Figure 7.

Hydrogen bonding in the G and U tetrads. The 2F_o − F_c maps are calculated at 2σ 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.