Structure of d(TGCGCG)·d(CGCGCA) in two crystal forms: effect of sequence and crystal packing in Z-­DNA

The sequence d(TGCGCG)·d(CGCGCA) crystallized in two crystal forms, orthorhombic and hexagonal, in the presence of cobalt hexammine chloride, a known inducer of the left-handed Z-form of DNA. The crystal structures have been solved and refined at 1.71 Å resolution in space group P2₁2₁2₁ and 2.0 Å resolution in space group P6₅. The orthorhombic structure contains one Z-DNA hexamer duplex, while the hexagonal structure contains two hexamer duplexes in the structure. Of the latter, one is situated on a crystallographic sixfold screw axis, leading to disorder. This paper reports the effects of sequence and crystal packing on the structure of Z-­type DNA. The structures lend additional support to the authors' earlier conclusion that a stretch of four C·G base pairs is sufficient to nucleate and define the regular model of the left-handed helix based on the structure of d(CGCGCG)₂.