Bezimidazolium bromide derivative inclusion complexes with native and modified beta-cyclodextrins

The interactions between four native and modified beta-cyclodextrins and a benzimidazolium bromide salt were analyzed through UV-Vis and NMR Spectroscopy. The new benzimidazolium salt was obtained by simple and efficient conversion of N-1 substituted 5,6dimethylbenzimidazole with phenacyl bromide in acetone. In all cases, the complexes stoichiometry was 1:1, as determined from UV-Vis titrations. Based on the values for association constants, the strength of the interactions with benzimidazolium bromide was weakest with the methyl substituted beta-cyclodextrin and strongest with the sulfobutylether substituted beta-cyclodextrin. Through-space NOE experiments were used to investigate the structural aspects of inclusion process. The obtained NOE correlations indicate coexistence of two inclusion modes: one with the phenacyl group inside the cyclodextrin cavity and the second one with dimethyl-substituted benzene ring inside the cavity. The imidazole ring and the ethyl substituent have been proven to remain outside the cyclodextrin cavity in both inclusion modes.