Lipid cation

Molecular mechanism of ion transport across membrane Cells are enclosed by a membrane of about 70 A thickness and composed of double layers of protein separated by lipids. Cation cannot pass through the lipid layer without encapsulation and thus the enclosed cation presents an organic, lipid soluble surface to the membrane. 

Since Feigner et al. first reported in 1987 on the in-vitro transfection of eukaryotic cells with cationic lipid, cationic liposomes have been used extensively for gene delivery [56]. To date, numerous cationic lipids have been synthesized and tested for gene delivery. In general, cationic lipids are composed of three parts (Fig. 5.4) a positively charged head group a hydrophobic tail group and a linker bond between the two. 

Most cubic phases in lipid-water systems exhibit unit cell parameters not larger than 20 mn, while the imit cell of cubic membranes is usually larger than 100 nm. Some exceptioi have been apparently found [131, 132] although at this stage such findings should be treated with caution, as the determination of lattice parameters is dependent on the indexing of diffraction patterns, based only on a small niunber of reflections. Further, in lipid-protein-water, lipid-poloxamer-water and lipid-cationic surfactant-water systems, cubic phases with cell parameters of the order of 50 nm have been observed [56,127, 128]. Due to the small number of reports dealing with the... 

Many substances cross biological membranes according to their lipid solubility. Other polar molecules, such as amino acids and glucose, cross the membranes more rapidly than expected according to their solubUity in lipids. Cations, such as Na" and K, also cross membranes rapidly in spite of their hydrophilic nature. This passive transport of substances at higher rates than predicted from their lipid solubility is termed facilitated diffusion. That proteins are directly involved in facilitated diffusion was shown by comparison of experiments with natural membranes and synthetic membranes produced with phospholipid films. With phospholipid films all molecules, except water, diffuse according to lipid solubility and molecular size. Ions are essentially impermeable. The addition of membrane proteins, however, frequently allowed many polar and charged species to penetrate the membrane at rates comparable to natural membranes. 

Soaps and surfactants, lipids, cationic ingredients, and even polymers or polymer association complexes have been used as conditioning ingredients in shampoos and/or conditioning products. Soaps deposit their hydrophobic salts on the hair or bind by metal bridging. Cationic surfactants and polymers attach substantively to hair by ionic bonds enhanced by Van der Waals attractive forces. The substantivity of most polymer association complexes is probably due to their hydrophobic nature, enhanced by Van der Waals forces (entropy) and possibly ionic bonds. 

Commercial s. contain 99% of the d.s. as -+starch polysaccharides and not more than 1% minor constituents (-> starch composition), such as crude protein, lipids, cations (Na", K", Mg , Ca" ), anions (Cr, SO3, S04, P04 , SiOz) and trace elements in cereal starches, most phosphorus is part of the lysophosphatide fraction of the lipids, which are bound to the amylose by eomplexing. 

Lipids in model systems are often found in asymmetric clusters (see Figure 9.8). Such behavior is referred to as a phase separation, which arises either spontaneously or as the result of some extraneous influence. Phase separations can be induced in model membranes by divalent cations, which interact with negatively charged moieties on the surface of the bilayer. For example, Ca induces phase separations in membranes formed from phosphatidylserine (PS)... 

Bilayer phase transitions are sensitive to the presence of solutes that interact with lipids, including multivalent cations, lipid-soluble agents, peptides, and proteins. 

Discuss the effects on the lipid phase transition of pure dimyris-toyl phosphatidylcholine vesicles of added (a) divalent cations, (b) cholesterol, (c) distearoyl phosphatidylserine, (d) dioleoyl phosphatidylcholine, and (e) integral membrane proteins. 

Knowles, B. H., Blatt, M. R., Tester, M., et al., 1989. A cytosolic 5-endo-toxin from Bacillus thurigiensis var. israelensis forms cation-selective channels in planar lipid bilayers. FEES Letters 244 259-262. 

This review addresses the issues of the chemical and physical processes whereby inorganic anions and cations are selectively retained by or passed through cell membranes. The channel and carrier mechanisms of membranes permeation are treated by means of model systems. The models are the planar lipid bilayer for the cell membrane, Gramicidin for the channel mechanism, and Valinomycin for the carrier mechanism. 

With respect to the carrier mechanism, the phenomenology of the carrier transport of ions is discussed in terms of the criteria and kinetic scheme for the carrier mechanism the molecular structure of the Valinomycin-potassium ion complex is considered in terms of the polar core wherein the ion resides and comparison is made to the Enniatin B complexation of ions it is seen again that anion vs cation selectivity is the result of chemical structure and conformation lipid proximity and polar component of the polar core are discussed relative to monovalent vs multivalent cation selectivity and the dramatic monovalent cation selectivity of Valinomycin is demonstrated to be the result of the conformational energetics of forming polar cores of sizes suitable for different sized monovalent cations. 

The Energetics Problem of Cation Transport Across Lipid Bilayer Membranes A qualitative perspective of the barrier presented by a lipid bilayer membrane can be obtained from the Bom expression2) for solvation energy, SE,... 

The prespective to be gained thus far is that in order to pass through a lipid layer an ion must have an appropriate polar shell provided in large part by the carrier or channel structure which by virtue of its conformation and by also having lipophilic side chains provides for the polar shell to lipid shell transition. While the relative permeability of monovalent vs divalent and trivalent ions can be qualitatively appreciated from the z2 term in Eqn 2, as indicated in Figure 1B, it is essential to know structural and mechanistic detail in order even qualitatively to understand anion vs cation selectivity and to understand selectivity among monovalent cations. 

C. Hydrated monovalent cation approaching an area of the membrane where an amphiphilic carrier is located with its lipid side in contact with the lipid layer and with polar oxygens directed outward into solution. On close approach to the carrier, water molecules in the first coordination shell become replaced by carrier oxygens. As the ion becomes enclosed, the carrier moves into the lipid layer. 

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