Binding Modes of a Phenylpyridinium Styryl Fluorescent Dye with Cucurbiturils.

Adrien Paudics,Mihály Kállay,András Szilágyi,Márton Bojtár,Dóra Hessz,István Bitter,Miklós Kubinyi,Benjámin Gyarmati

doi:10.3390/molecules25215111

Abstract

In order to explore how cucurbituril hosts accommodate an N-phenyl-pyridinium derivative guest, the complexation of the solvatochromic dye, 4-(4-(dimethylamino)styryl)-1-phenylpyridinium iodide (PhSt) with α,α′,δ,δ′-tetramethyl-cucurbit[6]uril (Me4CB6) and cucurbit[7]uril (CB7) was investigated by absorption spectroscopic, fluorescence and NMR experiments. In aqueous solutions, PhSt forms 1:1 complexes with both cucurbiturils, the complex with CB7 has a higher stability constant (Ka = 6.0 × 106 M−1) than the complex with Me4CB6 (Ka = 1.1 × 106 M−1). As revealed by NMR experiments and confirmed by theoretical calculations, CB7 encapsulates the whole phenylpyridinium entity of the PhSt cation guest, whereas the cavity of Me4CB6 includes only the phenyl ring, the pyridinium ring is bound to the carbonyl rim of the host. The binding of PhSt to cucurbiturils is accompanied by a strong enhancement of the fluorescence quantum yield due to the blocking of the deactivation through a twisted intramolecular charge transfer (TICT) state. The TICT mechanism in PhSt was characterized by fluorescence experiments in polyethylene glycol (PEG) solvents of different viscosities. The PhSt-CB7 system was tested as a fluorescence indicator displacement (FID) assay, and it recognized trimethyl-lysine selectively over other lysine derivatives.

Highlights

Cucurbiturils (CB-s), with their electron-rich carbonyl portals and hydrophobic cavities, are suitable hosts for the binding of organic cations [1]
CB6 complexes of alkylpyridinium cations, only the alkyl chain intrudes into the host cavity; the charged N interacts with the electron-rich CO groups of the CB
We studied the complexation of the styryl pyridinium dye PhSt with cucurbiturils Me4 CB6 and CB7 and demonstrated that the system CB7-PhSt functions as a displacement assay which discriminates lysine and its methylated derivatives

Summary

Introduction

Cucurbiturils (CB-s), with their electron-rich carbonyl portals and hydrophobic cavities, are suitable hosts for the binding of organic cations [1]. Quaternary pyridinium derivatives are one class of cationic species, the binding of which to CB-s has been studied extensively. In the CB6 (cucurbit[n]urils are denoted as CBn-s) complexes of alkylpyridinium cations, only the alkyl chain intrudes into the host cavity; the charged N interacts with the electron-rich CO groups of the CB portal [2,3,4]. CB7 includes only the alkyl chain of 4-naphthyl-N-alkylpyridinium cation [5]. CB8 can bind a pair of stacked N-benzylpyridinium [8] or 4-aryl-N-methylpyridinium derivative guests in its cavity [9]. Studies on the complexes of 4-pyrrolidino-N-alkylpyridinium cations with

Objectives

Methods

Results

Conclusion