High regularity of Z-DNA revealed by ultra high-resolution crystal structure at 0.55

The crystal structure of a Z-DNA hexamer duplex d(CGCGCG)2 determined at ultra high resolution of 0.55 A ̊ and refined without restraints, displays a high degree of regularity and rigidity in its stereochemis-try, in contrast to the more flexible B-DNA duplexes. The estimations of standard uncertainties of all individually refined parameters, obtained by full-matrix least-squares optimization, are comparable with values that are typical for small-molecule crys-tallography. The Z-DNA model generated with ultra high-resolution diffraction data can be used to revise the stereochemical restraints applied in lower reso-lution refinements. Detailed comparisons of the stereochemical library values with the present accur-ate Z-DNA parameters, shows in general a good agreement, but also reveals significant discrepancies in the description of guanine-sugar valence angles and in the geometry of the phosphate groups