Cyclodextrin complexing ability

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. 

Recently, NMR spectroscopy has been used to study the effect of substituents in structural PBN (402) and 5,5-dimethylpyrroline A-oxide (DMPO) (403) analogs on their complexing ability with natural (j-cyclodextrin (402, 403). 

By virtue of their complexing ability, CDs may influence the course of chemical reactions in respect of rates and/or product selectivity. In consequence, there is a large body of data in the literature on the effect of CDs on many types of reactions (Fendler and Fendler, 1975 Bender and Komiyama, 1978 Szejtli, 1982 Tabushi, 1982 Sirlin, 1984 Ramamurthy, 1986 Ramamurthy and Eaton, 1988). The present review concentrates on reactions for which sufficient kinetic data are available to allow quantification of the effects of CDs on transition state stability, in an attempt to understand how cyclodextrins influence reactivity in either a positive or negative sense. 

Some modifications to the cyclodextrin structure have also been found to improve their complexing ability. Casu and coworkers prepared 2,3,6-tri-O-methyl and 2,6-di-O-methyl derivatives of alpha and beta cyclodextrin. They observed that tri-O-methyl-alpha cyclodextrin shows an almost ten-fold increased stability of the complex with the guest, Methyl Orange, compared with the unmodified alpha cyclodextrin. A possible reason for this increase in stability is that the methyl groups are responsible for an extension of the hydrophobic cavity of the cyclodextrin. Other workers,however, observed a much smaller enhancement of stability of complexes on methylation of the cyclodextrin, and a decrease in stability has even been reportedfor the one host-two guests complex of tropaeolin with beta cyclodextrin. Thus, the effect of methylation on the stability of a complex varies with the guest species involved, and cannot be readily predicted. 

Among enzyme modified starch derivatives,cyclo dextrins behave as empty molecular capsules with the ability to entrap guest molecules of appropriate geometry and polarity.The included molecules are protected from surroundings light, heat,oxidation, etc. The flavor cyclodextrin com -plexes show the above advantageous properties while they are in the dry,solld state.On contact with water,cyclodextrin complexes release their flavor content. In Hungary,the spice flavor beta-cyclodextrin complexes have been on the market, since 1982. 

Cyclodextrins have a large range of industrial applications. The market for them is growing as a consequence of their unique inclusion properties and decomplexation kinetics in conjunction with their stability, non-toxicity and relative cheapness. Cyclodextrins are the main active ingredient in Procter and Gamble s deodorising product Febreze, for example, where their complexation ability binds molecules responsible for household odours. The principal areas of interest are summarised in Figure 6.26. Some 1,649 research papers were published with the word cyclodextrin in the title in 2006 alone. 

The cyclodextrins in particular are of considerable commercial importance in applications such as food additives, drug delivery and analytical separations because of their complexing ability in water and in the solid state, non-toxicity and chirality. 

In order to further profit from the dual complexing ability of CDs, we have studied Fenton-type processes in the presence of CDs. Several classes of compounds showed enhanced degradation rates in the presence of CDs in aqueous solution PCBs, PAHs, TNT, and chlorinated phenoxyacetic acids [38,102]. Dissolved natural organic matter typically inhibits Fenton degradation by sequestering the iron away from the pollutant [31,32]. However, addition of cyclodextrins overcame the inhibitory effect of the NOM and resulted in enhanced degradation rates [38]. 

Muller, B. W., and Brauns, U. (1986), Hydroxypropyl-P-cyclodextrin derivatives Influence of average degree of substitution on complexing ability and surface activity, J. Pharm. Set, 75, 571-572. 

Loftsson, T. Frioriksdottir, H. The effect of water-soluble polymers on the aqueous solubility and complexing abilities of P-cyclodextrin. Int. J. Pharm. 1998, 163 (1/2), 115-121. 

Sequential complexation was confirmed in reference 97, where it was reported that P-CD addition decreases the R value and increases the intensity of the two vibronic bands. Solutions of P-CD containing 13 always exhibited a biexponential decay the shortest, t, = 130 ns, has the same lifetime of 13 in water, the largest, Tj = 300 ns, indicates that 13 experiences a hydrophobic environment. The ratio of the preexponentials /42Mi grew monotonically with [/ -CD]. The data are consistent with a sequential complexation, as in the complexation of 13 with a-CD. In the 1 1 complex, the included pyrene has the same lifetime as 13 in water because a substantial portion of the molecule is still exposed to the solvent when 13 is encapsulated by two cyclodextrins, it experiences a low-polarity microenvironment and its lifetime consequently increases. In the same paper, the complexing ability of a polymer-supported P-CD of the general formula... 

NMR Parameters and the Complexing Ability of Cyclodextrins and Their Derivatives... 

Sakairi, N., Nishi, N. and Tokura, S. (1999) Cyclodextrin-linked chitosan synthesis and inclusion complexation ability, in Polysaccharide Applications Cosmetics and Pharmaceuticals (eds M.A. El-Nokaly and H.A. Soini), American Chemical Society, Washington, pp. 68-84. 

Dimeric j8-cyclodextrin complexes have been used as model compounds to study membrane diffusion transport through lipid bilayer membranes. a-Cyclodextrin is able to interact with some optically-active benzene derivatives. The ability of the compound with the benzene ring to be inserted into the central cavity of the a-cyclodextrin molecule provided the driving force for complex formation. The effect of cyclodextrins on the dissociation of some azo dyes showed that the apparent Ka values of the dyes increased with increasing cyclodextrin concentration. The effect was explained as a hydrophobic interaction between the dye and the cyclodextrin molecules. 

Cyclodextrin analogues based on 2-acetamido-2-deo3 -D-glucose have been prepared and their complexing abilities have been studied, and a S-(l- 3) linked... 

Investigating the assembled structures of macrocyclic compounds in the solid state is very important because such studies reveal the optical properties, mechanical and thermal stabilities in macrocyclic-based bulk materials, and the complexation ability of macrocyclic compounds. Assembled structures of macrocyclic compounds, such as cyclodextrins (CDs), calix[ ]arenes and cucutbit[ ]urils in the crystal state have... 

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