Molecular Selective Binding of Pyridinium Guest Ions by Water‐Soluble Calix[4]arenes

Abstract

The complex stability constants (KS) and thermodynamic parameters (ΔG°, ΔH°, and TΔS°) for 1:1 intermolecular complexation of water‐soluble calix[4]arene tetrasulfonate (CAS) and thiacalix[4]arene tetrasulfonate (TCAS) with pyridinium guest ions 1–10 have been determined by means of titration calorimetry in an acidic buffer solution (pH = 2.0) at 298.15 K, and their binding modes have been investigated by NMR spectroscopy. Possessing a smaller cavity and a higher π electron density than TCAS, CAS affords the stronger binding abilities for all guests examined, giving an exciting molecular selectivity of up to 354 for the 2,6‐dimethylpyridine/2,6‐pyridinedicarboxylic acid pair. The high binding affinities, attributed to the favorable enthalpic gains, are comprehensively discussed from the viewpoint of π–π stacking, electronic effect and size‐fit relationship between host and guest. The thermodynamic parameters that have been obtained, together with the NMR results, jointly demonstrate that the position, number, and type of substituent groups introduced onto the guest molecule are the key factors controlling the structural‐energetics correlation for the molecular selective binding of water‐soluble calixarenes. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Operational calixarene-based fluorescent sensing systems for choline and acetylcholine and their application to enzymatic reactions

“…43 The conformational rigidification, evident from the NMR-spectral changes, is less pronounced for 2 than for 1, presumably due to its larger size and higher intrinsic flexibility. [44][45][46] For calixarene 3, which has a more flexible cone compared to 1 47,48 and is more rigid compared to 2, 45 NMR spectra cannot directly reveal any conformational change due to the lack of bridgehead -CH 2protons (Fig. S6…”

Ionic Exchange in p-Sulfonatocalix[4]arene-Mediated Formation of Metal–Ligand Complexes

“…2 The absolute value of the complexation-induced chemical shifts increases in the order H4 > H5 > H3 > H6, indicating that the guest molecule should be incorporated into the SC4 cavity by the H4 position, with the N atom located at the upper rim, as also observed at pH 2.0. 7 This structure may enable the formation of ternary complexes with metal ions since the position of the N atom is correctly positioned to allow metal− ligand interactions with cations placed near the sulfonato groups. 27 Therefore, it was decided to investigate the influence of sodium and copper cations in the formation of complexes between 1 and SC4.…”

“…p -Sulfonato­calixarenes are a special class of water-soluble macrocycles recognized as potent model receptors for studying noncovalent interactions in aqueous solution with special attention to cation−π, CH, and π–π interactions. − Besides being very useful models to investigate fundamental aspects of supra­molecular chemistry, p -sulfonato­calixa­renes have proven to be highly attractive building blocks for sensing and supra­molecular self-assembly applications including displacement assays for monitoring enzymatic activity, supra­molecular polymers, supra-amphiphiles, etc. − Given the increasing interest and applications of these compounds, detailed knowledge of their binding mechanisms is mandatory for optimization and development of host–guest systems with increased binding affinity and selectivity.…”

Complexation of p‐Sulfonatocalixarenes with Local Anaesthetics Guests: Binding Structures, Stabilities, and Thermodynamic Origins

“…As previous studies without guest molecules have shown, the methylene protons in either SC4A or SC5A give a single NMR peak assigned to a rapid cone‐to‐cone interconversion of uncomplexed host, indicating that the host sulfonatocalixarenes exists in a flexible conformation. However, after complexation with guests, this single peak of SC4A broadens to the baseline or splits into two sets of double peaks with the same integration intensities, which means that SC4A adjusts itself into a more fixed cone conformation to adopt the size of the guest molecule; thus, the conformation changes of the host become slow on the NMR time‐scale following the inclusion of guest 6b. In contrast, the NMR signal assigned to the methylene protons in SC5A remain in their original shape after complexation with guest molecules, which means that the conformation of SC5A remains flexible during the course of complexation, presumably due to its larger size and higher intrinsic flexibility.…”