Membranes example

In supported liquid membranes, a chiral liquid is immobilized in the pores of a membrane by capillary and interfacial tension forces. The immobilized film can keep apart two miscible liquids that do not wet the porous membrane. Vaidya et al. [10] reported the effects of membrane type (structure and wettability) on the stability of solvents in the pores of the membrane. Examples of chiral separation by a supported liquid membrane are extraction of chiral ammonium cations by a supported (micro-porous polypropylene film) membrane [11] and the enantiomeric separation of propranolol (2) and bupranolol (3) by a nitrate membrane with a A/ -hexadecyl-L-hydroxy proline carrier [12]. 

PH domains consist of about 120 amino acid residues. They do not interact with other proteins, but associate with specific polyphosphoinositides. Consequently, PH domains appear to be important for localizing target proteins to the plasma membrane. Examples of PH domain-containing proteins include phospholipase C andpl20/RasGAP (Fig. 1). 

Terms such as symmetric and asymmetric, as well as microporous, meso-porous and macroporous materials will be introduced. Symmetric membranes are systems with a homogeneous structure throughout the membrane. Examples can be found in capillary glass membranes or anodized alumina membranes. Asymmetric membranes have a gradual change in structure throughout the membrane. In most cases these are composite membranes... 

Translocation of the effector molecule to the plasma membrane Example Grb2-Sos, Shc-Grb2... 

A primary function of the SH3 domains is to form fimctional oligomeric complexes at defined subcellular sites, frequently in cooperation with other modular domains. SH3 domains are foimd in many proteins associated with the cytoskeleton or with the plasma membrane. Examples are the actin binding protein a-spectrin and myosin lb. Furthermore, SH3 interactions are involved in signal transduction in the Ras pathway (see Chapter 9). 

Receptor-mediated transporters are excipients that serve as substrates to exploit specific receptors present on cell membranes. Examples of various receptors that have been explored for permeation enhancement include bile acids (45), vitamin Bi2 (46), amino acids (47), and folic acid (48). Most of the work in receptor-mediated transporters is conducted via the use of prodrugs. For example, a prodrug of acyclovir conjugated to bile acids was seen to have higher permeability as compared to the original drug, because of receptor-mediated transport of the prodrug via bile acid transporters (49). 

The antibiotics are compounds secreted by microbes that enhance the permeability of membranes to cations. One class functions by binding a metal to give a liposoluble complex that can then pass across the membrane. Examples are valinomycin, a cyclic peptide that binds K+ selectively, and monensin which binds Na+. These too are oxygen-donor ligands, and will be discussed in the following section. They function as antibiotics because they allow the concentrations of a cation across membranes to become equalized, and so cause the collapse of the membrane potential. 

Significant contributors to cell structure are those proteins that crosslink actin filaments or connect actin filaments to the cell membrane. Examples of such proteins can be found within the spectrin superfamily of cytoskeletal proteins. This discrete group is principally composed of the actin crosslinking protein o-actinin, and the membrane-associated actin-binding proteins spectrin and dystrophin. 

The barrier to paracellular diffusion potentially isolates the brain from many essential polar nutrients such as glucose and amino acids that are required for metabolism and, therefore, the BBB endothelium must contain a number of specific solute carriers (transporters) to supply the CNS with its requirements for these substances. The formation of tight junctions essentially confers on the BBB the properties of a continuous cell membrane, both in terms of the diffusional characteristics imposed by the lipid bilayer, and the directionality and properties of the specific transport proteins, and solute carriers (SLC) that are present in the cell membrane. Examples of BBB solute carriers (SLC transporters) are listed in Table 27.2. 

A proven approach to enhancing chemical conversion in the reaction zone is to carry out another reaction in the permeate zone to consume the permeate, thus creating a greater driving force for the permeate through the membrane. Examples have been given in... 

In the field of biology, the effects of hydration on equilibrium protein structure and dynamics are fundamental to the relationship between structure and biological function [21-27]. In particular, the assessment of perturbation of liquid water structure and dynamics by hydrophilic and hydrophobic molecular surfaces is fundamental to the quantitative understanding of the stability and enzymatic activity of globular proteins and functions of membranes. Examples of structures that impose spatial restriction on water molecules include polymer gels, micelles, vesicles, and microemulsions. In the last three cases since the hydrophobic effect is the primary cause for the self-organization of these structures, obviously the configuration of water molecules near the hydrophilic-hydrophobic interfaces is of considerable relevance. 

Permeation through silicalite-1 membranes examples and modelling... 

Because of their excellent adhesion and physiological safety, povidone K 30 and povidone K 90 are used as adhesives on the skin or mucous membranes. Examples include transdermal systems, oral adhesive gels, buccal adhesive patches or tablets [511,546,547,559,573,574], contact gels for electrocardiograph or electroencephalograph electrodes and adhesives for colostomy bags. Table 107 shows a formulation for a contact gel developed on a laboratory scale for ultrasonic scanning. 

The most simple form is a single, uniformly structured wall of a certain material, the so-called symmetric, stand-alone membranes. Examples are dense metal or oxide tubes and porous hollow fibres. To obtain sufficient mechanical strength, single-walled symmetric systems usually have a considerable thickness. 

Molecular models can considerably impact the chemical process industry. Obviously, numerous problems fall beyond the realm of conventional molecular simulation (see the example above on zeolitic membranes). Examples include dynamics of protein folding, diffusion through microporous membranes and human cells, formation of quantum dots in heteroepitaxial growth of semiconductors, and pattern formation on catalyst surfaces. 

The fleld of membrane structure and characterization is attracting the attention of a growing number of researchers. The basic research in this area builds upon studies performed on the simplest and minimal systems mimicking cell membranes, namely model membranes. Examples of such model membranes are... 

Partition equilibrium - Separation occurs between two phases rather than across a semi-permeable membrane. Examples include partition between aqueous and lipid phases or partition between a liquid and a sohd phase (e.g., where the binding sites are embedded on a sohd matrix). 

Devaux PF. 8tatic and dynamic lipid asymmetry in cell membranes. Biochemistry 1991 30 1163-1173. Dratz EA, Deese AJ. The role of docosahexaenoic acid (22 6w3) in biological membranes examples from... 

SH3 domains are found in many proteins associated with the cytoskeleton or with the plasma membrane. Examples are the actin-binding protein a-spectrin and myosin lb. 

Nonionic detergents - weaker groups for hydrogen bonds, apolar groups and are able to solubilize membranes examples are, Triton X-100, and Tween 20. 

Today, most bipolar ion exchange membranes are industrially used in continuous ion exchange reactions across the membrane by the use of H+ and OH-generated from the bipolar ion exchange membrane. Examples include separation of gluconic acid from gluconate,28 production of amino acids from amino acid salts,29 separation of citric acid from citrate,101 ion exchange of soybean protein,102 and conversion of lactate into lactic acid.82... 

Linking a lipid to a protein helps to provide an "anchor" for the protein inside of the lipid bilayer of cellular membranes. Examples include the following ... 

Membrane proteins have a variety of functions. Most, but not all, of the important functions of the membrane as a whole are those of the protein component. Transport proteins help move substances in and out of the cell, and receptor proteins are important in the transfer of extracellular signals, such as those carried by hormones or neurotransmitters, into the cell. In addition, some enzymes are tightly bound to membranes examples include many of the enzymes responsible for aerobic oxidation reactions, which are found in specific parts of mitochondrial membranes. Some of these enzymes are on the inner surface of the membrane, and some are on the outer surface. There is an uneven distribution of proteins of all types on the inner and outer layers of all cell membranes, just as there is an asymmetric distribution of lipids. 

Spin labels are stable, paramagnetic molecules that, by their structme, easily attach themselves to various biological macromolecular systems such as proteins or cell membranes. Examples of spin labels that can be covalently bonded to specific sites of biological systems include nitroxide derivatives of A-ethylmaleimide, which bind specifically to -SH groups, and nitroxide derivatives of iodoacetamide, which bind specifically to methionine, lysine, and arginine residues of amino acids. Nonco-valently bonded spin-labels that can be incorporated into biological systems include nitroxide derivatives of stearic acid, of phospholipids, and of cholesterol. 

Theory. The relationship of the chemical aspects of complexatlon reactions to the performance of facilitated transport membranes Is discussed by Koval and Reyes (108). They describe a procedure which can be used to predict and optimize the facilitated transport of gases, Including measurement of the appropriate equilibrium, transport, and kinetic parameters and structural modification of the carrier to Improve the performance of the membrane. Examples of this procedure and carrier modification are given for derivatives of Fe(II) tetralmlne complexes which reversibly bind CO In nitrile solvents (118). Experimental challenges In the measurement of the appropriate properties for other membrane configurations such as reactive Ion exchange membranes and reactive polymer membranes are also discussed. 

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