Precise tuning of cationic cyclophanes toward highly selective fluorogenic recognition of specific biophosphate anions

Cationic cyclophanes with bridging and spacer groups possess well-organized semirigid cavities and are able to encapsulate and stabilize anionic species through diverse molecular interactions. We highlight the precise tuning of functionalized cyclophanes toward selective recognition of AMP, GTP, and pyrophosphate (PPi) using fluorescence, NMR spectroscopy, and density functional theory (DFT).close0

Halides with Fifteen Aliphatic C-H center dot center dot center dot Anion Interaction Sites

Since the aliphatic C-H center dot center dot center dot anion interaction is relatively weak, anion binding using hydrophobic aliphatic C-H(C-ali-H) groups has generally been considered not possible without the presence of additional binding sites that contain stronger interactions to the anion. Herein, we report X-ray structures of organic crystals that feature a chloride anion bound exclusively by hydrophobic C-ali-H groups. An X-ray structure of imidazolium-based scaffolds using C-ali-H center dot center dot center dot A-interactions (A(-) = anion) shows that a halide anion is directly interacting with fifteen C-ali-H groups (involving eleven hydrogen bonds, two bidentate hydrogen-bond-type binding interactions and two weakly hydrogen-bonding-like binding interactions). Additional supporting interactions and/or other binding sites are not observed. We note that such types of complexes may not be rare since such high numbers of binding sites for an anion are also found in analogous tetraalkylammonium complexes. The C-ali-H center dot center dot center dot A-interactions are driven by the formation of a near-spherical dipole layer shell structure around the anion. The alternating layers of electrostatic charge around the anion arise because the repulsions between weakly positively charged H atoms are reduced by the presence of the weakly negatively charged C atoms connected to H atomsclos