GENERAL-ACID AND GENERAL-BASE CATALYSIS OF THE CLEAVAGE OF ALPHA-D-GLUCOPYRANOSYL FLUORIDE
GENERAL-ACID AND GENERAL-BASE CATALYSIS OF THE CLEAVAGE OF ALPHA-D-GLUCOPYRANOSYL FLUORIDE
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DOI：
10.1021/ja00021a022
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发表时间：
1991-10-09
影响因子：
15
通讯作者：
JENCKS, WP
中科院分区：
文献类型：
--
作者：
BANAIT, NS;JENCKS, WP
在30.0 ℃和mu = 2.0 M（KCl）条件下，在水和氧化氘中，在pH非依赖性区域中，磷酸盐和膦酸盐缓冲液可催化α-D-吡喃葡萄糖基氟化物的水解。 一般碱催化的二价阴离子占大部分的速率增加，但也有显着的一般酸催化的单阴离子。 磷酸盐缓冲液催化的溶剂同位素效应对于二价阴离子为k（H）/k（D）= 1.9，对于一价阴离子为k（H）/k（D）= 2.0。 布朗斯台德斜率很小，一般碱催化的β = 0.06，一般酸催化的α = 0.15;然而，如果包括H3 O+的速率常数，则α = 0.4。 通过L3 O+的催化显示出溶剂同位素效应k（H）/k（D）= 1.4;这不同于k（H）/k（D）全等值至0.5的值，该值预期用于具有底物的平衡质子化的特定酸催化。 LO-的催化作用和L_2O中未催化的溶剂分解反应分别表现出k（D）/k（H）= 1.5和k（D）/k（H）= 1.1的逆溶剂同位素效应。 在没有催化剂的情况下，没有观察到与甲醇的反应，但是在甲醇-水（45：55体积比）中将甲醇加入到α-D-葡萄糖基氟化物中，由磷酸盐缓冲液（50%二价阴离子）催化，并得到作为产物的1-O-甲基-β-吡喃葡萄糖苷。 有人建议，一般碱催化代表一个协调的亲核攻击和质子提取的机制，是强制执行的情况下，氟离子存在下的葡萄糖基阳离子的显着寿命，和一般酸催化发生氢键离开氟离子。 这两种催化机制都是通过阴离子催化剂和过渡态的正电荷之间的静电相互作用来促进的。 这些结果的意义，由糖苷酶的催化机制进行了讨论。
The hydrolysis of alpha-D-glucopyranosyl fluoride is catalyzed by phosphate and phosphonate buffers in the pH-independent region in water and in deuterium oxide at 30.0-degrees-C and mu = 2.0 M (KCl). General-base catalysis by the dianions accounts for most of the rate increase, but there is also significant general-acid catalysis by the monoanions. The solvent isotope effect for catalysis by phosphate buffers is k(H)/k(D) = 1.9 for the dianion and k(H)/k(D) = 2.0 for the monoanion. The Bronsted slopes are small, with beta = 0.06 for catalysis by general bases and alpha = 0.15 for catalysis by general acids; however, alpha = 0.4 if the rate constant for H3O+ is included. Catalysis by L3O+ shows a solvent isotope effect of k(H)/k(D) = 1.4; this differs from the value of k(H)/k(D) congruent-to 0.5 that is expected for specific-acid catalysis with equilibrium protonation of the substrate. Catalysis by LO- and the uncatalyzed solvolysis in L2O show inverse solvent isotope effects of k(D)/k(H) = 1.5 and k(D)/k(H) = 1.1, respectively. No reaction with methanol is observed in the absence of catalysts, but the addition of methanol to alpha-D-glucosyl fluoride in methanol-water (45:55 by volume) is catalyzed by phosphate buffer, 50% dianion, and gives 1-O-methyl-beta-glucopyranoside as the product. It is suggested that the general-base catalysis represents a concerted mechanism of nucleophilic attack and proton abstraction that is enforced by the absence of a significant lifetime for the glucosyl cation in the presence of fluoride ion, and that general-acid catalysis occurs by hydrogen bonding to the leaving fluoride ion. Both mechanisms of catalysis are facilitated by an electrostatic interaction between the anionic catalyst and the developing positive charge of the transition state. The significance of these results for the mechanism of catalysis by glycosidases is discussed.