Amplification of acetylcholine-binding catenanes from dynamic combinatorial libraries. 
Directed chemical synthesis can produce a vast range of molecular structures, but the intended product must be known at the outset.  In contrast, evolution in nature can lead to efficient receptors and catalysts whose structures defy prediction.  To access such unpredictable structures, we prepared dynamic combinatorial libraries in which reversibly binding building blocks assemble around a receptor target.  We selected for an acetylcholine receptor by adding the neurotransmitter to solutions of dipeptide hydrazones [proline-phenylalanine or proline-(cyclohexyl)alanine], which reversibly combine through hydrazone linkages.  At thermodynamic equilibrium, the dominant receptor structure was an elaborate [2]-catenane consisting of two interlocked macrocyclic trimers.  This complex receptor with a 100 nM affinity for acetylcholine could be isolated on a preparative scale in 67% yield.