Isotope effects on enzymatic and nonenzymaticreactions of phosphorothioates

Document Type

Article

Journal/Book Title

Nukleonika

Publication Date

2002

Volume

47

Issue

1

First Page

S17

Last Page

S23

Abstract

Kinetic isotope effects have been measured for the aqueous hydrolysis reactions of p-nitrophenyl phosphorothioate (pNPPT) and the diester ethyl p-nitrophenyl phosphorothioate, and for the alkaline phosphatase-catalyzed reaction with pNPPT. The results show that the transition states of the uncatalyzed reactions of the phosphorothioate mono- and diesters are very similar to those of the corresponding phosphate ester reactions. The secondary 18O nonbridge isotope effects in reactions of phosphate esters become more normal as the mechanism changes from dissociative, metaphosphate-like to associative, phosphorane-like. The opposite trend occurs in phosphorothioate esters, due to differences in the relative contributions of bond-order changes and bending modes to this isotope effect. The KIEs for the alkaline phosphatase-catalyzed reaction of pNPPT are most consistent with a tight, triester-like transition state, probably a result of perturbations resulting from the larger size of sulfur that lead to a nucleophile attack angle that is unfavorable for an in-line process with a loose transition state.

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