In Figure and Figure , the molecular structures of a subset of the strand separation mechanism are presented. One can observe the gradual increase in θ complementing the bond stretching patterns visible in Figure and 2b. Two key features that appear in Figure and Figure are the last image of the Watson-Crick sequence and the first image of the tautomeric sequence. In the last Watson-Crick image (at 2.0 Å separation), the angle of base separation has reverted to θ = 0 -the bond stretching is no longer asymmetric. This reversion to roughly the equilibrium angle is observed in Figure . The first tautomeric image represents only a single proton transfer, hence the product is a zwitterionic base pair.