Key points are not available for this paper at this time.
Abstract The water-soluble tetra- and hexasulfonated calix4 and calix6arenes bind very strongly the neurotransmitter acetylcholine and other quaternary ammonium cations with association constants Ks up to 4×105 M−1 for NEt4 +. The high affinities for choline and acetylcholine (Ks=5×104 to 8×104 M−1) are comparable to those of the biological recognition sites. The crystal structure of the choline-tetrasulfonated calix[4arene] complex was determined. Two complexes A and B were observed in the asymmetric unit. In each one, the choline inserts its N-terminal inside the cavity of the receptor. The calix4arene adopts a single cone conformation but the choline substrate is in an extended conformation in complex A and adopts two other folded conformations in complex B (75 % –25 %). The very high association constants and the crystal structure provide important information about the nature of the binding in this biologically most relevant complex.
Lehn et al. (Thu,) studied this question.