Using sodium p-nitrophenyl methylphosphonate as a model for aged (dealkylated) phosphonylated cholinesterase, a study was made of its rate of reaction with various alkylating reagents containing nucleophilic groups.In DMF the alkylation rate ranged widely between a second-order rate constant, k2 = 190 M~l hr-1, at 25°, to no observable reaction in 96 hr at 60°c orresponding to k2 <0.02 M~l hr-1. Aqueous hydrolysis rates at pH 7.15, 25°, were determined for 20 of the mixed esters. The hydrolysis rates also varied extensively, with ii" ranging from 106 sec to less than 5 sec. The exceptionally rapid aqueous hydrolysis of several activated -bromo ketones strongly supports a mechanism involving attack by OHon the CO group. The first example of the intramolecular participation of an amide group in phosphonate hydrolysis by attack on P was observed in carbamoylmethyl p-nitrophenyl methylphosphonate. An exceptionally small rate ratio of 0.025 was found for the reaction of phenacyl methylsulfonate vs. phenacyl bromide with sodium p-nitrophenyl methylphosphonate in DMF. It is suggested that this ratio reflects steric interference in the reaction of the sulfonate and indicates that caution must be exercised in using the Hoffman method for estimating the position of the transition state on the reaction coordinate in SNs reactions. Finally, it was found that the spontaneous reactivation of phenacyl methylphosphonylated eel acetylcholinesterase occurs about 100 times more rapidly than that of the corresponding ethyl methylphosphonylated enzyme.

Unter Verwendung von Natrium+p‐nitrophenylmethylphosphonat (I) als Modell für eine gealterte (desalkylierte) phosphonylierte Cholinesterase wurde dessen Reaktionsgeschwindigkeit mit verschiedenen Alkylierungsmitteln,die gleichzeitig nucleophile Gruppen enthalten, untersucht.