Inclusion of Short Guests into the a-Cyclodextrin Cavity

1 Inclusion of Short Guests into the a-Cyclodextrin Cavity 

Volume data for host-guest interactions of a-CD with diphenyl azo dyes (Fig. 11.8) were first reported by Merbach and co-workers [20]. Their kinetic study at variable temperature and NMR experiments have shown that a 1 1 complex is formed by directional binding of the dye, which is inserted by its sulfonate/sulfonamide 

High-pressure stopped-flow kinetic measurements were performed up to 200 MPa. The observed rate constants, ki,obs and k2,obs as a function of pressure and a-CD concentration, were fitted simultaneously according to the reaction scheme shown in Fig. 11.8 leading to the microscopic rate constants kij, k2,f, h,r and fe2,r-The results are summarized in the form of volume profiles in Fig. 11.10. 

An interpretation of the volume profiles in the context of the complexa-tion mechanism previously discussed, offers a global view of the inclusion process. The inclusion mechanism begins with the encounter of the dye and a-CD mainly due to hydrophobic interactions followed by a partial desolvation of the entering sulfonate/sulfonamide head of the dye. The latter interacts with the two activated water molecules inside the cavity of the free host, and their complete release through the smaller rim of the a-CD is retarded by the primary hydroxy groups. 

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