N-Piperidinopropyl-galanthamine (2) and N-saccharinohexyl-galanthamine (3) were used to investigate interaction sites along the active site gorge of Torpedo californica actylcholinesterase (TcAChE). The crystal structure of TcAChE-2 solved at 2.3 A showed that the N-piperidinopropyl group in 2 is not stretched along the gorge but is folded over the galanthamine moiety. This result was unexpected because the three carbon alkyl chain is just long enough for the bulky piperidine group to be placed above the bottleneck (Tyr121, Phe330) midway down the gorge. The crystal structure of TcAChE-3 at 2.2 A confirmed that a dual interaction with the sites at the bottom, and at the entrance of the gorge, enhances inhibitory activity: a chain of six carbon atoms has, in this class of derivatives, the correct length for optimal interactions with the peripheral anionic site (PAS).
