经常建议酶催化的磷酰基转移反应通过从其溶液对应物改变的过渡态进行。然而,先前对大肠杆菌碱性磷酸酶 (AP) 的研究表明,这种酶通过松散的解离过渡态催化磷酸单酯的水解,类似于溶液中的过渡态。尽管在溶液中磷酸二酯水解具有更紧密、更缔合的过渡态,但 AP 还表现出催化混杂,具有低水平的磷酸二酯酶活性。因为 AP 在进化上针对磷酸单酯水解进行了优化,所以活性位点环境可能会改变二酯水解的过渡状态,使其与传入和传出基团的键合更松散。为了测试这种可能性,我们已经测量了一系列取代的甲基苯基磷酸二酯的非酶促和 AP 催化的反应速率。beta(lg) 的值和其他数据表明 AP 催化的磷酸二酯水解的过渡态与溶液中的过渡态无法区分。AP 通过识别和稳定类似于水溶液中的过渡态来催化磷酰基转移反应,而不是改变过渡态结构。因此,AP 活性位点能够识别不同的过渡状态,这一特性有助于混杂活动的进化优化。beta(lg) 的值和其他数据表明
The reaction of the title substrate with a series of amines of varying pKa, viz. ammonia, ethanolamine, glycine, 1,2-diaminopropane, 1,3-diaminopropane, n-butylamine, piperidine, hydrazine, imidazole, and hydroxylamine is subjected to a kinetic study in aqueous medium, 25°C, ionic strength 0.1 M (KCl). Pseudo-first-order rate coefficients (kobs) are found throughout under amine excess at various pH
A comprehensive view of the gas-phase reaction of fluoride ions and acetic acid derivatives
作者:Jose M. Riveros
DOI:10.1039/c39900000773
日期:——
The gas-phase reaction of F– with several acetic acid derivatives (MeCOX) is shown to proceed primarily by proton abstraction in the collision complex, rather than nucleophilic displacement, leading to the facile formation of FHX– or CH2CFO–.
The kinetics of aminolysis of several substitutedphenylacetates by imidazole is studied in aqueous medium at 20°C and an ionic strength of 0.1 M (KCl). By following the leavinggroups spectrophotometrically (λmax = 272–401 nm), under excess free imidazole, pseudo-first-order rate constants (kobs) are obtained. For the esters with good nucleofuges, the reaction follows clean second-order kinetics
Heterolysis and homolysis energies for some carbon-oxygen bonds
作者:Edward M. Arnett、Kalyani Amarnath、Noel G. Harvey、Sampath Venimadhavan
DOI:10.1021/ja00176a039
日期:1990.9
Methods described previously for obtaining heterolysis (ΔH het ) and homolysis (ΔH homo ) enthalpies for bonds that can be cleaved to produce resonance-stabilized carbeniumions, anions, and radicals are extended to the study of carbon-oxygen bonds through the reactions of resonance-stabilized carbeniumions with substituted phenoxide ions. Titration calorimetry was used to obtain the heat of heterolysis
先前描述的获得异裂 (ΔH het ) 和均裂 (ΔH homo ) 焓的方法可以裂解以产生共振稳定的碳正离子、阴离子和自由基,通过共振反应扩展到碳-氧键的研究- 稳定的碳正离子与取代的酚盐离子。滴定量热法用于获得异解热,二次谐波交流伏安法(SHACV)法用于获得阴离子的可逆氧化电位
Lind; Shen; Eriksen, Journal of the American Chemical Society, 1990, vol. 112, # 2, p. 479 - 482