Molecular‐Recognition‐Assisted pK_a Shifts and Metal‐Ion‐Induced Fluorescence Regeneration in p‐Sulfonatocalix[6]arene‐Encapsulated Acridine

Abstract

The host-guest interactions of cationic (AcH(+) ) and neutral (Ac) forms of the dye acridine with the macrocyclic host p-sulfonatocalix[6]arene (SCX6) were investigated by using ground-state absorption, steady-state and time-resolved fluorescence, and NMR measurements. The cationic form undergoes significant complexation with SCX6 (Keq =2.5×10(4) M(-1) ), causing a sharp decrease in the fluorescence intensity and severe quenching in the excited-state lifetime of the dye. The strong binding of the AcH(+) form of the dye with SCX6 is attributed to ion-ion interactions involving the sulfonato groups (SO3 (-) ) of SCX6 and the positively charged AcH(+) at pH of approximately 4.3. Whereas, the neutral Ac form of the dye undergoes weak complexation with SCX6 (Keq =0.9×10(3) M(-1) ) and the binding constant is lowered by one order of magnitude compared with that of the SCX6-AcH(+) system. The strong affinity of SCX6 to the protonated form leads to a large upward pKa shift (≈2 units) in the dye. In contrast, strong emission quenching upon SCX6 interaction and the regeneration of fluorescence intensity of the dye in the presence of Gd(3+) through competitive binding have also been demonstrated.

Revisiting Fluorescent Calixarenes: From Molecular Sensors to Smart Materials

“…In a separate investigation, Mohanty et al investigated the effect of pH on the binding behavior of 192 (Figure ) with acridine. Fluorescence spectroscopy analysis revealed that calix 192 bound acridine most strongly in acidic media, where the protonated N atom (p K a of acridinium ion = 5.58) forms attractive Coulombic interactions with the host.…”

Inhibition and disintegration of insulin amyloid fibrils: a facile supramolecular strategy with p-sulfonatocalixarenes

“…SCXs are extensively used in enzymatic assays, metal-ion sensors/extractants, drug binding, medicine etc. 13,14 The portals of SCXs can bind small guest molecules through electrostatic and more specific interactions like H-bonding, p-stacking, etc. [14][15][16] and in the present context, the cavity can encapsulate the amino acid residue through hydrophobic interaction and helps in the stabilization the a-helices of the protein, which in turn prevents its transformation to the b-sheets.…”

“…13,14 The portals of SCXs can bind small guest molecules through electrostatic and more specific interactions like H-bonding, p-stacking, etc. [14][15][16] and in the present context, the cavity can encapsulate the amino acid residue through hydrophobic interaction and helps in the stabilization the a-helices of the protein, which in turn prevents its transformation to the b-sheets. 17 On the other hand, disintegration/dissolution of mature fibrils or plaques into smaller particles is another competent remedial approach to target fibril deposits/plaques.…”

Macrocycle encapsulation triggered supramolecular pKa shift: A fluorescence indicator for detecting octreotide in aqueous solution

“…This has led to related applications in the investigations of micro-heterogeneous and confined media such as micelles [35], cyclodextrins and calixarenes [36]. For example, Mohanty and co-workers revealed that the protonated form of Ac exhibits a stronger interaction with p-sulfonatocalix [6]arene than does the neutral form [37], which leads to a large upward pKa shift (∼2 units) for the dye.…”