Solvent Isotope Effect on the Reaction of Acetylcholinesterase with Alkanesulfonyl Halogenides

The effect of solvent isotope composition on acetylcholinesterase reaction with n-alkanesulfonyl halogenides was studied in order to establish the nature of the rate-limiting step of the reaction. The rate constant of the irreversible step of acetyleholinesterase reaction with methanesulfonyl fluoride was found to have a solvent isotope effect of 1.5 ± 0.1 that is close to that established for the second-order rate constant (1.38 ± 0.05). The dependence of the apparent second-order rate constant on the molar fraction of heavy water in the reaction buffer (proton inventory) was linear, implicating the transfer of one proton in the rate-limiting step of the reaction. The second-order rate constants for acetylcholinesterase reactions with methane-, ethane-, propane, and butanesulfonyl chlorides also depended on the isotope composition of the reaction medium with the average solvent isotope effect of 1.4 ± 0.2. Acetylcholinesterase reaction with alkanesulfonyl halogenides and the dependence of this reaction on the isotope composition of the medium is adequately described by a two-step kinetic scheme where the reversible binding step is immediately followed by the irreversible sulfonylation step. It can be expected that the same is true for the other anticholinesterase agents for which the value of the first-order rate constant for the irreversible reaction step is not significantly higher than that for methanesulfonyl fluoride (0.3 s−1).