Hydrophobic effects cyclodextrin complexes

Upon formulating these relationships, phenols with branched alkyl substituents were not included in the data of a-cyclodextrin systems, though they were included in (3-cyclodextrin systems. In all the above equations, the n term was statistically significant at the 99.5 % level of confidence, indicating that the hydrophobic interaction plays a decisive role in the complexation of cyclodextrin with phenols. The Ibrnch term was statistically significant at the 99.5% level of confidence for (3-cyclo-dextrin complexes with m- and p-substituted phenols. The stability of the complexes increases with an increasing number of branches in substituents. This was ascribed to the attractive van der Waals interaction due to the close fitness of the branched substituents to the (3-cyclodextrin cavity. The steric effect of substituents was also observed for a-cyclodextrin complexes with p-substituted phenols (Eq. 22). In this case, the B parameter was used in place of Ibmch, since no phenol with a branched... 

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. 

Natural enzymes use the hydrophobic effect as a binding force in forming the enzyme-substrate complex. Artificial enzymes can be used to bind substrates and enhance reactivities in water (Breslow, 1995). Cyclodextrins, which are cyclic compounds composed of glucose units, can be used as the artificial enzymes (Bender and... 

We also examined Diels-Alder reactions in a cyclodextrin complex. In the reaction of cyclopentadiene with 2-butenone (Figure 2.3) or with acrylonitrile we saw that there was catalysis in water with j8-cyclodextrin, but not with a-cyclodextrin, which was an inhibitor. The cyclopentadiene bound into both of the cyclodextrins, but only in the j3-cyclodextrin is there also room for the dienophile to fit. In this same publication we saw that the Diels-Alder reaction is also strongly accelerated simply in water solution, which favours the transition state for the reaction in which a smaller total amount of hydrophobic surface is exposed to the water solvent. This latter finding stimulated us, and others, to study many other examples of catalytic effects in water alone with hydrophobic reagents. However, the cyclodextrin effect was added on top of this in the case where both reagents could fit into the pocket. 

Orstan, A. Ross. J.B.A. Investigation of the 3-cyclodextrin-indole inclusion complex by absorption and fluorescence spectroscopies. J. Phys. Chem. 1987, 91. 2739-2745. Harrison, J.C. Eftink, M.R. Cyclodextrin-adamantanecar-boxylate inclusion complexes A model system for the hydrophobic effect. Biopolymers 1982. 21. 1153-1166. Sinanoglu. O, Molecular Associations in Biology Pullman. B., Ed. Academic New York. 1968 427. 

In 1980, Breslow (3) made the dramatic observation that the reaction of cyclopentadiene with butenone in water was more than 700 times faster than the same reaction in isooctane. The reaction rate in methanol is comparable to that in a hydrocarbon solvent. Such an unusual acceleration of the Diels-Alder reaction by water was attributed to the "hydrophobic effect (4) in which the hydrophobic interactions brought together the two nonpolar groups in the transition state. The use of P-cyclodextrin, which simultaneously forms an inclusion complex with the dioie and dienophile, and the use of 4.86 M LiCl aqueous solution as solvent, which salts out nonpolar materials dissolved in water (5), further enhanced the rate of aqueous Diels-Alder reactions. The second-order rate constant of the reaction between hydroxymethylanthiaceneandN-ethylmaleimideinwaterat 45°C was over 200 times larger than in acetonitrile (eq. 1). 

The lariat concept is extremely powerful. To take just one example, side arms have been appended to more complex systems, such as the members of the cyclodextrin family (Section 2.7.5), as in compound 2.25. The fluorescent indole group of the L-tryptophane-derived lariat arm in this compound is encapsulated within the cavity of the cyclodextrin in aqueous solution by hydrophobic effects. On the addition of a guest, the lariat arm is displaced and the guest takes its place within the cavity, as illustrated in Figure 2.7. 

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. 

It has long been considered that the addition of cyclodextrins (CDs) disfavors the self-assembly of surfactants in dilute solutions since the hydrophobic effect is destroyed upon the formation of the hydrophiphilic CD/surfactant inclusion complex. However, it was found that P-CD/ nonionic surfactant inclusion complexes are able to self-assemble into vesicles in dilute solutions, namely in solutions with concentration lower than the CMC of surfactants. When using Tween 20 as a model surfactant, H-NMR and MS measurements indicate that the building block for the vesicles is the channel type Tween 20 2(3-CD inclusion complex. ... 

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