Utilizing guanine-coordinated Zn²⁺ ions to determine DNA crystal structures by single-wavelength anomalous diffraction

The experimental phase determination of crystal structures of nucleic acids and nucleic acid–ligand complexes would benefit from a facile method. Even for double-stranded DNA, software-generated models are generally insufficiently accurate to serve as molecular replacement search models, necessitating experimental phasing. Here, it is demonstrated that Zn²⁺ ions coordinated to the N7 atom of guanine bases generate sufficient anomalous signal for single-wavelength anomalous diffraction (SAD) phasing of DNA crystal structures. Using zinc SAD, three crystal structures of double-stranded DNA oligomers, 5′-AGGGATCCCT-3′, 5′-GGGATCCC-3′ and 5′-GAGGCCTC-3′, were determined. By determining the crystal structure of one of these oligomers, GAGGCCTC, in the presence of Mg²⁺ instead of Zn²⁺, it was demonstrated that Zn²⁺ is not structurally perturbing. These structures allowed the analysis of structural changes in the DNA on the binding of analogues of the natural product mithramycin to two of these oligomers, AGGGATCCCT and GAGGCCTC. Zinc SAD may become a routine approach for determining the crystal structures of nucleic acids and their complexes with small molecules.