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An Assessment of Desolvation on Rates of Acetyl Transfer: Insights into Enzyme Catalysis

Rucker, Victor C. and Byers, Larry D. (2000) An Assessment of Desolvation on Rates of Acetyl Transfer: Insights into Enzyme Catalysis. Journal of the American Chemical Society, 122 (35). pp. 8365-8369. ISSN 0002-7863. http://resolver.caltech.edu/CaltechAUTHORS:20170425-123232383

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Abstract

Enzymes greatly enhance the rate of reactions by a variety of physical organic mechanisms. One of the more contentious of these has been desolvation. To get a quantitative assessment of this contribution, we examined acetyl transfer to oxydianions. This is a model reaction for enzymes in which a high-energy acyl phosphate is formed. The aqueous reaction of phosphate or phosphonates with p-nitrophenyl acetate (pNPA) shows a substantial negative deviation from the Brønsted correlation obtained with monoanionic nucleophiles and from the reactivity of other, larger, oxydianions (molybdate, arsenate and vanadate). This, and other data, suggests a significant contribution of desolvation to the activation energy. To further investigate this we studied the effect of various DMSO (dimethyl sulfoxide)/H_2O mixtures on the reaction of chloromethylphosphonate, and of molybdate, on the reaction with pNPA. Increasing the DMSO concentration from 1% to 90% (v/v) increases the second-order rate constant for each of these reactions by over a factor of 5000. This is over a 1000 times greater than the enhancing effect on the reaction of phenoxides and over 105 times the (inhibiting) effect on the reaction with neutral nucleophiles (imidazole). Extrapolation to pure DMSO yields a rate enhancement of ∼10^5, relative to the reaction in water, for the oxydianions. This enhancement is over 3 orders of magnitude greater than that seen with monoanionic phenoxide nucleophiles. This suggests a significant role for desolvation in the reactions in the enzyme-catalyzed nucleophilic reactions of inorganic phosphate but a modest role in reactions with less highly charged nucleophiles.


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http://dx.doi.org/10.1021/ja993341pDOIArticle
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Additional Information:© 2000 American Chemical Society. Received September 14, 1999. Publication Date (Web): August 18, 2000.
Record Number:CaltechAUTHORS:20170425-123232383
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20170425-123232383
Official Citation:An Assessment of Desolvation on Rates of Acetyl Transfer:  Insights into Enzyme Catalysis Victor C. Rucker and Larry D. Byers Journal of the American Chemical Society 2000 122 (35), 8365-8369 DOI: 10.1021/ja993341p
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:76912
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:25 Apr 2017 19:43
Last Modified:25 Apr 2017 19:43

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