Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India.
J Phys Chem B. 2013 Jul 18;117(28):8603-10. doi: 10.1021/jp403476n. Epub 2013 Jul 5.
2-(4'-Pyridyl)benzimidazole (4PBI) can exist in several states of protonation, having three basic nitrogen atoms. The equilibria involving these states, in ground as well as in excited states, are found to be affected significantly by cyclodextrins (CDs). The formation of inclusion complexes of this compound with all three varieties of cyclodextrins is observed to be more favorable at pH 9 than at pH 4, due to the predominance of the neutral form of dye at pH 9. The binding affinity of 4PBI to CDs is found to be governed by two factors: (i) the size of the host and (ii) the mode of insertion of 4PBI. We find that, for the host with a smaller cavity (α-CD), insertion of the dye with a pyridyl face is favored, whereas, for γ-CD, the preference is shifted toward the benzimidazole face of the dye. For β-CD, the binding affinity of the dye is maximum due to perfect cavity matching with the guest. A combination of steric factor and hydrogen bonding interaction is found to be responsible for modulation of the protonation-deprotonation equilibria of the guest molecule in the inclusion complex. Surprisingly, a protonated form is found to be promoted upon inclusion in cyclodextrins, under certain conditions. This is an unusual behavior and has been rationalized by prototropism involving the hydroxyl protons of cyclodextrin molecules.
2-(4'-吡啶基)苯并咪唑（4PBI）可以存在于几种质子化状态，具有三个碱性氮原子。这些状态的平衡，无论是在基态还是激发态，都被发现受到环糊精（CDs）的显著影响。该化合物与所有三种环糊精的包合复合物的形成被发现更有利于在 pH 9 下形成，而不是在 pH 4 下形成，这是由于在 pH 9 下染料的中性形式占主导地位。4PBI 与 CDs 的结合亲和力受两个因素的控制：（i）主体的大小和（ii）4PBI 的插入模式。我们发现，对于具有较小空腔的主体（α-CD），插入具有吡啶面的染料是有利的，而对于 γ-CD，偏好则偏向于染料的苯并咪唑面。对于 β-CD，由于与客体的完美空腔匹配，染料的结合亲和力最大。发现空间位阻因素和氢键相互作用的组合负责调节客体分子在包合物中的质子化-去质子化平衡。令人惊讶的是，在某些条件下，发现质子化形式在包含在环糊精中时得到促进。这是一种不寻常的行为，已经通过涉及环糊精分子的羟基质子的质子迁移解释了。
