Cyclodextrin aqueous

Fig, 2. Left and right parts show the fluorescence dynamics of C522 in water and in p-cyclodextrin aqueous solution for the fluorescence wavelengths of 500 (squares), 520 (circles), and 540 (triangles) nm, respectively. The experimental and simulation data are scatter and line, respectively. 

Keywords. Plant cell culture, In situ extraction, In situ adsorption, Cyclodextrin, Aqueous two-phase systems... 

Carbohydrates - Starches and cyclodextrins Aqueous Inclusion complexes... 

In conventional reversed phase HPLC, differences in the physicochemical interactions of the eluate with the mobile phase and the stationary phase determine their partition coefficients and, hence, their capacity factor, k. In reversed-phase systems containing cyclodextrins in the mobile phase, eluates may form complexes based not only on hydrophobicity but on size as well, making these systems more complex. If 1 1 stoichiometry is involved, the primary association equilibrium, generally recognized to be of considerable importance in micellar chromatography, can be applied (11-13). The formation constant, Kf, of the inclusion complex is defined as the ratio of the entrance and exit rate constants between the solute and the cyclodextrin. Addition of organic modifiers, such as methanol, into the cyclodextrin aqueous mobile phase should alter the kinetic and thermodynamic characteristics of the system. This would alter the Kf values by modifying the entrance and exit rate constants which determine the quality of the separation. 

The extreme influence water can exert on the Diels-Alder reaction was rediscovered by Breslow in 1980, much by coincidence . Whale studying the effect of p-cyclodextrin on the rate of a Diels-Alder reaction in water, accidentally, the addition of the cyclodextrin was omitted, but still rate constants were observed that were one to two orders of magnitude larger than those obtained in organic solvents. The investigations that followed this remarkable observation showed that the acceleration of Diels-Alder reactions by water is a general phenomenon. Table 1.2 contains a selection from the multitude of Diels-Alder reactions in aqueous media that have been studied Note that the rate enhancements induced by water can amount up to a factor 12,800 compared to organic solvents (entry 1 in Table 1.2). 

While the previous receptors are typically used in organic solvents, except for the cyclodextrins, there are special cases of cyclophane receptors supphed with peripheral charges (ammonium units) (107—12) or ionizable groups (carboxylate functions) (113,114) (Fig. 17) to allow substrate recognition, as in nature, in an aqueous medium, profiting from the solvophobic effects of water (115). 

The main supramolecular self-assembled species involved in analytical chemistry are micelles (direct and reversed), microemulsions (oil/water and water/oil), liposomes, and vesicles, Langmuir-Blodgett films composed of diphilic surfactant molecules or ions. They can form in aqueous, nonaqueous liquid media and on the surface. The other species involved in supramolecular analytical chemistry are molecules-receptors such as calixarenes, cyclodextrins, cyclophanes, cyclopeptides, crown ethers etc. Furthermore, new supramolecular host-guest systems arise due to analytical reaction or process. 

Purification of C q from a C(,q/C-,q mixture was achieved by dissolving in an aqueous soln of y (but not p) cyclodextrin (0.02M) upon refluxing. The rate of dissolution (as can be followed by UV spectra) is quite slow and constant up to lO M of C o- The highest concn of C o in H2O obtained was 8 x 10 M and a 2 y-cyclodextrin C q clathrate is obtained. C ) is extracted from this aqueous soln by toluene and C oof >99 purity is obtained by evaporation. With excess of y-cyclodextrin more C g dissolves and the complex precipitates. The ppte is insol in cold H2O but sol in boiling H2O to give a yellow soln. [J Chem Soc, Chem Commun 604 7922.]... 

Recently, two examples of the separation of enantiomers using CCC have been published (Fig. 1-2). The complete enantiomeric separation of commercial d,l-kynurenine (2) with bovine serum albumin (BSA) as a chiral selector in an aqueous-aqueous polymer phase system was achieved within 3.5 h [128]. Moreover, the chiral resolution of 100 mg of an estrogen receptor partial agonist (7-DMO, 3) was performed using a sulfated (3-cyclodextrin [129, 130], while previous attempts with unsubstituted cyclodextrin were not successful [124]. The same authors described the partial resolution of a glucose-6-phosphatase inhibitor (4) with a Whelk-0 derivative as chiral selector (5) [129]. 

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. 

Comparisons of LC and SFC have also been performed on naphthylethylcar-bamoylated-(3-cyclodextrin CSPs. These multimodal CSPs can be used in conjunction with normal phase, reversed phase, and polar organic eluents. Discrete sets of chiral compounds tend to be resolved in each of the three mobile phase modes in LC. As demonstrated by Williams et al., separations obtained in each of the different mobile phase modes in LC could be replicated with a simple CO,-methanol eluent in SFC [54]. Separation of tropicamide enantiomers on a Cyclobond I SN CSP with a modified CO, eluent is illustrated in Fig. 12-4. An aqueous-organic mobile phase was required for enantioresolution of the same compound on the Cyclobond I SN CSP in LC. In this case, SFC offered a means of simplifying method development for the derivatized cyclodextrin CSPs. Higher resolution was also achieved in SFC. 

Several intermolecular interactions have been proposed and discussed as being responsible for the formation of cyclodextrin inclusion complexes in an aqueous solution 6-10). They are... 

Water plays a crucial role in the inclusion process. Although cyclodextrin does form inclusion complexes in such nonaqueous solvents as dimethyl sulfoxide, the binding is very weak compared with that in water 13 Recently, it has been shown that the thermodynamic stabilities of some inclusion complexes in aqueous solutions decrease markedly with the addition of dimethyl sulfoxide to the solutions 14,15>. Kinetic parameters determined for inclusion reactions also revealed that the rate-determining step of the reactions is the breakdown of the water structure around a substrate molecule and/or within the cyclodextrin cavity 16,17). 

Table 1. Thermodynamic parameters for the association of cyclodextrin with alcohol in an aqueous solution at 25 °C...

To circumvent these difficulties, a preparation of water-soluble coelenterazine has been developed (Teranishi and Shimomura, 1997a). The preparation contains coelenterazine and 50-times (by weight) of hydroxypropyl-P-cyclodextrin. To prepare this material, 0.1 ml of 3.0 mM coelenterazine in methanol and 0.2 ml of 45 mM solution of the cyclodextrin are mixed and dried under reduced pressure. The dried residue is extracted with 1.0 ml of lOmM phosphate buffer, pH 7.0, containing 2 mM EDTA (if needed), and the extract (after centrifugation) is again dried under reduced pressure. With this preparation, an aqueous solution containing up to 3 mM coelenterazine can be made. 

Recently, 6-alkylsulphinyl / -cyclodextrins 6 were obtained from the corresponding sulphides by oxidation with dilute aqueous hydrogen peroxide19 (equation 2). 

A prehminary study of the use of larch AGs in aqueous two-phase systems [394] revealed that this polysaccharide provides a low-cost alternative to fractionated dextrans for use in aqueous two-phase, two-polymer systems with polyethylene glycol (PEG). The narrow molecular-weight distribution (Mw/Mn of 1-2) and low viscosity at high concentration of AG can be exploited for reproducible separations of proteins under a variety of conditions. The AG/PEG systems were used with success for batch extractive bioconversions of cornstarch to cyclodextrin and glucose. 

Cyclodextrins (CDs) are cyclic a-l,4-linked D-(- -)-glucopyranose units (a-CD = six units jS-CD = seven units) that form inclusion complexes with a variety of hydrophobic molecules in aqueous medium [64]. 

The Diels-Alder reaction of nonyl acrylate with cyclopentadiene was used to investigate the effect of homochiral surfactant 114 (Figure 4.5) on the enantioselectivity of the reaction [77]. Performing the reaction at room temperature in aqueous medium at pH 3 and in the presence of lithium chloride, a 2.2 1 mixture of endo/exo adducts was obtained with 75% yield. Only 15% of ee was observed, which compares well with the results quoted for Diels-Alder reactions in cyclodextrins [65d]. Only the endo addition was enantioselective and the R enantiomer was prevalent. This is the first reported aqueous chiral micellar catalysis of a Diels-Alder reaction. 

The theory and development of a solvent-extraction scheme for polynuclear aromatic hydrocarbons (PAHs) is described. The use of y-cyclodextrin (CDx) as an aqueous phase modifier makes this scheme unique since it allows for the extraction of PAHs from ether to the aqueous phase. Generally, the extraction of PAHS into water is not feasible due to the low solubility of these compounds in aqueous media. Water-soluble cyclodextrins, which act as hosts in the formation of inclusion complexes, promote this type of extraction by partitioning PAHs into the aqueous phase through the formation of complexes. The stereoselective nature of CDx inclusion-complex formation enhances the separation of different sized PAH molecules present in a mixture. For example, perylene is extracted into the aqueous phase from an organic phase anthracene-perylene mixture in the presence of CDx modifier. Extraction results for a variety of PAHs are presented, and the potential of this method for separation of more complex mixtures is discussed. 

Cyclodextrins can solubilize hydrophobic molecules in aqueous media through complex formation (5-8). A nonpolar species prefers the protective environment of the CDx cavity to the hulk aqueous solvent. In addition, cyclodextrins create a degree of structural rigidity and molecular organization for the included species. As a result of these characteristics, these macrocycles are used in studies of fluorescence and phosphorescence enhancement (9-11), stereoselective catalysis (.12,13), and reverse-phase chromatographic separations of structurally similar molecules (14,15). These same complexing abilities make cyclodextrins useful in solvent extraction. 

Cyclodextrin-modified solvent extraction has been used to extract several PAHs from ether to an aqueous phase. Data evaluation shows that the degree of extraction is related to the size of the potential guest molecule and that the method successfully separates simple binary mixtures in which one component does not complex strongly with CDx. The most useful application of cyclodextrin-modified solvent extraction is for the simplification of complex mixtures. The combined use of CDx modifier and data-analysis techniques may simplify the qualitative analysis of PAH mixtures. 

Dendritic hosts can be used in aqueous solution to encapsulate water-soluble fluorescent probes. Changes in the photophysical properties of these encapsulated probes are useful to understand the properties of the microenvironment created by the dendritic interior. For example, adamantyl-terminated poly(pro-pylene amine) dendrimers from the first to the fifth generation (36 represents the third generation) can be dissolved in water at pH<7 in the presence of -cyclodextrin because of encapsulation of the hydrophobic adamantyl residue inside the /1-cyclodextrin cavity and the presence of protonated tertiary amine units inside the dendrimer [72]. Under these experimental conditions, 8-anifi-... 

As described above, the enzymatic polymerization of phenols was often carried out in a mixture of a water-miscible organic solvent and a buffer. By adding 2,6-di-0-methyl-(3-cyclodextrin (DM-(3-CD), the enzymatic polymerization of water-insoluble m-substituted phenols proceeded in buffer. The water-soluble complex of the monomer and DM-(3-CD was formed and was polymerized by HRP to give a soluble polymer. In the case of phenol, the polymerization took place in the presence of 2,6-di-O-methyl-a-cyclodextrin (DM-a-CD) in a buffer. Only a catalytic amount of DM-a-CD was necessary to induce the polymerization efficiently. Coniferyl alcohol was oxidatively polymerized in the presence of a-CD in an aqueous solution. ... 

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