Base-pair induced shifts in the tautomeric equilibrium of a modified DNA base

We have examined the base-pairing properties of N ⁴ -methoxycytosine (mo ⁴ C), a mutagenic base analog, in DNA by nuclear magnetic resonance spectroscopy. Unlike standard bases, the tautomeric equilibrium of mo ⁴ C could be strongly influenced by base-pair formation. Paired with A, mo ⁴ C is found predominantly in the imino configuration in Watson-Crick geometry. However, when paired with G, two structurally distinct configurations are observed in equilibrium with one another. In one configuration, mo ⁴ C is in the amino form paired with G in Watson-Crick geometry. In the second species, mo ⁴ C is in the imino configuration paired with G in a wobble geometry. This is the first demonstration of base-pair induced tautomeric shifts in DNA and supports the hypothesis that rare tautomeric forms may be involved in mutagenesis.