Catalytic recruitment in the inactivation of acetylcholinesterase by Soman: temperature dependence of the solvent isotope effect

Abstract

3,3-Dimethyl-2-butyl methylphosphonofluoridate (soman) recruits at least 60-70% of the catalytic power of acetylcholinesterase during phosphonylation of the active-site serine, as compared with the acetylation by acetylcholine. The solvent isotope effect is 1.34 +/- 0.11 and is temperature independent within experimental error between 5 and 45/sup 0/C. This observation is quite similar to what has been found for the reactions of aryl acetates and anilides. Average activation parameters for the same temperature range are ..delta..H* = 7.3 +/- 0.6 kcal/mol and ..delta..S* = -4 +/- 2 eu (H/sub 2/O) and ..delta..H* = 7.1 +/- 0.4 kcal/mol and ..delta..S* = -6 +/- 1 eu (D/sub 2/O). A rate-limiting process with 50% contribution of an induced-fit conformational change is supported by the data.