Dear Dr. Myakishev-Rempel,

from your proposed drawings, I would say it is correct to say that tautomers 1a/1b are aromatic and tautomers 2/3 are not aromatic.

However, I don't know, if your key statement (oscillation between the aromatic and non-aromatic tautomer) is true, for the following reason:

While the case is clear if drawn on paper, the reality of electron density distribution and aromatic ring-currents is usually not binary (on/off), but something in-between. Thus, my guess would be that non of the drawn tautomers describe the reality well enough and therefore I would be careful claiming this, unless you have good evidence.

Nonetheless, I don't have solid expertise regarding this particular case and I hope someone else can provide you with solid answers.

All the best

Dear Max,

You can write non-aromatic tautomers for the pyrimidine ring but keeping aromaticity in the imidazolinic ring in the purine, as ii is the case in tautomer 3. Actually, this structure for G is actually the one that form H-bond with C in DNA. The reason is that the energetic change by the loss of aromaticity is compensated by two facts: on one side, the lactam-type bond is quite stable and , in the other hand, the H-bond with C also stabilize this structure.

For the case of adenine, the aromaticity of the pyrimidine ring is kept since the alternative structure with an amidine-like bond is less stable that the cognate lactam.

It is very unlikely that purines are oscilating betwwen aromatic and non-aromatic tautomers.

Bernardo