Lipophilicity/lipophobicity

A note to the terms hydrophobicity and hydrophilicity these are frequently employed in characterizing adsorbents. Hydrophilicity/hydrophobicity is a qualitative measure of an adsorbent, characterizing its behavior towards water. The term lipophilicity/lipophobicity is applied to characterize the polarity of a compound. Table 3.4 summarizes the different column packings and stationary phases used in preparative chromatography. 

Kaneko et al. (1993) have described a group of lipophilic ascorbic-acid analogues that have been studied in cultured human umbilical vein endothelial cells that were first incubated with test drug and then exposed to lipid hydroperoxides. Although ascorbate itself did not protect the endothelial cells, derivatives like CV3611 protected. Pretreatment was necessary. CV3611 was synergistic with vitamin E. The authors concluded that these lipophilic antioxidants incorporate into endothelial cell membranes where they are effective inhibitors of lipid peroxidation. In contrast, lipophobic antioxidants were not effective in their hands (Kaneko et al., 1993). 

Surfactants, sometimes called surface active agents or detergents, are amphiphilic materials which contain both apolar, hydrophobic (lipophilic) and polar, hydrophilic (lipophobic) groups (Hartley, 1948, 1977 Fendler and Fendler, 1975 Fendler, 1982 Lindman and Wennerstrom, 1980 Sudholter... 

In summary, for the general population, the common routes of exposure to environmental compounds are ingestion, dermal contact and inhalation. Many PEAS are environmentally persistent but not lipophilic rather they have mixed lipophobic and hydrophobic properties. The exposure scenario is complex as PEAS have a large variety of applications. Gral exposure from materials other than food, inhalation and dermal contact may be important exposure routes for certain segments of the population. Dust inhalation could also be a possible source of exposure. However, the information on concentrations of PEAS in indoor dust is very limited and the bioavailability of the current compounds from dust is unknown. 

Mesogen composed of molecules consisting of two parts of contrasting character that are hydrophilic and hydrophobic or lipophobic and lipophilic. 

Fig. 16. Schematic representation of a fluorinated vesicle obtained from perfluoroalkylated phospholipids showing separated nanometer-thick domains within their bilayer membrane. The central hydrophobic and lipophobic fluorous core of the membrane is flanked by two lipophilic shells, then by the hydrophilic outermost and innermost layers of polar heads [4].

This is a lipophilic weak base (pKa 4), which crosses cell membranes easily at physiological pH. It is a prodrug, which at the acidic intracellular pH in the gastric parietal cell converts to the active sulfenamide form, which is lipophobic and so is trapped and preferentially concentrated in the parietal cells. Omeprazole is highly protein-bound. Hepatic metabolism is rapid, with a plasma half-life of 0.5-1.5 h, but because of covalent binding to the H-1-/K-1- ATPase enzyme, the duration of action exceeds 24 h. 

The biochemical structure of a membrane is that of a lipid bilayer composed of phospho- and sphingolipids, as well as cholesterol. These lipids are amphipathic in nature, that is, they each have a polar and a nonpolar end. In water the nonpolar (hydrophobic, lipophilic) ends will seek to avoid the polar solvent and aggregate into a bilayer with the polar (hydrophilic, lipophobic) ends oriented towards the outside of the bilayer. As this structure extends in all directions the exposed nonpolar regions will close up and form a sphere (or ellipsoid) with water trapped inside and excluded outside. See Figures 2a and 2b. 

Phenobarbital poisoning is exacerbated by its ionized forms also. Its pKa is about 7.2. At a urine pH of 7.4 there will exist about a 50 50 mixture of lipophilic and lipophobic species. Once again the uncharged form will be reabsorbed and the remaining barbiturate molecules will redistribute themselves according to the equilibrium eventually leading to the reabsorption of the virtually all of the phenobarbital. 

Microbes also have a plasma membrane that resides adjacent to their cell wall. Polymyxins are amphipathic agents (containing both nonpolar, lipophilic and polar, lipophobic groups) that interact with phospholipids in microbial cell membranes. The result is disruption of the membrane and increased permeability. However, because microbial and mammalian cell membranes are not exceedingly dissimilar, polymixins can produce significant toxicity in humans (i.e., they have low selective toxicity). This is also true for the related drug nystatin. This is why these particular antibiotics are not generally used systemically and are usually restricted to topical application. 

These non-SEC modes use high efficiency silica-based columns and binary, ternary and quaternary miscible mixtures of organic solvents and water to achieve fast, selective separations (Figure 9.6). In general, a reverse phase mode of operation is most suitable for lipophilic samples, whilst normal partition or adsorption is used for samples which are lipophobic. 

Usually hydrophobicity is encoded by - steric descriptors such as molar or - molecular volume which account satisfactorily for nonpolar interactions polarity can be described by polar terms which are negatively related to lipophilicity. An important factorization of lipophilicity is provided by the - solvatochromic parameters. Moreover, a measure of the global polarity of a given solute was proposed by Testa and coworkers [Testa and Seiler, 1981 El Tayar and Testa, 1993 Vallat et al., 1995] and called the interactive polar parameter A (or the Testa lipophobic constant). It is defined as the difference between the experimentally measured lipophilicity and that estimated for a hypothetical n-alkane of the same molecular volume V as ... 

Dietary lipids may affect the toxicity of environmental chemicals by delaying or enhancing their absorption. The absorption of lipophobic substances would be delayed and that of lipophilic substances accelerated. 

Another remarkable feature of rod—coil copolymers is their amphiphilic characteristics that show the tendency of their lipophilic and lipophobic parts to segregate in space into distinct microdomains. Depending on the solvent content and polarity, rod— coil copolymers self-assemble into a wide variety of different supramolecular structures. Because of the... 

Treatment of LPS with detergents result in a reversible dissociation of LPS into subunits (33). Both Ribi, et (34) and Horisberger and Dentam (35) observed that LPS treated with sodium deocycholate, followed by dialysis, did not alter the size of the particles, yet resulted in a more uniform preparation. Polymyxin B will dissociate and degrade LPS components. These effects have been suggested to be caused by the interactions of the lipophilic and lipophobic groups of the antibiotic with those of the LPS (36). 

Testa lipophobic constant = interactive polar parameter lipophilicity descriptors... 

From the above, a surfactant capable of being both an efficient and an effective yow reducer should have a balanced structure (R 1) in the system and under the conditions of use, with a considerable amount of both hydrophilic and lipophilic character (large values of Acw and ACo) The value of R 1 and large values of Acw and Aco will cause reduction of jow to a very low value, i.e., make it an effective reducer of yow. A surfactant of this type will also have limited solubility in a hydrophilic solvent because of the large lipophilic (hydrophobic) portion of the molecule and limited solubility in a lipophilic solvent because of the large hydrophilic (lipophobic) portion of the molecule. This limited solubility in both liquids will cause the surfactant to adsorb strongly at the interface and consequently to be a very efficient reducer of yow. 

Sometimes chemical reaction or solvation chemistry is exploited by the extraction process. Often, coordination reactions render a lipophobic species lipophilic by complexation with suitable organic ligands, which cluster around the lipophobic moiety in the complex and enable it to transfer into the solvent phase. The oily organic or solvent phase in such instances usually consists of an... 

In their similar action on the cell membrane the tyrocidines, gramicidins and the polymyxins resemble other surface active agents. Like these, they contain also lipophilic and lipophobic groups, which, as has been revealed by studies on gramicidins, may be separated in the molecule by being fixed on different sides of the molecular plane. 

Barrier membrane process. Processes in which one component, in a mixture, interacting with internal or external nanospaces of a membrane, present a barrier for another components, so that only the interacting component passes through the membrane. In the simplest case, a surface-active micro-filtration membrane, if saturated with a wetting liquid, is the barrier for a gas or a nonwetting liquid. Using lipophilic and lipophobic micro-filtration membranes, this phenomenon is applied in multiphase membrane contactors [7] and in the production of fine emulsions. 

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