Evaluation of complex forming ability of hydroxypropyl-β-cyclodextrins

The complex forming ability of hydroxypropyl-β-cyclodextrins (HP-β-CDs) is highly influenced by the distribution of substituents and the average degree of substitution (DS), both the size of the cavity and the reactivity of CDs are altered when the hydroxyl groups are substituted. On the other hand, the guests themselves influence these interactions by their sizes and configurations. In the present study, 9 HP-β-CDs with different substitution patterns and DS, which have been investigated by the reductive-cleavage method and methylation analysis, were chosen. The interactions among HP-β-CDs and phenolphthalein (as a model for ‘larger spheriform’ guests) or p-methyl red (as a model for ‘smaller linear’ guests) were studied for determining the complex forming ability of HP-β-CDs. The results indicated that, compared with parent β-CD, HP-β-CDs have a lower ability to form inclusion complexes with the ‘larger spheriform’ guest molecules. With regard to the ‘smaller linear’ guest molecules, HP-β-CDs have a higher complex forming ability, especially the low DS value (<6.5) HP-β-CDs which have a ratio of DS (2 + 3) to DS (6) close to 1.