Leaving group effects in thiolester hydrolysis. Part 2. On the possibility of an elimination–addition (keten) mediated pathway in S-acetylcoenzyme a basic hydrolysis and acetyl transfer
作者:Kenneth T. Douglas、Norbert F. Yaggi、Cynthia M. Mervis
DOI:10.1039/p29810000171
日期:——
alkyl leaving groups, ascribed to the lower steric sensitivity of thiolacetate esters compared with the oxygen analogues. The small value of βl.g. and the lower reactivity of S- than O-esters are offered as evidence of a bimolecular associative (BAc2) mechanism for basic hydrolysis. The E2 route is excluded by the lack of deuterium incorporation into the (acetate) product of hydrolysis. In spite of
一系列S-烷基和S-芳基硫代乙酸酯(包括S-乙酰辅酶A )在25°C水溶液中的碱性水解速率(k HO–)与共轭酸的p K a相关(以对数形式)硫醇盐离去基团的β-基团的斜率(βl.g.)为–0.33。与相应的氧酯相比,就给定离去物质的碱性而言,硫酯对氢氧根离子的反应性低1-2个数量级,并且几乎不分散到芳基和烷基离去基团中,这归因于硫羟乙酸盐的较低空间敏感性。酯与氧类似物相比。β的小值l.g.S-的反应性比O-酯低,这是碱性水解的双分子缔合(B Ac 2)机理的证据。所述ë 2路由由缺乏氘掺入到水解的(乙酸盐)的产品中排除。尽管硫醇乙酸盐具有可接受的酸度,但即使在高的非生理pH下,在水溶液中仍形成动力学上无足轻重的酯缀合物碱,因此S-乙酰辅酶A不会通过E 1cB途径水解。