Using DFT calculations, we propose that the double proton transfer tautomer of the adenine-thymine base pair (A*-T*) can exist in an energetically stable state on the condition that the DNA duplex is separated by more than 0.444 Å. As the DNA strands separate further from one another, the A*-T* state becomes more stabilised due to the increase in the energy barriers of the tautomerisation reaction. However, this increasing energy barrier also diminishes the probability of the tautomerisation reaction occurring. Before the separation that stabilises the A*-T* state is reached, only single proton transfer products are stable, but these reactions are not biologically relevant to the spontaneous mutagenesis process.