High-resolution structure of an extended A-tract: [d(CGCAAATTTGCG)]2.

The crystal structure of [d(CGCAAATTTGCG)]2 has been determined to 1.5 A resolution, representing the first high-resolution structure of this DNA fragment. The ion interactions are novel. A spermine molecule replaces a Mg2+ observed in analogous structures. Unlike lower-resolution structures, the minor groove is narrow and the major groove lacks extra Watson-Crick hydrogen bonds. In addition, a monolayer of solvent sites, including a "spine of hydration", is visible in the minor groove. The crystal of [d(CGCAAATTTGCG)]2 was grown from a solution containing spermine, magnesium, and lithium. The conformation recapitulates that of "monovalent-minus" DNA.