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

Abstract

The stability of inclusion complexes (i.e. the measure of molecular recognition between a guest and (β-cyclodextrin, CD) is highly influenced by the 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 influence these interactions by their sizes, shapes and functional groups. On the other hand, both the size of the cavity and the reactivity of β-CDs are altered when the hydroxy groups are substituted. As best models, the interactions among hydroxypropylated CDs of different average degree (and pattern) of substitution (DS) and phenolphthalein (as a model for ‘large’ guests) or p-nitrophenol/ p-nitrophenolate ( p-NP/ p-NPate) couple (as for ‘small’ ones) have been studied. The formation constants of phenolphthalein–hydroxypropyl-β-CD (HP-CD) complexes are continuously decreasing, while those of p-NP and p-NPate ones show a maximum with increasing DS. Similarly, the pattern of substitution has a significant effect on the quality of the interaction, too. The change of the DS on O(6) position alters the type of the interactions most, and a series of different findings prove that this change 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 characterizing the substitution pattern.