature regulation. Normal body temperature is 37°C (98°F), at which all BLM function efficiently. 

In order to give a homogeneous distribution of enzyme molecules inside the membrane, it was necessary to synthesize the membrane and to incorporate the enzymes at the same time. The co-cross-linking of enzyme molecules with an inert protein appears to be a proper solution. Purely active proteic films were created by using this procedure.11-12 These artificial enzyme membranes can be used in the study of heterogeneous enzyme kinetics and for modeling biological membranes. The phenomena in the enzyme membranes can be classified in two parts. 

At the most fundamental level, monolayers of surfactants at an air-liquid interface serve as model systems to examine condensed matter phenomena. As we see briefly in Section 7.4, a rich variety of phases and structures occurs in such films, and phenomena such as nucleation, dendritic growth, and crystallization can be studied by a number of methods. Moreover, monolayers and bilayers of lipids can be used to model biological membranes and to produce vesicles and liposomes for potential applications in artificial blood substitutes and drug delivery systems (see, for example, Vignette 1.3 on liposomes in Chapter 1). 

In electrophysiology modeling biological membranes are typically treated as capacitors with constant capacitance. The basic equation for a capacitor is ... 

Seelig J, Seelig A. Lipid conformation in model membranes and biological membranes. Quart. Rev. Biophys. 1980 13 19-61. Brown MF, Thurmond RL, Dodd SW, Otten D, Beyer K. Elastic deformation of membrane bilayers probed by deuterium NMR relaxation. J. Am. Chem. Soc. 2002 124 8471-8484. 

Most of the properties attributed to living organisms (e.g., movement, growth, reproduction, and metabolism) depend, either directly or indirectly, on membranes. All biological membranes have the same general structure. As previously mentioned (Chapter 2), membranes contain lipid and protein molecules. In the currently accepted concept of membranes, referred to as the fluid mosaic model, membrane is a bimolecular lipid layer (lipid bilayer). The proteins, most of which float within the lipid bilayer, largely determine a membrane s biological functions. Because of the importance of membranes in biochemical processes, the remainder of Chapter 11 is devoted to a discussion of their structure and functions. 

Recently, Zawis2a et al. [84-86] and Bin et al. [87, 88] demonstrated that quantitative PM IRRAS has a very important application in biomimetic research. This technique provides unique information concerning potential-induced changes in the orientation and conformation of molecules in a model biological membrane supported at an electrode surface. This point is illustrated by the ap-phcation of PM IRRAS to study the stracture of a bilayer of DMPC formed at the Au(lll) electrode surface by fusion of unilamellar vesicles [87]. 

Since the first attempt of Chapman and co-workers in 1966 [860], IR spectroscopy has become one of the most frequently used tools for elucidating lipid properties and the mutual effects of different lipids and proteins, which are of interest for different aspects of bioscience and biosensor design (see Refs. [333, 748, 861-864] for review). The IR methods used are transmission, ATR (MIR), and IRRAS for model monolayer, bilayer, and multibilayer membranes and biological membranes. To perform in situ measurements on the membranes of intact individual cells (e.g., as a function of cell membrane potential), planar miniature waveguides can be used instead of the ATR optics [865]. PM-IRRAS has been applied to obtain high-performance spectra of model membranes at the AW interface [866-875]. The experimental data focus mainly on the correlation between the structure of the matrix amphiphile or phospholipid film and the structure of the constituent species, the subphase composition, the surface pressure, and other external conditions, as well as the interaction of such monolayers with peptides and proteins (for reviews, see Refs. [332-334, 876, 877]). 

Conventional liposomes are MLVs made of lipids having neutral or negative charge. These liposomes are large. Because of their surface characteristics they are readily cleared from blood circulation by reticuloendothelial system cells, and hence they have a short biological half-life [2]. Conventional liposomes are most conunonly used in research to investigate the entrapment of compounds and their release profiles. Moreover, they are commonly studied as model biological membranes [1]. 

Saiz L, Klein ML (2002) Computer simulation studies of model biological membranes. Acc Chem Res 35(6) 482- 89... 

Molecular Dynamics Simulation of Continuous Current Flow Through a Model Biological Membrane Channel. 

As a point of interest, it is possible to form very thin films or membranes in water, that is, to have the water-film-water system. Thus a solution of lipid can be stretched on an underwater wire frame and, on thinning, the film goes through a succession of interference colors and may end up as a black film of 60-90 A thickness [109]. The situation is reminiscent of soap films in air (see Section XIV-9) it also represents a potentially important modeling of biological membranes. A theoretical model has been discussed by Good [110]. 

Biological membranes provide the essential barrier between cells and the organelles of which cells are composed. Cellular membranes are complicated extensive biomolecular sheetlike structures, mostly fonned by lipid molecules held together by cooperative nonco-valent interactions. A membrane is not a static structure, but rather a complex dynamical two-dimensional liquid crystalline fluid mosaic of oriented proteins and lipids. A number of experimental approaches can be used to investigate and characterize biological membranes. However, the complexity of membranes is such that experimental data remain very difficult to interpret at the microscopic level. In recent years, computational studies of membranes based on detailed atomic models, as summarized in Chapter 21, have greatly increased the ability to interpret experimental data, yielding a much-improved picture of the structure and dynamics of lipid bilayers and the relationship of those properties to membrane function [21]. 

Hydrated bilayers containing one or more lipid components are commonly employed as models for biological membranes. These model systems exhibit a multiplicity of structural phases that are not observed in biological membranes. In the state that is analogous to fluid biological membranes, the liquid crystal or La bilayer phase present above the main bilayer phase transition temperature, Ta, the lipid hydrocarbon chains are conforma-tionally disordered and fluid ( melted ), and the lipids diffuse in the plane of the bilayer. At temperatures well below Ta, hydrated bilayers exist in the gel, or Lp, state in which the mostly all-trans chains are collectively tilted and pack in a regular two-dimensional... 

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]. 

Simulations of water in synthetic and biological membranes are often performed by modeling the pore as an approximately cylindrical tube of infinite length (thus employing periodic boundary conditions in one direction only). Such a system contains one (curved) interface between the aqueous phase and the pore surface. If the entrance region of the channel is important, or if the pore is to be simulated in equilibrium with a bulk-like phase, a scheme like the one in Fig. 2 can be used. In such a system there are two planar interfaces (with a hole representing the channel entrance) in addition to the curved interface of interest. Periodic boundary conditions can be applied again in all three directions of space. 

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