One host-two guests complexation

In order to determine the mechanism of complex-formation, however, kinetic methods must be used. Consider one host-two guests complex-ation. The two possible mechanisms are dimerization of the guest outside the cyclodextrin cavity followed by inclusion, and dimerization within the cyclodextrin cavity. Equilibrium measurements alone cannot distinguish between these two possibilities. The same is the case for 2 2 complex-formation, where a larger number of possible mechanisms exist. 

However, if the dye is of an appropriate size to form a one host-two guests complex with beta or gamma cyclodextrin, it was found that, on addition of the cyclodextrin, dye solutions often show spectral changes... 

The dependence of 1/t on concentration can be explained by an extension to the equation for the two-step mechanism of one host-two guests complex-formation, that is,... 

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. 

Hirai, H., Toshima, N., and Uenoyama, S. (1985) Inclusion Complex Formation of y-Cyclodextrin. One Host-Two Guest Complexation with Water-soluble Dyes in Ground State, Bull. Chem. Soc. Jpn. 58, 1156-1164. 

A. Ueno, K. Takahashi, T. Osa, One host-two guests complexation between y-cyclodextrin and sodium-a-naphthylacetate as shown by excimer fluorescence, J. Chem. Soc., Chem. Commun., 1980, 921-922. 

The formation of a one host-two guests inclusion-complex could proceed by way of two main mechanisms (/) dimerization of the guest, followed by inclusion and (2) stepwise inclusion of two guest molecules. In order to distinguish between these two possibilities, several temperature-jump relaxation kinetic studies have been carried us-... 

Gyclodextrin cavities form the early models of host molecules involved in supramolecular assemblies. There are many other molecules known as cryptands which can be designed to offer a cavity of fairly precise dimensions to accommodate various ions or metal complexes. It may be possible to locate not just one, but two, guest molecules inside a cryptand cavity, and this may lead to new electron transfer reactions in restricted environments another step towards synthetic photoinduced biochemical reactions. 

Checking the literature of the second half of the 1980s for applications of crown complexes of arenediazonium salts for synthetic purposes, one gets the impression that these complexes are not widely used. The two comprehensive reviews by Izatt et al. (1985, 1991) show a definite decline of interest in host —guest complexation of arenediazonium ions in the second half of the 1980s. The subject of arenediazonium... 

Another related host-guest complex has been constructed from citric acid, boric acid, and a strontium salt in a 2 1 1 stoichiometry. In this complex the strontium cation is surrounded by four water molecules, two monodentate carboxyl groups and one oxygen atom of the BO4 unit [161]. 

The maximum observed free energy difference between two enantiomeric host-guest complexes in which one 1,1 -dinaphthyl element is the only source of chirality in the crown ether is about 0.3 kcal mol-1. Improvement of the free energy difference can be achieved by introduction of two such elements. Unfortunately crown ethers with three 1,1 -dinaphthyl groups did not form complexes with primary ammonium salts (de Jong et al., 1975). The dilocular chiral crown ether [294] forms complexes of different stability with R- and 5-cr-phenylethylammonium hexafluorophosphate. The (J )-J J -[284] complex was the more stable by 0.3 kcal mol-1 at 0°C (EDC value 1.77) (Kyba et al., 1973b). Crown ether [284] also discriminates between the two enantiomers of phenylglycine methyl ester hexafluorophosphate and valine methyl ester... 

Fig. 18. [18]Crown-6 urea 1 5 stoichiometry with 1 2 host/guest complexes. One of the two independent [18]crown-6 molecules is shown with the two complexed guest urea molecules 31)...

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