Cyclodextrins associated with

Although the chiral recognition mechanism of these cyclodexttin-based phases is not entirely understood, thermodynamic and column capacity studies indicate that the analytes may interact with the functionalized cyclodextrins by either associating with the outside or mouth of the cyclodextrin, or by forming a more traditional inclusion complex with the cyclodextrin (122). As in the case of the metal-complex chiral stationary phase, configuration assignment is generally not possible in the absence of pure chiral standards. 

Figures 4 and 5 show the plots of log 1/Kd vs. log Pe for branched or cyclic alcohol-cyclodextrin systems. Both of the plots showed considerable scatter in contrast to the plots for 1-alkanol systems (solid lines). However, a remarkable trend was found by comparing both plots. Most of the plots for an a-cyclodextrin system (Fig. 4) are located below the straight line due to Eq. 5, whereas those for a P-cyclodextrin system (Fig. 5) are located above the straight line given by Eq. 6. This shows that it is general for a bulky alcohol to associate with a-cyclodextrin less strongly and with P-cyclodextrin more strongly than a rod-like 1-alkanol if the log Pe values are the...

The above two models together with Tabushi s cyclodextrin bis(histamine)23) are really elabolate ones, each having a substrate binding cavity, but their catalytic activities are yet far behind of those of natural enzymes. They suggest the difficulties associated with the design of a metal ion center inside of a cavity which activates both substrate and catalytic groups. 

Similarly, cyclodextrin accelerates the cleavage of pyrophosphates by about 200-fold. This enhancement is associated with a simultaneous transfer of a phenylphosphate group to the host by the vicinal action of the hydroxy groups [see Figure 5.4] (Hennrich Cramer, 1965). In this case the product monophenylphosphate also forms an inclusion complex and thus product inhibition occurs. Because of this, the system is not truly catalytic. 

Commercial LASs are complex mixtures of four individual LASs (C10-C13) with 20 possible positional isomers. Isomeric separation can be achieved by solvophobic association with SDS or host-guest interaction with cyclodextrins. Complete resolution of 19 isomers was achieved using 10 mM phosphate buffer (pH 6.8) containing 40 mM SDS and 30% acetonitrile [4]. LAS isomers in technical products were separated using a-cyclodextrin, but complete resolution of all isomers was not achieved [5]. 

ILs have been used to separate and determine the purity of anthraqui-nones. Rapid and sensitive determination of anthraquinones in Chinese herb using l-butyl-3-methylimidazolium-based IL with p-cyclodextrin (p-CD) as a modifier in CZE was provided by Qi et al. [49]. Successful separation and identification of four anthraquinones of Paedicalyx attopevensis Pierre ex Pitard extracts has been achieved. In the running electrolyte the anthraquinones may associate with the imidazolium ions or with the p-CDs. They may be entirely or partly embedded in the cavity of the p-CDs, so the association with the free imidazolium ions in the bulk solution was weak and those analytes, that were not embedded in the cavity of the p-CDs had rather stronger association with the imidazolium ions in the system. The mechanism of separation is illustrated in Figure 6.6. 

The aqueous-organic two-phase system was successfully applied to perform hydrocarboxylation.300 Palladium complexes of trisulfonated triphenylphosphine ligands were shown to exhibit high activity.301-303 The application of cosolvents and modified cyclodextrins allow to eliminate solubility problems associated with the transformation of higher alkenes.304... 

The hetero-dimerization behavior of dye-modified -cyclodextrins with native CDs was investigated by means of absorption and induced circular dichroism spectroscopy in aqueous solution [43], Three types of azo dye-modified /i-CDs show different association behavior, depending on the positional difference and the electronic character of substituent connected to the CD unit in the dye moiety. p-Methyl Red-modified fi-CD (1), which has a 4-(dimethylamino)azobenzene moiety connected to the CD unit at the 4 position by an amido linkage, forms an intramolecular self-complex, inserting the dye moiety in its / -CD cavity (Figure 13). 1 also associates with native a-CD by inserting the dye residue into the a-CD cavity. The association constants for such hetero-dimerization are 198 M"1 at pH 1.00 and 305 M 1 at pH 6.59, which are larger than the association constants of 1 for / -CD (43 M 1 at pH 1.00). 

The CD obtained after mono-, di-, and triazanaphthalenes were complexed with 8-cyclodextrin has been correlated with theoretical results obtained using the CNDO-S-CI approximation [38]. As shown in Figure 5, the induced CD associated with the lowest energy ir->ir band system of various azanaphthalenes is very sensitive to the position and number of aza nitrogens contained in the ring system. Correlation with the theoretical results permitted deductions to be made regarding the orbital character of the bands involved in the observed CD, and several LCAO-MO selection rules for the signs of the induced CD bands were obtained. 

The CD induced in 2-(2,4,6-cycloheptatrien-l-ylidine)-4-cyclopentene-l,3-dione upon its inclusion in 6-cyclodextrin was used to obtain full assignment of the observed absorption bands [49]. It was concluded that the low energy band system was composed of three electronic transitions, two of which were polarized perpendicular, and one parallel, with respect to the long molecular axis of the solute. The higher energy band system was found to also consist of three allowed electronic transitions, but two of these were polarized parallel, and one perpendicular, to the long axis of the solute molecule. These deductions were supported by a series of CNDO/S-CI calculations performed on the system. A similar approach was taken to assign the polarizations of the various transitions associated with 2-thioxo-l,3-benzodithiole and 2-selenoxo-1,3-benzodithiole [50]. 

It was found that formation of the 6-cyclodextrin inclusion complex improved the compound solubility from less than 0.5 mg/mL to 160 mg/mL, eliminating the formulation problems associated with solubility concerns [65]. Although retinoic acid does exhibit intrinsic CD when dissolved in dimethyl sulfoxide, the CD spectrum is drastically upon formation of the cyclodextrin inclusion complex (see Figure 10). That CD can be measured in the absorption bands located at 350 and 400 nm can be taken as evidence that it is the conjugated sidechain of the retinoic acid solute which becomes included in the cavity of the cyclodextrin host system. 

The ability of some components of nucleic acids, especially those with an adenine base, to form complex with 8-cyclodextrin, can also be readily used for chromatographic separations of various nucleotides and nucleosides (59). A substantial problem associated with application of cyclodextrin polymer gels, is that the accessibility of the cyclodextrin cavities on the surface and within the interior of the polymer particle is rather different. The rate of entrapment and release of solutes from the streaming liquid is obviously a diffusion controlled process. Consequently, a longer time is needed to reach an equilibrium within the particle than on its surface. The accessibility of the cyclodextrin rings will be more uniform, if the cyclodextrin is immobilized on the surface of non-complexing polymer particles (polyacrylamide, agarose (60,61) cellulose (62), and silica (63)). Therefore, a better separation (however lower capacity) is expected. 

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