Reinvestigation of Calixarene-Based Artificial-Signaling Acetylcholine Receptors Useful in Neutral Aqueous (Water/Methanol) Solution

It was shown that an artificial acetylcholine (neurotransmitter) detection system using resorcin[4]arene (1:  Inouye et al. J. Am. Chem. Soc. 1994, 116, 5517) has substantial problems:  both acetylcholine and a fluorescent receptor molecule decompose in 0.01 M KOH/MeOH solution. In their system the strong alkaline medium was necessitated to dissociate OH groups in 1 and to enhance the fluorescence quenching ability. To solve this problem we exploited a new artificial acetylcholine detection system using water-soluble calix[n]arene-p-sulfonates which are useful even in aqueous (water/methanol) neutral solution. This achievement is due to several outstanding properties of these calix[n]arenes such as low pKa values, cation−π interactions, high water-solubility, etc.