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

Abstract

Host‐guest complexes of protonated (AcH+) and neutral (Ac) forms of biologically important acridine dye with p‐sulfonatocalix[4]arene (SCX4) and their responses towards acetylcholine (AcCh) as a competitive binder has been investigated using photochemical studies. Unlike Ac, the AcH+ undergoes strong binding with SCX4 and their differential binding results in a large upward pKa shift for the bound dye. Dye binding to SCX4 causes a drastic fluorescence quenching, witnessing a strong fluorescence “turn OFF”, which is switched to strong fluorescence “turn ON” by the presence of neurotransmitter, AcCh, acts as a stimulus cum competitive binder. This is a unique system showing controlled binding and release for both AcH+ and Ac forms of the dye triggered by AcCh, convincingly established through absorption, pKa tuning, fluorescence modulation and NMR shift. Fluorescence “OFF/ON” switching observed in this study demonstrates an efficient indicator displacement methodology, achieving a control over the selectivity in binding and release of the dye/analyte, having prospective uses in designing new optical sensors and smart functional materials for analytical and biological applications.

pH‐Responsive Interaction of Fluorogenic Antimalarial Drug Quinine with Macrocyclic Host Cucurbit[7]uril: Modulations in Photophysical and Acid‐Base Properties

“…In the host‐guest inclusion complexes, as the encapsulated dye/drug experiences a much lower micropolarity than in the bulk water phase, the absorption and/or fluorescence spectra of the bound dye normally display a hypsochromic shift ,,,,. In the present case the bathochromic shifts observed in the absorption spectra for both QNH 2 2+ and QNH + forms on their interactions with CB7 host are not only intriguing but also quite unusual.…”

“…Since 430–550 nm spectral region is very similar to the broad emission band of QNH 2 2+ form ( cf .Figure a), we feel that the new shoulder band arises due to partial protonation of QNH + ‐CB7 complex in the excited state, which is further supported by the appearance of long fluorescence decay tail for this system that corresponds nicely with the fluorescence decay for the QNH 2 2+ ‐CB7 system at pH 2 (discussed latter in subsection on “time‐resolved (TR) fluorescence studies”). We thus infer that the binding to the CB7 host causes a large upward shift in the pK a (1) value of the drug,,,,,,, especially in the excited state, resulting the QNH + ‐CB7 complex (as well as the free QNH + ) to undergo partial protonation in the excited S 1 state, even at pH 7, although the ground state pK a (1) of the free drug is just 4.3.…”

“…The blue shift observed is understandably due to inclusion of the drug into the hydrophobic CB7 cavity such that it experiences a much lower micropolarity compared to that in bulk aqueous phase. This is expected because a lower solvent polarity in general causes a blue shift in the emission spectra for most chromophoric dyes ,,,,,…”

“…The increase in the fluorescence intensity for QNH 2 2+ on its interaction with CB7 can be rationalized considering the confinement effect imposed by the host cavity such that the vibrational and rotational motions of the encapsulated dye are largely retarded compared to those of the free dye in bulk aqueous phase. Such confinement effect understandably causes a significant reduction in the non‐radiative deexcitation rates for the excited dye molecules and thereby enhances their fluorescence yield ,,,,,…”

Sulfated β‐Cyclodextrin Templated Assembly and Disassembly of Acridine Orange: Unraveling Contrasting Binding Mechanisms and Light Off/On Switching

“…To explore the stoichiometry of the complexes formed in the studied AOH + ‐SCD system, job's plot measurements were carried out by varying the mole fraction of the dye and host components in the solution, keeping the sum of their concentrations constant ,,,. In this study, Job's plots were constructed following both fluorescence and absorbance changes independently as are shown in Figure .…”