pH Independent Complexation of 2-Aminobenzimidazole with β-cyclodextrin

The spectral characteristics of 2-aminobenzimidazole (2ABZ) in aqueous and β-cyclodextrin media has been investigated using absorption, steady state and time resolved fluorescence techniques. The stoichiometric ratio of the inclusion complex of neutral and monocationic forms of 2ABZ in β-CDx medium is found to be 1:1. There is no significant differences between ground and the excited state pKa values of 2ABZ under both the media and thus the amino group of 2ABZ molecule is not much affected by the complexation process i.e., it lies outside the β-CDx cavity. Considering the shape and dimensions of β-CDx, it is clear that the 2ABZ molecule cannot be completely included in the β-CDx cavity. Because, the overall height of 2ABZ is 10.6 Å, but the overall height of β-CDx is only 7.8 Å. Hence, it is possible to locate half of the 2ABZ molecule only permitted inside the β-CDx cavity as interpreted using experimental data.