Comparative studies of high resolution Z-DNA crystal structures. Part 1: Common hydration patterns of alternating dC-dG.

The water structure in three crystal forms of the left-handed Z-DNA hexamer [d(CGCGCG)]2 has been analyzed. Several common motifs have been found in the first hydration shells. On the convex surface, the major groove of the left-handed conformation, water molecules bridge the guanine O-6 keto groups at GpC steps. Cytosine N-4 nitrogens of opposite strands are hydrated by tandem water molecules. At the bottom of the minor groove, a string of water molecules connects the cytosine O-2 keto groups. Across the minor groove guanine N-2 nitrogens are bridged to phosphate oxygens of cytosine and guanine residues by one or two water molecules. In contrast to the very regular geometry of the water structure around the bases, the arrangement of water molecules between phosphate groups appears to be less ordered. However, there is a strong correlation between the interphosphate distances and the number of water molecules or ions which link the phosphate groups. In all three structures various ions, such as sodium and magnesium ions, as well as the protonated amino and imino groups of the polycation spermine displace and replace water molecules in the first hydration shell. Nevertheless, the analysis reveals that numerous first hydration shell water molecules in Z-DNA crystals can be regarded as part of the DNA structure. Their positions and thermal parameters are generally independent of changes in the local crystallographic environment.