Binding Modes of a Phenylpyridinium Styryl Fluorescent Dye with Cucurbiturils

• Main Authors: Adrien Paudics, Dóra Hessz, Márton Bojtár, Benjámin Gyarmati, András Szilágyi, Mihály Kállay, István Bitter, Miklós Kubinyi
• Format: Article
• Language: English
• Published: MDPI AG 2020-11-01
• Series: Molecules
• Subjects: molecular recognition; host–guest complex; hydrophobic interactions; size selectivity
• Online Access: https://www.mdpi.com/1420-3049/25/21/5111

In order to explore how cucurbituril hosts accommodate an <i>N</i>-phenyl-pyridinium derivative guest, the complexation of the solvatochromic dye, 4-(4-(dimethylamino)styryl)-1-phenylpyridinium iodide (PhSt) with <i>,</i><i>′</i><i>,</i><i>δ</i><i>,</i><i>δ</i><i>′</i>-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 (<i>K</i>_a = 6.0 × 10⁶ M⁻¹) than the complex with Me₄CB6 (<i>K</i>_a<i> </i>= 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.