Cyclodextrins molecules

Packing of the cyclodexthn molecules (a, P, P) within the crystal lattice of iaclusion compounds (58,59) occurs in one of two modes, described as cage and channel stmctures (Fig. 7). In channel-type inclusions, cyclodextrin molecules are stacked on top of one another like coins in a roU producing endless channels in which guest molecules are embedded (Fig. 7a). In crystal stmctures of the cage type, the cavity of one cyclodextrin molecule is blocked off on both sides by neighboring cyclodextrin molecules packed crosswise in herringbone fashion (Fig. 7b), or in a motif reminiscent of bricks in a wall (Fig. 7c). 

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. 

Figure 12.2 Schematic representations of an a-cyclodextrin molecule showing the glucose rings and the overall topology...

Polar interaction between guest and cyclodextrin molecules, 32 428-432,434-435 Polarized X-ray absorption spectroscopy, 35 15-17... 

Yamamoto and coworkers used two-dimensional, nuclear Over-hauser effect experiments (NOESY) to determine the proximity of particular protons situated on an included p-nitrophenolate ion to particular protons of a host alpha cyclodextrin molecule. The experiments showed cross-peaks connecting the H-3 resonance of alpha cyclodextrin to both meta and ortho proton resonances of the p-nitrophenolate ion, whereas H-5 of the alpha cyclodextrin gave a cross-peak only with the resonance of the meta proton thereof. As a consequence, it was unequivocally confirmed that the p-nitrophenolate ion is, in solution, preferentially included with its nitro group oriented to the narrow end of the alpha cyclodextrin... 

Pyronine Y (PY) with beta cyclodextrin by using the temperature-jump technique. It was found that the kinetic data could be adequately explained by a mechanism involving the stepwise association of two beta cyclodextrin molecules (C) to the dye. Thus,... 

Thus, the formation of a one host-two guests inclusion-complex seems to proceed by way of the stepwise inclusion of two guest molecules, with the inclusion of the second guest molecule being rate-determining. The formation of a two hosts-two guests complex may then proceed by the rapid addition of a second cyclodextrin molecule. No evidence has been presented that supports dimerization of the guest prior to inclusion. 

A chiral selector can also be dissolved in the IL solvent and be subsequently coated on the capillary wall [38]. In this approach, the achiral [C4CiIm]Cl was used to dissolve permethylated p-cyclodextrin (p-PM) and dimethylated P-cyclodextrin (p-DM). The chromatographic separations obtained from these two columns were compared to two commercially available CSPs based on p-PM and p-DM dissolved in polydimethylsiloxane. From a set of 64 chiral molecules separafed by fhe commercial p-PM column, only 21 of the molecules were enantioresolved by the IL-based p-PM column. Likewise, from a collecfion of 80 analytes separated by the p-DM column, only 16 analytes could be separated on the IL-based p-DM column. The authors also noted a considerable enhancement in the separation efficiency of fhe IL-based CSPs. This resulf, coupled to fhe loss of enantioselecfivify for mosf separations, suggests that the imidazolium cation may occupy the cavity of the cyclodextrin preventing the analyte-cyclodextrin inclusion complex-ation that is crucial for chiral recognition. The ability for ILs to form inclusion complexes wifh cyclodextrin molecules has been recently studied by Tran and coworkers using near-infrared spectromefry [39]. 

Hydrogen bonds between guest and cyclodextrin molecules (43). 

Figure 3.11—Separation on a cyclodextrin-boimd stationary phase. Chromatogram of a racemic mixture chemical formula of /f-cyclodextrin (diameter, 1.5 nm cavity, 0.8 nm height, 0.8 nm) partial representation of cyclodextrin bonded to a silica gel bead through an alkyl chain linker arm side view of a cyclodextrin molecule with a hydrophobic cavity.

The torus-like cyclodextrin molecules have an outer polar surface and an inner nonpolar surface. 

Fig. 8 (a) Schematic representation of an ordered assembly of cyclodextrin molecules with each one binding a part of the template, (b) Binding modes of (a) cholesterol, (b) 4-hydroxyazobenzene and (c) 4-phenylphenol to the guest-binding sites in the cholesterol-imprinted beta-cyclodextrin polymer. Reprinted with permission from [89]. Copyright 1999 American Chemical Society... 

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