Site Selectivity of Fluorescent Bisquaternary Phenanthridinium Ligands for Acetylcholinesterase

摘要

This synthesis of decidium and hexidium diiodides, their spectroscopic properties, and association with acetylcholinesterase from Torpedo californica are described and compared with those for propidium. Decidium, hexidium, and propidium, bisquaternary analogs of the fluorescent phenanthridinium ligand ethidium, contain 10, 6, and 3 methylene carbons, respectively, interposed between the exocyclic and endocyclic quaternary nitrogens. The three ligands exhibit linear competitive inhibition of enzyme carbamylation by M-methyl-7-dimethylcarbamoxquinolinium. Association of the three ligands is characterized by a stoichiometry of one fluorescent ligand per 80-kDa molecular weight subunit and occurs with respective 6.5-, 4.5-, and 3-fold increases in both quantum yield and fluorescence lifetime. Decidium and hexidium, in marked contrast with propidium, are dissociated by ligands selective for the active center and undergo pronounced reduction in affinity upon modification of the active center with pyrenebutyl methylphosphonofluoridate.