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What tautomers base pair with what bases?

I know that adenine pairs with thymine and guanine pairs with cytosine normally. My question is what does the imino form of adenine base pair with? Regular cytosine? What does the enol form of guanine base pair with? Regular form of thymine?

I cannot find any books discussing this issue and Google is of no use. However, my professor wants us to know how tautomers base pair.

I tried base pairing cytosine with adenine but does the -NH2 group on cytosine really hydrogen bond with the imino -NH group on the adenine tautomer?

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The tautomers are rare but they can form and it is suggested that tautomerization can lead to mutations because of non-cognate base pairing.

Khuu & Ho (2009), have inferred the presence of adenine and thymine tautomers from the crystal structure of an in-vitro assembled holliday junction. They infer that imino-Adenine base pairs with amino-cytosine (normal form) and enol-thymine base pairs with keto-guanine (normal form).

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Reproduced from: Khuu & Ho (2009)


Khuu, P., & Ho, P. S. (2009). A rare nucleotide base tautomer in the structure of an asymmetric DNA junction. Biochemistry, 48(33), 7824-7832.

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