Studies of Transition-State Structures in Phosphoryl Transfer Reactions of Phosphodiesters of p-Nitrophenol
摘要:
Heavy-atom kinetic isotope effects have been used to study the transition states for a number of phosphoryl transfer reactions of the phosphodiesters p-tert-butylphenyl p-nitrophenyl phosphate (1) and 3,3-dimethylbutyl p-nitrophenyl phosphate (2). The alkaline and acid hydrolysis reactions and the reaction with phosphodiesterase I from snake venom were studied with each substrate. In addition, the hydrolysis reactions of 1 catalyzed by bis-(imidazolyl)- and by mono(imidazolyl)-beta-cyclodextrins were studied. The isotope effects measured were the primary O-18 isotope effect in the p-nitrophenyl leaving group, the secondary O-18 isotope effect in the nonbridge oxygen atoms, and the N-15 isotope effect in the leaving group. The data indicate similar early transition-state structures for the aqueous hydrolysis reactions of the two compounds with little bond cleavage to the leaving group. In contrast, significant differences in transition-state structure between the substrates are seen in their reactions with phosphodiesterase I. Compound 1 is a substrate for the imidazolyl-beta-cyclodextrin catalysts, which operate as simple general base catalysts for this substrate. Transition-state bond cleavage to the leaving group is much further advanced in these reactions than in the uncatalyzed aqueous reactions.
亚硝酸催化三氟乙酸水溶液中H 15 NO 3催化的对硝基苯酚硝化反应在形成的2,4-二硝基苯酚的15 N nmr光谱中的2- 15 NO 2基团反应期间产生强发射信号:最多可提高600倍。反应伴随着原始4-硝基向11位的11%迁移,并在4位被标记的硝基取代。在15使用相关反应在n个核磁共振谱15 Ñ标记p硝基苯酚和未标记的硝酸表明4-迁移15 NO2位到2位引起在形成的2,4-二硝基苯酚中2-15 NO 2基团的吸收信号大大增强。对于在叠氮化钠存在下对硝基苯酚与H 15 NO 3的反应,也发现该组的吸收信号大大增强。当通过扩散或单重态前体形成自由基对时,核对极化的反应就可以解释完整的结果。对硝基苯酚催化了hydr酸与硝酸之间的反应。
Mechanistic Study of Protein Phosphatase-1 (PP1), A Catalytically Promiscuous Enzyme
作者:Claire McWhirter、Elizabeth A. Lund、Eric A. Tanifum、Guoqiang Feng、Qaiser I. Sheikh、Alvan C. Hengge、Nicholas H. Williams
DOI:10.1021/ja803612z
日期:2008.10.15
the alkaline hydrolysis of 4NPMP that is similar to that of diesters with the same leaving group. For the enzymatic reaction of 4NPMP, the KIEs are indicative of a transition state that is somewhat looser than the alkaline hydrolysis reaction and similar to the PP1-catalyzed monoester reaction. The data cumulatively point to enzymatic transition states for aryl phosphate monoester and aryl methylphosphonate
Isotope Effects on Enzyme-Catalyzed Acyl Transfer from <i>p</i>-Nitrophenyl Acetate: Concerted Mechanisms and Increased Hyperconjugation in the Transition State
作者:Robert A. Hess、Alvan C. Hengge、W. W. Cleland
DOI:10.1021/ja973413h
日期:1998.4.1
0002. Uncatalyzed acyl transfer from PNPA to oxygen and sulfur nucleophiles proceeds by a concertedmechanism. All of the enzymatic reactions showed isotopeeffects consistent with a concertedmechanism like that seen in uncatalyzed aqueous reactions, but exhibited smaller inverse β-deuterium isotopeeffects than seen in the nonenzymatic aqueous reac...
The mechanism of nitration by 4-methyl-4-nitro-2,3,5,6-tetrabromocyclohexa-2,5-dienone
作者:Robert G. Coombes、John H. Ridd
DOI:10.1039/c39920000174
日期:——
Nitration of phenol by 4-methyl-4-nitro-2,3,5,6-tetrabromocyclohexa-2,5-dienone in diethyl ether is a radical process involving reaction between the phenoxyl radical and NO2˙ that has escaped from a radical pair in which it was formed by homolytic fission of the C–N bond.
and during the reaction of 1 with 15N-enriched nitric acid and nitrous acid, emission due to the nitration products o- and p-nitrophenol (2a, 2b) is observed in the 15N-NMR spectra. The CIDNP effects are built up by radical pairs formed by the encounters of the radicals NO2· and 1+· or PhO·. During the reaction of 1 with nitrous acid, 2b is formed, in part due to a non-radical reaction, via oxidation