Cyclodextrins specificity

Several procedures are used to control the ratios of cyclodextrins produced. One is addition of a substance to the reaction mixture that can gready affect the formation of one specific cyclodextrin over another. For example, in the presence of 1-decanol and 1-nonanol, a-cyclodextrin is produced almost exclusively whereas hexane or toluene promote the production of P-cyclodextrin. Conversely both cyclodextrins are produced simultaneously in the presence of 1-heptanol (2,4). 

Cyclodextrin stationary phases utilize cyclodextrins bound to a soHd support in such a way that the cyclodextrin is free to interact with solutes in solution. These bonded phases consist of cyclodextrin molecules linked to siUca gel by specific nonhydrolytic silane linkages (5,6). This stable cyclodextrin bonded phase is sold commercially under the trade name Cyclobond (Advanced Separation Technologies, Whippany, New Jersey). The vast majority of all reported hplc separations on CD-bonded phases utilize this media which was also the first chiral stationary phase (csp) developed for use in the reversed-phase mode. 

Enantioresolution in capillary electrophoresis (CE) is typically achieved with the help of chiral additives dissolved in the background electrolyte. A number of low as well as high molecular weight compounds such as proteins, antibiotics, crown ethers, and cyclodextrins have already been tested and optimized. Since the mechanism of retention and resolution remains ambiguous, the selection of an additive best suited for the specific separation relies on the one-at-a-time testing of each individual compound, a tedious process at best. Obviously, the use of a mixed library of chiral additives combined with an efficient deconvolution strategy has the potential to accelerate this selection. 

Template effects have been used in rotaxane synthesis to direct threading of the axle through the wheel. Since macrocycHc compounds such as cyclodextrins, crown ethers, cyclophanes, and cucurbiturils form stable complexes with specific guest molecules, they have been widely used in the templated synthesis of rotax-anes as ring (wheel) components. Here, we briefly discuss macrocycles used in the synthesis of rotaxane dendrimers and their important features. 

Cyclodextrins as catalysts and enzyme models It has long been known that cyclodextrins may act as elementary models for the catalytic behaviour of enzymes (Breslow, 1971). These hosts, with the assistance of their hydroxyl functions, may exhibit guest specificity, competitive inhibition, and Michaelis-Menten-type kinetics. All these are characteristics of enzyme-catalyzed reactions. 

Carbohydrates not only act as ligands, but they can also provide scaffolds for molecular recognition processes. It is well known that cyclodextrins (CDs) are able to form an inclusion complex with specific guest molecules. In the last years, NMR experiments combined with other techniques have been used to highlight different recognition events. 

The PO mode is a specific elution condition in HPLC enantiomer separation, which has received remarkable popularity especially for macrocyclic antibiotics CSPs and cyclodextrin-based CSPs. It is also applicable and often preferred over RP and NP modes for the separation of chiral acids on the cinchonan carbamate-type CSPs. The beneficial characteristics of the PO mode may arise from (i) the offset of nonspecific hydrophobic interactions, (ii) the faster elution speed, (iii) sometimes enhanced enan-tioselectivities, (iv) favorable peak shapes due to improved diffusive mass transfer in the intraparticulate pores, and last but not least, (v) less stress to the column, which may extend the column lifetime. Hence, it is rational to start separation attempts with such elution conditions. Typical eluents are composed of methanol, acetonitrile (ACN), or methanol-acetonitrile mixtures and to account for the ion-exchange retention mechanism the addition of a competitor acid that acts also as counterion (e.g., 0.5-2% glacial acetic acid or 0.1% formic acid) is required. A good choice for initial tests turned out to be a mobile phase being composed of methanol-glacial acetic acid-ammonium acetate (98 2 0.5 v/v/w). 

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). 

The monograph of levocarbastine has already been revised. The determination of the related substances is performed by means of MEKC using an electrolyte solution composed of sodium dodecyl sulfate as a micelle-forming agent in addition to hydroxypropyl-/ -cyclodextrin in a boric acid buffer of pH 9.0. Due to the very good specificity and robustness the method is able to baseline separate the nine specified and detectable impurities and the drug substance. It is easy to meet the system suitability (Rs>4) the resolution between levocarbastine and impurity D was found to be 6.4 and the content of related substances less than 0.5% (see Figure lA and B). 

It was pointed ouF that while ionomer-modified electrodes have greater selectivity than naked electrodes, ionomers provide only a general form of selectivity (charge type and mass or hydrophobicity selectivity), and special methods were suggested to improve specific selectivity. For example, it was thought that applying an additional cellulose acetate layer over the Nafion film would make the electrode selective with respect to dopamine specifically (in contrast to other neurotransmitters). To make the electrode system specifically selective to o-nitrophenol (in contrast to /(-isomer), a complexation with cyclodextrin was performed, which increased the selectivity ratio one order of magnitude with respect to the o-isomer. ... 

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