This effect is present in purines and pyrimidines, a fact which initially confused Watson and Crick;
This effect is present in purines and pyrimidines, a fact which initially confused Watson and Crick; for example, adenine can base pair with cytosine if adenine is present as a lactim.  It occurs very rarely in DNA, because hydrogen bonds and the attachment to the sugar prevent access to the acid or base which catalyzes it through protonation or deprotonation.  Due to aromatic stability, it can only be induced in phenols with great difficulty.  This is in sharp contrast to ketones and aldehydes, where contact with a glass surface can catalyze interconversion between keto and enol forms.  The result of a migration of a proton and a double bond, FTP, name this type of isomerism which involves a chemical equilibrium between two forms.
This effect is present in purines and pyrimidines, a fact which initially confused Watson and Crick; for example, adenine can base pair with cytosine if adenine is present as a lactim.  
tautomerism