Hydrogen-deuterium exchange in nucleosides and nucleotides. A mechanism for exchange of the exocyclic amino hydrogens of adenosine.

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.