| Abstract: | The water‐soluble tetra‐, hexa‐ and octasulfonated calix4]arenes, calix6]arenes, and calix8]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 acetylcholinesterase and butyrylcholinesterase. Further studies have suggested that calix8]arene forms a ditopic complex by binding concomitantly to both the cationic choline moiety and the aromatic photolabile group of A , whereas calix4]arene and calix6]arene form monotopic complexes with A . The ditopic complex between calix8]arene and A results from mutually induced fitting process, while the monotopic complexes between calix4]arene and A can be regulated by pH: at neutral pH, calix4]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. |
