Association with cyclodextrins

Du Pont Performance Products, Zonyl Fluorosurfactants, Du Pont, Wilmington DE(1987). 

lUPAC Manual of Symbols and Terminology, Pure Appl. Chem. 31. 577 (1972). 

Tanford, The Hydrophobic Effect Formation of Micelles and Biological Membranes, p. 36, Wiley, New York (1973). 

Hartley, Aqueous Solutions of Paraffin Chain Salts, Hermann et Cie, Paris (1936). 

in Colloidal Dispersions and Micellar Behavior. K. Mittal, ed.. Plenum, New York (1975). 

---

ABSTRACT. Studies of chiroptical properties need separation or at least enrichment of enantiomers. The separation is difficult with chiral con-formers of molecules with very low barriers of internal rotation. However, in association with cyclodextrin, these labile molecules can exhibit a strong Cotton effect in solution one chiral conformer is favoured by complexation. The cyclodextrins have the advantage to yield inclusion complex in solution as well as crystalline clathrates. Therefore, the absolute configuration of the guest can be obtained by determining the structure of the cyclodextrin. In this work, the first CD spectrum of 4-helicene is recorded And crystal structures of several clathrates of labile molecules are studied. 

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. 

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

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. 

Akiyoshi K, Ueminami A, Kurumada S et al (2000) Self-association of cholesteryl-bearing poly(L-lysine) in water and control of its secondary structure by host — guest interaction with cyclodextrin. Macromolecules 33 6752-6756... 

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 case of benzoin alkyl ethers illustrated in Figure 8.15 is a remarkable example of the effect of complexation with cyclodextrins. Such molecules normally undergo homolytic dissociation in solution (the Norrish type 1 process described in section 4.4) and there is practically no intramolecular hydrogen atom abstraction (Figure 8.15). When the benzoin alkyl ethers are complexed with a cyclodextrin to form a 1 1 association, it can be shown that one of the phenyl rings fits inside the cyclodextrin cavity in aqueous solution. When the solid complex is irradiated only the photoproducts resulting from hydrogen atom transfer are detected the opposite behaviour from irradiation of the crystal of benzoin alkyl ether as well as of solutions in benzene. 

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). 

---