A multinuclear NMR study of the restricted rotation in a bi-imidazole nucleoside

Ethenoadenosine, a common nucleoside adduct known to be formed by the actio n of several industrial and environmental genotoxic compounds on adenosine, undergoes ring-opening in aqueous NaOH solution with the consequent loss o f one carbon atom (C5). The prototropic equilibrium and protonation of the resultant bi-imidazole nucleoside was studied by H-1, C-13, and N-15 NMR sp ectroscopy and it was found that in the neutral form an intramolecular H-bo nding is in effect between N3" of the outer imidazole ring and the N6 proto ns of the inner imidazole ring, giving rise to degenerate tautomeric forms. In the protonated form it is the pyridine-type nitrogen (N3") of the outer imidazole ring where the proton is predominately located and the reduced i ntramolecular H-bonding results in essentially unrestricted rotation of the imidazole rings with respect to one another.