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 pK_a values, cation - π interactions, high water-solubility, etc.