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Looking at the keto and enol forms of guanine, it would seem as if the enol form is more stable, due to the fact that aromaticity is established with the enol form. However, according to a question on Chem.SE here: LINK there is a $\pu{8.7 kcal/mol}$ free energy difference between the enol and keto forms in aqueous solution.

Why is this? Shouldn't the enol form be much more stable due to aromatic stabilization?

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    $\begingroup$ Both structures are aromatic if you consider the keto tautomer’s mesomeric structure of $\ce{\overset{+}{C}-\overset{-}{O}}$. $\endgroup$ – Jan Aug 21 '17 at 6:28
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    $\begingroup$ The structure on the right is indeed a very tempting one, however, @jan is correct that both structures will be aromatic. If you look closer at possible resonance structures, it will become clear immediately. $\endgroup$ – Martin - マーチン Aug 21 '17 at 8:51
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    $\begingroup$ Note that the only difference between these two structures is where the hydrogen is bonded. So, the question then becomes is there a 8.7 kcal/mol difference between the basicity of a nitrogen versus an oxygen? $\endgroup$ – Zhe Aug 22 '17 at 2:01

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