title: Hydrogen-deuterium exchange in nucleosides and nucleotides. A mechanism for exchange of the exocyclic amino hydrogens of adenosine.
abstract: 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.
mesh_terms: D000241:Adenosine; D000249:Adenosine Monophosphate; D000588:Amines; D001665:Binding Sites; D003903:Deuterium; D004151:Dipeptides; D005998:Glycine; D006859:Hydrogen; D006863:Hydrogen-Ion Concentration; D007093:Imidazoles; D007097:Imines; D007700:Kinetics; D008433:Mathematics; D008968:Molecular Conformation; D013056:Spectrophotometry, Ultraviolet; D013696:Temperature; D013816:Thermodynamics; D013997:Time Factors
chemical_list: D000588:Amines; D004151:Dipeptides; D007093:Imidazoles; D007097:Imines; D000249:Adenosine Monophosphate; D006859:Hydrogen; D003903:Deuterium; D000241:Adenosine; D005998:Glycine