Lipophilic layer

Hurst (19) discusses the similarity in action of the pyrethrins and of DDT as indicated by a dispersant action on the lipids of insect cuticle and internal tissue. He has developed an elaborate theory of contact insecticidal action but provides no experimental data. Hurst believes that the susceptibility to insecticides depends partially on the cuticular permeability, but more fundamentally on the effects on internal tissue receptors which control oxidative metabolism or oxidative enzyme systems. The access of pyrethrins to insects, for example, is facilitated by adsorption and storage in the lipophilic layers of the epicuticle. The epicuticle is to be regarded as a lipoprotein mosaic consisting of alternating patches of lipid and protein receptors which are sites of oxidase activity. Such a condition exists in both the hydrophilic type of cuticle found in larvae of Calliphora and Phormia and in the waxy cuticle of Tenebrio larvae. Hurst explains pyrethrinization as a preliminary narcosis or knockdown phase in which oxidase action is blocked by adsorption of the insecticide on the lipoprotein tissue components, followed by death when further dispersant action of the insecticide results in an irreversible increase in the phenoloxidase activity as a result of the displacement of protective lipids. This increase in phenoloxidase activity is accompanied by the accumulation of toxic quinoid metabolites in the blood and tissues—for example, O-quinones which would block substrate access to normal enzyme systems. The varying degrees of susceptibility shown by different insect species to an insecticide may be explainable not only in terms of differences in cuticle make-up but also as internal factors associated with the stability of oxidase systems. 

These aspects of solvent property similarly apply to precoated impregnated silica gel plates, e.g., by ammonium sulfate, silver nitrate, or magnesium acetate, as well as to microcrystalline cellulose precoated plates. On preparative RP phases, water has the lowest elution power. Therefore, more polar or aqueous solvents should be preferred. In contrast to HPTLC RP-18 layers, on which such aqueous solutions remain as a drop on the surface and are not able to penetrate through the lipophilic layer, on preparative RP phases, pnre aqneons application solutions can be apphed owing to the minor degree of C18 modification. 

The Zn-N3imide interaction has been used to selectively extract imide-containing nucleosides and nucleotides into lipophilic media (39). Hexadecyl-derivatized Zn2+-cyclen was shown to extract dT from an aqueous solution containing a mixture of C, A, and G nucleobases. The antiviral agent AZT (3 azido-3 deoxythymidine) could also be extracted into CHCI3 from neutral aqueous solutions. Transport across a lipophilic layer was also shown, using acidic conditions, to promote the release of dT and AZT (Fig. 9). 

In contrast, vehicle that are immiscible with water and those with a high proportion of oils have occlusive effects. They reduce both insensible perspiration and the release of sweat. The sweat collects as droplets at the opening of the glands, but does not spread as a film between the hydrophobic skin surface and the hpophific base because the free surface energy of the vehicle-skin interface is smaller than that between water and skin. If a lipophilic layer of vehicle is present, this is not spontaneously replaced by the water-skin layer if sweat is secreted. 

Diethyl ether is also frequently used to delipidate extracts which are subsequently analyzed by conventional methods [46,55,67,175]. It has been reported that extraction using diethyl ether allows the quantitative removal of monomers from acidified aqueous wood extracts, while dimeric proanthocyanidins remain in the aqueous phase [121]. Recovery studies of (-)-epicatechin and (+)-catechin in synthetic wine solutions only yielded 24 % and 46 %, respectively, of the added amounts. Procyanidin B2, however, could not be detected in the lipophilic layer. Analogous results... 

Reaction of dendrimers 38-[G2] with L-hexadecylamino-l-deoxylactitol led to the formation of the cationic dendrimers 39-[G2] (Scheme 27). A spontaneous supramolecular self-assembly leading to the formation of vesicles was observed by transmission electron microscopy (TEM). It was shown that the size of these vesicles was around 100 nm, and that their shell was constituted by a bilayer (thickness 10 nm) made of two lipophilic layers (thickness 3 nm) separated by a hydrophilic layer. Furthermore, preliminary experiments show that these cationic dendrimers possess an anti-HIV activity in vitro [41]. 

If an enzyme does not tolerate to be modified by covalent attachment of PEG residues, various lipids, such as simple long-chain fatty acids or amphiphilic compounds can be attached onto its surface by mild adsorption. It has been estimated that about 150 lipid molecules are sufficient to cover an average protein with a lipophilic layer, which makes it soluble in organic solvents [62, 483]. This so-called lipid-coating seems to be applicable to various t5 es of enzymes, such as lipases [484], phospholipases [438], glycosidases [485], and catalase [486]. 

Type IV materials exhibit unique ion-exchange properties supplemented by RP characteristics, so that they may be considered as RP-modified ion-exchange materials. Lammerhofer et al. [59] have developed a number of RP/weak anion exchanger (WAX) materials consisting of a selector immobilized onto thiol-modified silica. In these phases, the WAX site is located at the top of the lipophilic layer and is linked to the silica support via a lipophihe spacer with polar-embedded amide or sulfide groups. These columns, however, are not commercially available. 

To separate the nonlipid impurities, add approximately 0.7 ml of 0.88% KCl (Christie, 1982) to the tube. Agitate the mixture by hand or with a Vortex mixer. This procedure will produce a biphasic mixture consisting of a top hydrophilic layer and a bottom lipophilic layer. If an emulsion forms in the tube, allow to stand at room temperature or in a refrigerator at 4°C for up to 4 h. 

This aggregation might also explain the incredibly hl solubility of the caipounds in saturated hydrocarbons (practiccil miscibility), as being due to a caipact oxide structure surrounded by a lipophilic layer of adkoxide groups. It also accounts for the electronic delocalization put in evidence by spectroscopic and magnetic measuranents. 

No structural or other physical evidence exists to support this claim. By increasing the addition of PGPR, low viscosity is achieved at low shear rates. Its synergistic effects with lecithins and ammonium phosphatide are known and documented. However, its mechanism of action is not yet well studied. The manufacturers claim that all three emulsifiers form a monolayer around the nonfat particles, especially around the sugar particles. The sugar particles will therefore be covered by an outer lipophilic layer that will bind to the oil (or fat). These... 

HoUow structures can also be prepared by the self-assembly of rod-like molecules into bundle-shaped frameworks. This is also an efficient strategy for the formation of transmembrane channels by rod-like proteins of y -sheets or -helices of amphipathic character in biological systems. The central cavity of the hollow structures is hydrophilic, while their outside area is lipophilic, which match with the lipophilic layer of the lipid membrane. A natural example of this type of proteins is a-hemolysin [31], a bacterium toxin self-assembled from seven identical subunits in cell membranes. This assembly gives rise to a mushroom-shaped structure, whose trunk is formed by a yS-barrel that is inserted into the cell membrane. The resulting channel has a diameter of 13 A at its narrowest point and can transport ions and other solutes. 

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