para-Sulfonated calixarenes used as synthetic receptors for complexing photolabile cholinergic ligand

摘要

The water-soluble tetra-, hexa- and octasulfonated calix[4]arenes. calix[6]arenes, and calix[8]arenes 1-3, respectively, were investigated as potential synthetic receptors for photolabile cholinergic ligand A. a photolytic precursor of choline. Ligand A is a bifunctional molecule carrying a photolabile 2-nitrobenzyl group at one end and a choline moiety at the other end. Results from NMR studies have shown that calixarenes 1-3 form stable 1 : 1 complexes with A. having similar binding potential to that observed with the cholinergic enzymes acetvicholinesterase and butyrylcholinesterase. Further studies have suggested that calix[8]arene forms a ditopic complex by binding concomitantly to both the cationic choline moiety and the aromatic photolabile group of A, whereas calix[4]arene and calix[6]arene form monotopic complexes with A. The ditopic complex between calix[8]arene and A results from mutually induced fitting process, while the monotopic complexes between calix[4]arene and A can be regulated by pH: at neutral pH, calix[4]arene specifically binds the cationic choline moiety, while, at acidic pH, it complexes unselectively both the cationic choline moiety and the aromatic group of A. Our results show that para-sulfonated calixarenes are versatile artificial receptors which bind in various ways to the bifunctional photolabile cholinergic ligand A, depending on their size, geometry, and state of protonation.