Phosphorane intermediate vs. leaving group stabilization by intramolecular hydrogen bonding in the cleavage of trinucleoside monophosphates: implications for understanding catalysis by the large ribozymes
作者:Tuomas Lönnberg、Maarit Laine
DOI:10.1039/b912042d
日期:——
Hydrolysis of 2′,3′-O-methyleneadenosin-5′-yl 5′-O-methyluridin-2′-yl 5′-O-methyl-2′-trifluoroacetamido-2′-deoxyuridin-3′-yl phosphate (1b) has been followed by HPLC over a wide pH range to study the effects of potential hydrogen bonding interactions of the 2′-trifluoroacetamido function on the rate and product distribution of the reaction. At pH < 2, decomposition of 1b (and its 3′,3′,5′-isomer 1a) is first-order in hydronium-ion concentration and cleavage of the P–O3′ bond of the 2′-trifluoroacetamido-modified nucleoside is slightly favored over cleavage of the P–O5′ bond. Between pH 2 and 4, the overall hydrolysis is pH-independent and the P–O3′ and P–O5′ bonds are cleaved at comparable rates. At pH 5, the reaction becomes first-order in hydroxide-ion concentration, with P–O3′ bond cleavage predominating. At 10 mmol L−1 aqueous sodium hydroxide, no P–O5′ bond cleavage is observed. Compared to the 2′-OH counterpart 2, a modest rate enhancement is observed over the entire pH range studied. The absence of P–O5′ fission under alkaline conditions suggests hydrogen bond stabilization of the departing 3′-oxyanion by the neighboring 2′-trifluoroacetamido function.
2',3'-O-亚甲基腺苷-5'-基 5'-O-甲基尿苷-2'-基 5'-O-甲基-2'-三氟乙酰氨基-2'-脱氧尿苷-3'-基磷酸酯的水解 ( 1b) 在较宽的 pH 范围内采用 HPLC 进行跟踪,以研究 2'-三氟乙酰胺基官能团的潜在氢键相互作用对反应速率和产物分布的影响。在 pH < 2 时,1b(及其 3',3',5'-异构体 1a)的分解在水合氢离子浓度和 2'-三氟乙酰胺修饰核苷的 P–O3' 键裂解方面呈一级分解比 P-O5' 键的裂解稍微有利。在 pH 2 和 4 之间,整体水解与 pH 无关,并且 P-O3' 和 P-O5' 键以相当的速率裂解。在 pH 5 时,氢氧根离子浓度为一级反应,其中 P-O3′ 键断裂占主导地位。在 10 mmol L−1 氢氧化钠水溶液中,没有观察到 P–O5' 键断裂。与 2'-OH 对应物 2 相比,在研究的整个 pH 范围内观察到速率适度增强。碱性条件下不存在 P-O5' 裂变,表明离去的 3'-氧阴离子通过邻近的 2'-三氟乙酰胺基官能团实现氢键稳定。