The molecular structure of the left-handed Z-DNA double helix at 1.0-A atomic resolution. Geometry, conformation, and ionic interactions of d(CGCGCG)

The structure of d(CGCGCG) crystallized in the presence of magnesium and sodium ions alone is compared to that of the spermine form of the molecule. The very high resolution nature of these structure determinations allows the first true examination of an oligonucleotide structure in fine detail. The values of bond distances and angles are compared to those derived from small molecule crystal structures. In addition, the interactions of cations and polyamines with the Z-DNA helix are analyzed. In particular, multiple cationic charges appear to offer enhanced stabilization for the Z-DNA conformation. The location of spermine molecules along the edge of the deep groove and also spanning the entrance to the groove emphasizes the importance of polyamines for stabilizing this left-handed structure. On averaging, we obtained very similar structural parameters for the two different structures with standard deviations generally smaller than the deviations of the crystallographic model from ideal values. This indicates a high degree of accuracy of the two structures, which have been refined using different data and different refinement methods. The derived bond lengths and angles may thus be more representative of this polymeric DNA structure than those derived from mono- and dinucleotide structures at a similar accuracy.