Question

How do you identify the difference between keto/enol and amino/imino forms of nucleobases? Which is favored in what situations?

Answer 1

Standard keto and rare enol forms of thymine and guanine differ by a spontaneous proton shift between the adjacent carbon and nitrogen molecules. The molecular formula does change, only the configuration of bonds among toms: the keto and enol forms are therefore tautomers of each other.

Adenine and cytosine can exist in either amino or imino forms as these tautomeric forms of each base exist in eqilibrium but the amino and lactam tautomers are more stable and therefore predominate under the conditions found inside most cells.