Cyclodextrin membrane

Addition of a chiral carrier can improve the enantioselective transport through the membrane by preferentially forming a complex with one enantiomer. Typically, chiral selectors such as cyclodextrins (e.g. (4)) and crown ethers (e.g. (5) [21]) are applied. Due to the apolar character of the inner surface and the hydrophilic external surface of cyclodextrins, these molecules are able to transport apolar compounds through an aqueous phase to an organic phase, whereas the opposite mechanism is valid for crown ethers. 

Armstrong and Jin [15] reported the separation of several hydrophobic isomers (including (l-ferrocenylethyl)thiophenol, 1 -benzylnornicotine, mephenytoin and disopyramide) by cyclodextrins as chiral selectors. A wide variety of crown ethers have been synthesized for application in enantioselective liquid membrane separation, such as binaphthyl-, biphenanthryl-, helicene-, tetrahydrofuran and cyclohex-anediol-based crown ethers [16-20]. Brice and Pirkle [7] give a comprehensive overview of the characteristics and performance of the various crown ethers used as chiral selectors in liquid membrane separation. 

Lancrajan, 1. et al.. Carotenoid incorporation into natural membranes from artificial carriers liposomes and P-cyclodextrins, Chem. Phys. Lipids, 112, 1, 2001. 

In the recent past separation of isomers has been attempted using aqueous liquid membranes based on p-cyclodextrin. Thus, separation of a mixture of o- and p-nitroaniline (in 80% i-octanol, 20% -heptane) has been studied, with the p-isomer showing a selectivity of 5 at 0.7 molar p-cyclodextrin. Even stereoisomers of stilbene cis and trans) were separated using a 0.02 to 0.2 M cyclodextrin solution, but the selectivity was less than 2 (Mandal et al, 1998). 

Spin trapping methods were also used to show that when carotenoid-P-cyclodextrin 1 1 inclusion complex is formed (Polyakov et al. 2004), cyclodextrin does not prevent the reaction of carotenoids with Fe3+ ions but does reduce their scavenging rate toward OOH radicals. This implies that different sites of the carotenoid interact with free radicals and the Fe3+ ions. Presumably, the OOH radical attacks only the cyclohexene ring of the carotenoid. This indicates that the torus-shaped cyclodextrins, Scheme 9.6, protects the incorporated carotenoids from reactive oxygen species. Since cyclodextrins are widely used as carriers and stabilizers of dietary carotenoids, this demonstrates a mechanism for their safe delivery to the cell membrane before reaction with oxygen species occurs. 

Rhodopsin is a seven ot-helix trans-membrane protein and visual pigment of the vertebrate rod photoreceptor cells that mediate dim light vision. In this photoreceptor, retinal is the chromophore bound by opsin protein, covalently linked to Lys296 by a Schiff base linkage. Kpega et al.64 have studied NMR spectra of Schiff bases being derivatives of all-frans retinal and amino-p-cyclodextrins as a model of rhodopsin, where p-cyclodextrin plays a role of a binding pocket. On the basis of analysis of the chemical shift differences for the model compound in the presence and in the absence of adamantane carboxylate, it has been shown that the derivative of 3-amino-p-cyclodextrin forms dimer in water and retinoid is inserted into p-cyclodextrin cavity [31]. 

The water-soluble methyl-P-cyclodextrin (mpCD) is known to form soluble inclusion complexes with cholesterol, leading to depletion of cholesterol from the plasma membrane (16,46,47). As a result, cholesterol-rich microdomains, which are involved in caveolae-mediated as well as clathrin-mediated endocytosis, are destroyed. mpCD therefore decreases both clathrin- and caveolae-mediated uptake. The two other well-known cyclodextrins [a-, and y-cyclodextrin (6 and 8 units of a-1,4 glucose)] do not bind cholesterol effectively (both are not specific for cholesterol, but might remove phospholipids from the plasma membrane) and have no significant effect (46). 

Steck, T.L., Ye, J., Lange, Y. Probing red cell membrane cholesterol movement with cyclodextrin. Biophys. J. 2002, 83, 2118-25. 

The complexation of fullerenes and y-cyclodextrin was found to take place more efficiently by the use of a HSVM. Thus, when Cgo was vigorously shaken with 4 molar equivalents of y-cyclodextrin for only 10 min by HSVM and the resulting mixture dissolved in 4 mL of water, a magenta-colored solution of Cgo was obtained by filtration with a membrane filter (Scheme 1) [16]. The concentration of Cgo was 1.4x10 mol L which is the highest value for Cgo dissolved in water. When this aqueous solution was let stand for 2 weeks, the Cgo-y-cy-clodextrin 1 2 complex was obtained as purple crystals. 

Motoyama et al. [182] applied on-line SPE for the enantioseparation of pindolol in urine and sernm. The samples were injected into the system after filtration through a membrane filter. A cation exchanger with a mobile phase of 100 mM NH4AC (pH 5.0) AcN (90 10) was connected to a phenylcarbamate-P-cyclodextrin colnmn. A mobile phase of AcN H20 (50 50) with lOmM NH4AC... 

This paper is not a review covering the entire field of carbohydrate-recognition in any organized system. Many excellent papers have already been devoted to supramolecular systems such as cyclodextrins, podands, coronands or cryptants able to entrap carbohydrate molecules [1]. This article only deals with the molecular recognition of mono and oligosaccharides in organized self-assemblies of amphiphilic carbohydrates (possibly blended with other lipids) in aqueous medium i.e. in assemblies mimicking the cell membrane. 

Cyclodextrin-substituted molecular channel approaches have now been extended to include acyl substituents through a covalent bond formation. Stearoyl and methyl cholate-substituted cyclodextrins 10 and 11, respectively, have been synthesized. It may be worthwhile commenting on the molecular design of methyl cholate-substituted a-cyclodextrin. All of the ether groupings are convergent at the inner side of the steroidal backbone of a bent structure to make the molecule amphiphilic. Once the cyclodextrin derivative is incorporated into the membrane phase, it may easily be expected that the ether parts are assembled inside the channel in the sea of hydrophobic lipid molecules and the hydrophobic steroidal skeletons cover its outside to stabilize the inner hydrophilic pore (Figure 13). 

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