Influence of the guests, the type and degree of substitution on inclusion complex formation of substituted beta-cyclodextrins

The stability of inclusion complexes (i.e. the measure of molecular recogni tion between a guest and (beta-cyclodextrin, CD) is highly influenced by th e fit of the guest into the cavity of the host and by secondary bonds among the functional groups getting in close connection. The guests themselves i nfluence these interactions by their sizes, shapes and functional groups. O n the other hand, both the size of the cavity and the reactivity of beta-CD s are altered when the hydroxy groups are substituted. As best models, the interactions among hydroxypropylated CDs of different average degree (and p attern) of substitution (DS) and phenolphthalein las a model for 'large' gu ests) or p-nitrophenol/p-nitrophenolate (p-NP/p-NPate) couple (as for 'smal l' ones) have been studied. The formation constants of phenolphthalein-hydr oxypropyl-beta-CD (HP-CD) complexes are continuously decreasing, while thos e of p-NP and p-NPate ones show a maximum with increasing DS. Similarly, th e pattern of substitution has a significant effect on the quality of the in teraction, too. The change of the DS on O(6) position alters the type of th e interactions most, and a series of different findings prove that this cha nge is the basis in the chiral selectivity of different CD derivatives, too . The ratio of the average DS on primary and secondary hydroxy rims [R-DS = DS(6)/DS(2,3)] is recommended as the simplest possibility for characterizi ng the substitution pattern. (C) 1999 Elsevier Science B.V. All rights rese rved.