Cross D G, Brown A, Fisher H F
Biochemistry. 1975 Jun 17;14(12):2745-9. doi: 10.1021/bi00683a029.
The pH dependence of the apparent first-order rate constant for the exchange of the exocyclic amino hydrogens of adenosine with deuterium from the solvent was measured by stopped-flow ultraviolet spectroscopy. This dependence shows acid catalysis, base catalysis, and spontaneous exchange at neutral pH values. A study of the effect of several buffers on the rates of exchange reveals both general acid and general base catalytic behavior for the exchange process. We propose a general mechanism for the exchange which requires N-1 protonated adenosine as an intermediate for the acid-catalyzed exchange and amidine anion for the base-catalyzed exchange. In both cases the rate-limiting step is the base-catalyzed abstraction of a proton from the exocyclic amino moiety. Evaluation of the rate constants predicts the equilibrium for the exocyclic amino/imino tautomers to be 6.3 times 10(3):1.
通过停流紫外光谱法测量了腺苷环外氨基氢与溶剂中氘交换的表观一级速率常数对pH的依赖性。这种依赖性显示了酸催化、碱催化以及在中性pH值下的自发交换。对几种缓冲剂对交换速率影响的研究揭示了交换过程中的一般酸催化和一般碱催化行为。我们提出了一种交换的一般机制，该机制要求N-1质子化的腺苷作为酸催化交换的中间体，以及脒阴离子作为碱催化交换的中间体。在这两种情况下，限速步骤都是碱催化从环外氨基部分夺取质子。速率常数的评估预测环外氨基/亚氨基互变异构体的平衡为6.3×10³:1。
