Cyclodextrin - A Naturally Occurring Cyclic Host

As mentioned above, some naturally occurring cyclic hosts that possess molecular recognition capabilities were known before crown ethers (the first artificial host molecules) were discovered. For example, the cyclic oligopeptide valinomycin and the cyclic oligosaccharide cyclodextrin were found to bind to specific guest molecules. The chemical modification of cyclodextrin was particularly well-researched, and artificially modified cyclodextrins became one of the most important compoimds used in host-guest chemistry. 

Cyclodextrins provide a hydrophobic micromedium in an aqueous phase. This characteristic is analogous to the reaction pockets of enzymes. Enzymes provide size-selective hydrophobic cavities and catalyze the reactions of bound substrates. As one might therefore expect, artificial enzymes based on cy- 

Another example of a cyclodextrin-based artificial enzyme is shown in Fig. 2.15. A phosphodiester is hydrolyzed through cooperative interactions with two imidazolyl groups. The relative positions of the imidazolyl groups and the inclusion geometry of the hydrophobic substrate are structurally well-matched. This artificial enzyme can be regarded as a model of ribonucle-ase A. 

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Cyclodextrin - A Naturally Occurring Cyclic Host Cyclodextrins are cyclic hosts made from oligosaccharides. They provide a hydrophobic microenvironment in an aqueous phase. 

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