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Four compounds were prepared: 3-oxo-1-methylquinuclidinium iodide (I), 2-hydroxyiminomethyl-1,3-dimethylimidazolium iodide (II) and two conjugates of I and II linked by -(CH2)3- (III) and -CH2-O-CH2- (IV). The aim was to evaluate separately the properties of I and II as opposed to III and IV, which contain both moieties in the same molecule. All four compounds were reversible inhibitors of acetylcholinesterase (AChE; EC 3.1.1.7). The enzyme/inhibitor dissociation constants for the catalytic site ranged from 0.073 mM (II) to 1.6 mM (I). The dissociation constant of I for the allosteric (substrate inhibition) site was 4.8 mM. Possible binding of the other compounds to the allosteric site could not be measured because II, III and IV reacted with the substrate acetylthiocholine (ATCh) and at high ATCh concentrations the non-enzymic reaction interfered with the enzymic hydrolysis of ATCh. The rate constants for the non-enzymic ATCh hydrolysis were between 23 and 37 l/mol per min. All four compounds protected AChE against phosphorylation by Soman and VX. The protective index (PI) of I (calculated from binding of I to both, catalytic and allosteric sites in AChE) agreed with the measured PI; this confirms that allosteric binding contributes to the decrease of phosphorylation rates. The PI values obtained with III and IV were higher than those predicted by the assumption of their binding to the AChE catalytic site only. The toxicity (i.p. LD50) of compounds I, II, III and IV for mice was 0.21, 0.68, 0.49 and 0.77 mmol/kg body wt. respectively. All four compounds protected mice against Soman when given (i.p.) together with atropine 1 min after Soman (s.c.). One-quarter of the LD50 dose fully protected mice (survival of all animals) against 2.52 (IV), 2.00 (I and III) and 1.58 (II) LD50 doses of Soman.