Evidence of 2-aminopurine-cytosine base mispairs involving two hydrogen bonds

It is shown that the mutagen base analogue 2-aminopurine is hydrogen-bonded at its 1-ring position when annealed with cytosine in DNA. The presence of stably hydrogen-bonded regions proximal to the 2-aminopurine-cytosine base mispair is a prerequisite for the occurrence of two hydrogen bonds coupling the bases at their 1-3- and 2-2-positions. We consider the possibility that, in the resulting heteroduplex base mispair, 2-aminopurine or cytosine may be present as a disfavored imino tautomer.