On the existence of the H3 tautomer of adenine in aqueous solution. Rationalizations based on hybrid quantum mechanics/molecular mechanics predictions.

The (15)N NMR spectrum of adenine in aqueous solution has been modeled using high-level combined density functional theory/molecular mechanics techniques coupled to a dynamical averaging scheme. The explicit consideration of the three lowest-energy tautomers of adenine-H9, H7 and H3-allows for a well-founded comparison to experimental data. Based on a very good agreement between the predicted and measured (15)N NMR spectrum of adenine, we have estimated the populations of the H9 and H7 tautomers to be around 83 and 17%, respectively-in good agreement with experimental data-and thus concluded that the H3 tautomer is likely not to be formed in aqueous solution. In addition to the (15)N NMR spectrum we have also predicted the (13)C NMR spectrum and (2)J((15)N-H) indirect spin-spin coupling constants. Finally we have calculated and rationalized the general shape of the low-resolution UV absorption spectrum of adenine in water.