Selective host–guest chemistry investigated by mass spectrometry: Which of the two, choline or acetylcholine, is the preferred one by the 3iPO triphosphonate-cavitand?

Triphosphonate cavitand, 3iPO, has been designed for the recognition of ammonium in solution. This Letter presents a study of the selective guest–host recognition between Acetylcholine and Choline ions, ACh+ and Ch+, and the host. We are able to observe that the ACh+ ion replaces the Ch+ ion in the 3iPO-Ch+ complexes with a concomitant exchange/substitution constant of 7 × 104 M−1. Conversely a replacement of the ACh+ ions by the Ch+ ions in the 3iPO-ACh+ complexes has been also observed with a concomitant exchange constant of 5.8 × 103 M−1. The structure of the 3iPO-ACh+ complex is determined by single crystal X-ray crystallography.