Figure 7 - full size

 

Figure 7.
Fig. 7. Hydrogen bonding in the G and U tetrads. The 2F[o] F[c] maps are calculated at 2 level and shown in blue. Top views are shown in the upper row and side views in the lower row. (a and b) G tetrads. The guanines are hydrogen bonded to each other by clockwise Hoogsteen-like pairing (N1H O4) with near ideal planarity. (c and d) The U6/U12 tetrad. It is formed with a single hydrogen bond between N3 and O4 atoms in a counterclockwise pattern. (e and f) The U24 tetrad is flipped over in a clockwise N3-H---O4 hydrogen-bonding pattern.