Modulation of Protonation–Deprotonation Processes of 2-(4′-Pyridyl)benzimidazole in Its Inclusion Complexes with Cyclodextrins

Abstract

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.

Excited‐State Proton Transfer and Conformational Relaxation of 2‐(4′‐Pyridyl)benzimidazole in Nafion Films

“…4PBI has earlier been used by our group as a fluorescent probe to differentiate between two kinds of cation exchanged Nafion 117 membranes, in which protons have been replaced by two different cations; namely, Na + and Me 4 N + . It has also been used to study the microenvironment in microheterogeneous media . Other similar fluorophores with different ground‐ and excited‐state properties depending upon the external pH, have also been reported .…”

pH‐Responsive Indicator Displacement Assay of Acetylcholine Based on Acridine–p‐Sulfonatocalix[4]arene Supramolecular System: Fluorescence Off/On Switching and Reversible pKa Shift

“…Supramolecular host‐guest chemistry is an extremely active research area due to their diverse applications in the fields of drug delivery, drug formulations, food industry, pharmaceuticals, nanomedicines, photodynamic therapy, photostabilization, catalysis, nanotechnology, functional materials, optical sensors, on–off switches, etc . Host‐guest assemblies are formed by combining discrete molecular components in ordered manner involving dynamic noncovalent interactions . Numerous weak noncovalent interactions, such as hydrophobic, van der Waals, hydrogen‐bonding, electrostatic, dipole–dipole, etc., can collectively contribute in the molecular recognition for the hosts towards the guest, dynamically leading to the development of various smart host−guest systems with excellent molecular recognition properties and strong responses towards various external stimuli, such as desired analyte molecules, pH, ionic strength, temperature, light, etc .…”

“…Host‐guest assemblies are formed by combining discrete molecular components in ordered manner involving dynamic noncovalent interactions . Numerous weak noncovalent interactions, such as hydrophobic, van der Waals, hydrogen‐bonding, electrostatic, dipole–dipole, etc., can collectively contribute in the molecular recognition for the hosts towards the guest, dynamically leading to the development of various smart host−guest systems with excellent molecular recognition properties and strong responses towards various external stimuli, such as desired analyte molecules, pH, ionic strength, temperature, light, etc . Studies on stimuli responsive “ON/OFF” optical sensors, involving macrocyclic hosts and fluorogenic guests in combination with suitable external stimuli as the triggers, are currently undergoing great advances, displaying binding and release mechanism through fluorescence indicator displacement methodology …”

pH-Assisted control over the binding and relocation of an acridine guest between a macrocyclic nanocarrier and natural DNA

“…1,2,[13][14][15][16] Cyclodextrins (CDs) as macrocyclic hosts have been very successfully used as drug carriers in many drug formulations. 7,13,[18][19][20][21] Depending upon the number of D-glucopyranose units present, different CD homologues with varying cavity sizes exist, namely aCD, bCD and gCD, containing 6, 7, and 8 monomer units, respectively. 7,13,[18][19][20][21] Depending upon the number of D-glucopyranose units present, different CD homologues with varying cavity sizes exist, namely aCD, bCD and gCD, containing 6, 7, and 8 monomer units, respectively.…”