Figure shows the quasi-linear reaction asymmetry of both the single and double proton transfer reactions. The reaction asymmetry depicts the variation in the stability of both the single and double proton transfer products in relation to the canonical state and one observes that the canonical state is the most stable. Figure shows that the base pair structure at each increment has access to a single proton transfer minimum but this is not the case for the double proton transfer which can only begin to obtain stability of the double proton transfer tautomer at ≥0.444 Å strand separation (the third imposed strand separation increment). The reaction asymmetry defines the difference between the energy of the product (tautomeric state) and the energy of the reactant (canonical state). For all data points, there is a single proton transfer product and a, beginning at the third data point, there is a double proton transfer product. (c) The reaction energy barrier between the forwards and backwards tautomerisation reaction as a function of separation distance for both single proton transfer and double proton transfer tautomerisation. The two proton transfer events along bonds B1 and B2 are asynchronous.