Transition-State Structures for Phosphoryl- TransferReactions of p-Nitrophenyl Phosphate

Authors

Alvan C. Hengge, Utah State UniversityFollow
William A. Edens
Hannah Elsing

Document Type

Article

Journal/Book Title

Journal of the American Chemical Society

Publication Date

1994

Volume

116

First Page

5045

Last Page

5049

Abstract

Heavy-atom isotope effects have been used to characterize the transition states for the aqueous hydrolysis reactions of the p-nitrophenyl phosphate dianion and monoanion, for the reaction of the dianion in neat tert-butyl alcohol, and for the reaction catalyzed by alkaline phosphatase. The primary oxygen-18 isotope effect at the phenolic oxygen (¹⁸k_bridge), the secondary nitrogen-15 effect (¹⁵k) in the nitrogen atom of the leaving group, and the secondary oxygen-18 isotope effects in the nonbridge oxygen atoms of the phosphoryl group (¹⁸k_nonbridge) have been measured. The isotope effects for the dianion reaction in water at 95 °C were ¹⁵k = 1.0028 ± 0.0002, ¹⁸k_bridge = 1.0189 ± 0.0005, and ¹⁸k_nonbridge = 0.9994 ± 0.0005. The dianion reaction in tert-butyl alcohol at 30 °C gave values of ¹⁵k = 1.0039 ± 0.0003, ¹⁸k_bridge = 1.0202 ± 0.0008, and ¹⁸k_nonbridge = 0.9997 ± 0.0016. When corrected for temperature, the results are very similar, indicating similar late transition state structures for the two reactions with little or no change in bond order between the phosphorus and the nonbridge oxygen atoms. The isotope effects on the aqueous reaction of the monoanion were ¹⁵k = 1.0004 ± 0.0002, ¹⁸k_bridge = 1.0087 ± 0.0003, and ¹⁸k_nonbridge = 1.0184 ± 0.0005, suggesting both proton transfer and bond cleavage are rate-limiting. The isotope effects on the alkaline phosphatase reaction are all near unity, indicating that a nonchemical step is rate-limiting for the enzymatic reaction.

Recommended Citation

A. C. Hengge, W.A. Edens, H. Elsing. “Transition-State Structures for Phosphoryl- Transfer Reactions of p-Nitrophenyl Phosphate.” J. Am. Chem. Soc.1994, 116, 5045-5049.