Cyclodextrin is the name of a class of cyclic oligomers of glucose whose members are a-(hexamer), ß-(heptamer), 7-(octamer) cyclodextrin, etc. Cyclodextrin1 is a well-constructed miniature of an enzyme in the sense that it has a hydrophobic cavity of appropriate size, sites for introduction of catalytic groups at juxta positions, and satisfactory water solubility. The basic understanding of specific binding and catalysis of enzyme action is one of the most significant targets of cyclodextrin chemistry. Further development of the concept of inclusion catalysis from simple enzyme models into the more artificial, well-organized catalytic system of "superenzyme" activity may be one of the future targets.Crystallographic studies of cyclodextrins,2 their derivatives,3 and inclusion complexes4 reveal that water molecules occupy the hydrophobic cavity in the absence of a guest molecule. However, a specific "guest" molecule, when added into the cyclodextrin solution, drives water molecules out of the cavity by occupying the cavity by itself (Figure 1). Importantly, the conformational change of cyclodextrin during this process is not too serious, and for this reason, a conformational difference between cyclodextrin hydrate in the crystalline state and cyclodextrin in aqueous solution should be very small. This is even more the case for /3-cyclodextrin where the crystalline hydrate has a conformation very close to the expected torus in contrast to the -hydrate's partly tilted torus. Thus, /3-cyclodextrin is free of the rather difficult conformational problems involved in complex formation.The driving force (-AG0) which brings a hydrophobic guest molecule into the cavity of cyclodextrin and drives water molecules out seems to be a complex composite of different "elemental" forces such as activated (less hydrogen-bonded) water,5 a change in degrees of motional freedoms, conformation energy,6 etc. But the major driving force binding a nonpolar guest molecule is the so-called "hydrophobic interaction", which dominates when electrostatic or coordination interactions are not important.A hydrocarbon-like molecule dissolved in water is surrounded by water assemblies along its exposed surface. This process is entropically very unfavorable, as is typically shown for transfer of methane from the gaseous to the aqueous phase. The hydrophobic interaction, most simply expressed, is the tendency of a Iwao Tabushi is Professor in the Department of Synthetic Chemistry at Kyoto University, where he received his doctorate. He was a postdoctoral fellow at Harvard and held faculty appointments at Kyoto and Kyushu Universities before assuming his present post at Kyoto. His research interests include enzyme models, blomlmetlc synthesis, artificial photosynthesis, macrocycllc polydentates, especially for U02 binding from sea water, artificial cell, nucleotide transport, and synthesis.

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Abstract— The phosphorescence of 1‐bromonaphthalene and 1‐chloronaphthalene is readily observable in nitrogen purged aqueous solutions containing ß‐cyclodextrin. Addition of acetonitrile increases both the phosphorescence intensity and lifetime. The quenching of halonaphthalene phosphorescence in aqueous solution by nitrite is substantially inhibited upon addition of ß‐cyclodextrin, as a result of a guest‐host complex. The rate constants for formation and dissociation of the l‐bromonaphthalene/ß‐cyclodextrin complex are evaluated from an analysis of the dependence of phosphorescence lifetimes on nitrite concentration.

A comprehensive model of the inclusion process of -cyclodextrin is presented herein, van der Waals interaction energy, Allinger's conformation energy, solvation energy of apolar solute in water, hydrogen bond energy of water molecules in the cavity of -cyclodextrin hexahydrate, and all other possible energies were taken into account for the calculation of freeenergy change by complexing an apolar guest molecule by -cyclodextrin. Motional freedoms of all of the particles relevant to the inclusion process were taken into consideration. The calculated values of the net stabilization free energy of a-cyclodextrin complexes with benzene, methyl orange, and p-iodoaniline were 4.50, 6.20, and 8.99 kcal mol-1, respectively, which were in good agreement with the observed values. The largest value of 10.09 kcal mol-1 was obtained as the van der Waals stabilization energy of the -cyclodextrin-p-iodoaniline complex which was most easily understood as a result of large atomic polarizability of iodine atom and/or better fit of the iodide substrate to -cyclodextrin's cavity than the other substrates. The value of the solvation energy (-A//scluster) ranges from 4.1 to 7.1 kcal mol-1, depending upon the size of the apolar moiety of the guest molecule, or the number of water molecules in the first layer of water clusters. The increase in entropy which accompanies breaking of the water clusters around the apolar guest has been found to be a nonnegligible term which contributes to the net stabilization of the inclusion complex. A true aspect that -cyclodextrin in the complex has a larger conformation energy by ca. 4 kcal mol-1 than that in the hexahydrate is also pointed out. Therefore, a speculation presented early by others that the strain relief is the main driving force of the formation of the inclusion complexes of -cyclodextrin is to be corrected based on the present results.

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