Role of water in defining the structure and properties of B-form DNA

DNA in the cell is rarely naked but normally protein-bound in nucleosomes. Of special interest is the DNA bound to other factors that control its key functions of transcription, replication, and repair. For these several transactions of DNA, the state of hydration plays an important role in its function, and therefore needs to be defined in as much detail as possible. High-resolution crystallography of short B-form duplexes shows that the mixed polar and apolar surface of the major groove binds water molecules over the broad polar floor of the groove in a sequence-dependent varied manner. In contrast, the narrower minor groove, particularly at AT-rich segments, binds water molecules to the polar groups of the bases in a regular double layer reminiscent of the structure of ice. This review is largely devoted to measurements made in solution, principally calorimetric, that are fully consistent with the location of water molecules seen in crystals, thereby emphasizing the substantial difference between the hydration patterns of the two grooves.