Abstract
AbstractThe rates of alkaline hydrolysis of the enantiomers of isopropyl p‐nitrophenyl methylphosphonate have been measured with and without varying concentrations of α‐cyclodextrin (cyclohexa‐amylose) in aqueous 1.0 M KCl solution at 25° in order to determine the origin of the acceleration of phosphate ester hydrolysis by cyclodextrins.The observed kinetics can be analyzed in terms of a non‐stereospecific binding of both enantiomers to α‐cyclodextrin prior to a nucleophilic reaction of α‐cyclodextrin predominantly with the (R)‐(‐)‐enantiomer. Dissociation constants (Kdiss) of the inclusion complexes and rate constants (k2) of the nucleophilic reaction are reported for the (R)‐(‐)‐enantiomer in dilute alkaline media.From the pH‐dependency of the rate of hydrolysis of the (R)‐(‐)‐enantiomer a kinetic pKa value of 12.6 has been evaluated for the ionization of α‐cyclodextrin.The results also support the suggestion—proposed in the first paper of this series—to use the α‐cyclodextrin‐organophosphorus compound system as a simple model for inhibition of cholinesterases by organophosphorus compounds.
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