Crystal structure of pyrogallol[4]arene complex with phosphocholine: A molecular recognition model for phosphocholine through cation–π interaction

Single crystal X-ray analysis of the complex formed between pyrogallo[4]arene (1) and phosphocholine (PCh), [Ca(H2O)3(1)2]·(PCh−)2·2EtOH·5H2O, a model compound that mimics PCh–receptor recognition in biological systems, has shown that PCh is incorporated into the bowl-shaped cavity of 1 through cation–π interactions between the quaternary cationic trimethylammonium group of PCh and π-rings of 1. The hepta-coordinated [Ca(H2O)3]2+ ion is directly bound to two molecules of 1 through phenolic oxygens to form a dimeric molecule of 1, [1{Ca(H2O)3}1]2+. Conformational data of PCh and its derivatives observed in known crystal structures are compiled, focusing on the conformation about the C–C bond in the P–O–C–C–N+ backbone with a reemphasis that the usually observed gauche conformation could be stabilized by an intramolecular N+–C(methyl group)–H⋯O hydrogen bonding.