Jordan F, Niv H
Nucleic Acids Res. 1977 Mar;4(3):697-709. doi: 10.1093/nar/4.3.697.
The log kobs vs. pH profiles were determined in the intermediate acidity region for the glycosyl hydrolysis of guanosine and its 8-amino, 8-monomethylamino, 8-dimethylamino and 8-bromo derivatives. The decreased rate of the 8-amino and enhanced rate of the 8-bromo compound compared to guanosine support an A type mechanism: base protonation followed by glycosyl bond cleavage. All three 8-amino guanosines exhibited log kobs - pH profiles clearly showing that both mono and di-base protonated nucleosides undergo hydrolysis. The 700 fold rate acceleration of 8-N(CH3)-guanosine compared to 8-NHCH3-guanosine and the 110 fold rate acceleration of 8-N(CH3)2-adenosine compared to 8-NHCH3-adenosine could be unequivocally attributed to the fixed syn glycosyl conformation of both 8-dimethylamino compounds and relief of steric compression upon hydrolysis in these molecules. The lack of anomerization of all substrates during the course of the reaction supports an A rather than a Schiff-base mechanism.
在中等酸度区域测定了鸟苷及其8-氨基、8-单甲基氨基、8-二甲基氨基和8-溴衍生物糖基水解反应的表观一级速率常数（log kobs）与pH的关系曲线。与鸟苷相比，8-氨基化合物水解速率降低，而8-溴化合物水解速率加快，这支持了A型反应机理：碱基质子化后糖基键断裂。所有三种8-氨基鸟苷的log kobs - pH关系曲线清楚地表明，单碱基和双碱基质子化的核苷都会发生水解。与8-NHCH3-鸟苷相比，8-N(CH3)-鸟苷的水解速率加快了700倍；与8-NHCH3-腺苷相比，8-N(CH3)2-腺苷的水解速率加快了110倍，这可以明确归因于两种8-二甲基氨基化合物均具有固定的顺式糖基构象，并且在这些分子水解时空间压缩得到缓解。反应过程中所有底物均未发生异头化，这支持了A型反应机理而非席夫碱机理。
