Figure 6.

Stick drawings of dinucleotide steps favouring cross-strand and intra-strand hydrogen bonds in RY steps. a) AC/GU step with N-H..N intra-strand and N-H..O cross-strand hydrogen bonds is shown. The distance between 3′-Cyt-N4 and 5′-Ade-N6, as well as 5′-Ade-N6 and 5′-Gua-O6 are marked. b) AU/AU step, showing an N-H..N cross-strand hydrogen bond between the N6 groups of both 5′-Ade. Other details are as in Figure  5. See Additional file 2 for details of the structures selected and hydrogen bond parameters.