Tautomerism of Uracil and Thymine

Like cytosine, uracil (or thymine, its 5-methyl derivative) can exist in six tautomeric forms 27-32. A large amount of experimental evidence shows that uracil and thymine have the diketo (dilactam) 

Mizuno, N. Nakanishi, T. Fujiwara, K. Tomita, T. Tsukihara, T. Ashida, andM. Kakudo, Biochem. Biophys. Res. Commun. 41, 1161 (1970). 

Sobell, in The Purines—Theory and Experiment (E. D. Bergmann and B. Pullman, eds.), p. 124. Israel Acad. Sci. Humanities, Jerusalem (distributed by Academic Press, New York), 1972. 

Regarding the ionized forms of uracil (or thymine), Sinsheimer et al,327 suggested that uridylic and thymidylic acids ionized at the C-4 and C-2 

Saenger, and J. Hobbs, Biochim. Biophys. Acta 259,157 (1972). 

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