Opening Image: Van der Waals surface of d(GCGAATTCG)2. The model shows seven magnesium ions and one chloride ion, and 152 ordered water molecules. Although many of the hydrogen atoms of the water molecules cannot be located in the electron density map, the DNA hydrogens have been located in the model.
At 0.9-angstrom resolution, this model of a self-complementary DNA is the best crystallographic structure of the right-handed B-DNA double helix to date; we'll return to this DNA molecule in a later tutorial on the role of water in protein-nucleic acid interactions. However, our focus here is the flexibility of the sugar-phosphate backbone, with an emphasis on the structure and function of aptamers.