Lipid water monolayer

The presence of a transporter can be assessed by comparing basolateral-to-apical with apical-to-basolateral transport of substrates in polarized cell monolayers. If P-gp is present, then basolateral-to-apical transport is enhanced and apical-to baso-lateral transport is reduced. Transport experiments are in general performed with radioactively labeled compounds. Several studies have been performed with Caco-2 cell lines (e.g. Ref. [85]). Since Caco-2 cells express a number of different transporters, the effects measured are most probably specific for the ensemble of transporters rather than for P-gp alone. P-gp-specific transport has been assayed across confluent cell layers formed by polarized kidney epithelial cells transfected with the MDR1 gene [86], Figure 20.11 shows experimental data obtained with these cell lines. A rank order for transport called substrate quality was determined for a number of compounds [86]. The substrate quality is a qualitative estimate, but nevertheless allows an investigation of the role of the air/water (or lipid/water) partition coefficient, log Kaw, for transport as seen in Fig. 20.11(A). For most of the compounds, a linear correlation is observed between substrate quality and log Kaw- However, four compounds are not transported at all despite their distinct lipophilicity. A plot of the substrate quality as a function of the potential of a... 

Since lipases act on lipids at lipid-water interfaces, preparation of substrates in a suitable physical form for maximal lipase activity is very important. Preparation methods include emulsification with an emulsifying agent incorporation into a gel dissolution in a water-soluble organic solvent, such as 2-methoxyethanol or tetrahydrofuran, followed by addition to an aqueous reaction mixture sonication, with or without emulsifier and formation of a thin film or monolayer. 

Besides the asymmetry between monolayers in cytomembranes, two of the more obvious differences between cubic phases and membranes are the unit cell size and the water activity. It has been argued that tire latter must control the topology of the cubic membranes [15], and hence tiiat the cubic membrane structures must be of the reversed type (in the accepted nomenclature of equilibrium phase behaviour discussed in Chapters 4 and 5 type II) rather than normal (type I). All known lipid-water and lipid-protein-water systems that exhibit phases in equilibrium with excess water are of the reversed type. Thus, water activity alone cannot determine the topology of cubic membranes. Cubic phases have recently been observed with very high water activity (75-90 wt.%), in mixtures of lipids [127], in lipid-protein systems [56], in lipid-poloxamer systems [128], and in lipid A and similar lipopolysaccharides [129,130]. 

Fig. 7. The increase in surface pressure of phospholipid monolayers as a function of signal-peptide concentration for the various E. coli LamB synthetic signal sequences (from Briggs, 1986). A monolayer of egg phosphatidylethanolamine and egg phosphatidylgly-cerol (65 35) was spread from a benzene solution onto 5 mM Tris buffer, pH 7.3, yielding a hnal surface pressure of 20 dyn/cm after evaporation of the benzene. The peptide was added by injecting a concentrated solution below the lipid-water interface. The surface pressure was measured by the du Noiiy ring method with a Fisher Autotensiomat equipped with a platinum-iridium ring. The plateau values are plotted as a function of the peptide concentration for the wild-type (O), Pro— Leu pseudorevertant (A), and deletion-mutant ( ) peptides.

Non-lamellar lipid mesophases (Fig. 4) may also be identified by their characteristic small-angle diffraction pattern. The structure of the inverse hexagonal lipid-water mesophase (denoted as Hu) is based on cylindrical water rods, which are surrounded by lipid monolayers. The rods are packed in a two-dimensional hexagonal lattice with Bragg peaks positioned at... 

Figure 2. Assembly of lipids into more complex structures. At low concentrations, lipids form monolayers, with the polar head group (represented as a circle) associating with the water, while the hydrophobic tails (represented as lines) associate with the air. As the concentration of lipid increases, either miscelles or bilayers form, depending upon the lipid and conditions.

To some authors, it seems to be apparent that the microlayer consists of a surface monolayer of adsorbed organic matter of thickness —20 A diluted by a vast excess, 10 —10 times as much, subsurface seawater. It should be noted that a monolayer thickness of this magnitude applies mostly to mono-layers of simple surfactants such as fatty lipids. Water-soluble surfactants of the wet variety (MacIntyre, 1974) can form monolayer films of much greater thickness, with hydrophobic parts of the molecule attached to the interface and hydrophilic parts extending by as much as —1 pm into the aqueous phase. The results of Baler et al. (1974), to be discussed shortly, show that films in a dry state on germanium prisms used for the Blodgett (1934, 1935) type of sampling method have thicknesses determined by ellip-... 

These close relationships existing between different monolayer phases and three-dimensional polymorphic forms occurring in non-polar lipids are considered here as conclusive evidence for the occurrence of the same structures. In the case of polar lipids, however, the interaction with water means that a relationship between a monolayer phase and a bulk phase can only be expected when the monolayer phases are compared with lipid-water phases, such as L - and gel phases. 

Lipids which form liquid-crystalline phases with water give II-A isotherms of two types. The first type is obtained above the chain melting temperature in the actual lipid-water system. Under these conditions there is only one monolayer phase which exists up to a film pressure of about 40 mN m . As discussed above the structure is considered to be the same as in the lipid monolayer of the L -phase. Below the chain transition temperature the second type of II-A isotherm is obtained. A phase with liquid chains is formed at low surface pressure but at a certain film pressure it is transformed into a monolayer phase with crystalline chains. It is also possible that other phases with crystalline chains can be formed under further compression as shown in Section 8.10. 

Lipid bilayers, 321,322,329,335,383 Lipid dermatoarthritis, 548 Lipid extraction, Bligh and Dyer, 272 Lipid film, spreading pressure, 338 Lipid monolayers, 338 Lipid multilayers, 341 Lipid polymers, 284 Lipid-protein interactions, 382-84 Lipid proteinosis, 548 Lipid structure, 322 see also Chain packing Lipid-water properties, 327-32, 356,... 

Fig. 1.17 Lipid-water systems, (a) A monolayer, (b) A micelle, (c) A bilayer.

Fig. XV-4. Schematic drawing of four streptavidin molecules bound to biotinylated lipid in a monolayer above heavy water. The scattering length density for neutron reflectivity is shown at the side. (From Ref. 30.)...

Mixing fatty acids with fatty bases can dissolve films as the resulting complexes become water-soluble however, in some cases the mixed Langmuir film is stabilized [128]. The application of an electric field to a mixed lipid monolayer can drive phase separation [129]. 

Anotlier metliod applicable to interfaces is tlie detennination of tlie partial molecular area (7 of a biopolynier partitioning into a lipid monolayer at tlie water-air interface using tlie Langmuir trough [28]. The first step is to record a series of pressure 71-area (A) isotlienns witli different amounts of an amphiphilic biopolynier spread at tlie interface. 

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). 

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