The structure of the tetramer d(CpGpCpG) has been solved by x-ray analysis in two different crystal forms with and without spermine cations. The molecules crystallize in hexagonal unit cells and they form a left-handed double helix of Z-DNA similar to that previously reported for the hexamer d(CpGpCpGpCpG). In the crystal lattice the molecules stack together to form a virtually continuous left-handed double helix in which every fourth phosphate group is missing. The stacking of bases upon each other is similar to that seen in the hexamer. However, the base pairs have a slightly different orientation in that the cytosine residues are slightly removed from the axis of the molecule compared to the position they occupy in the hexamer. The structures are similar in two crystal forms with and without spermine cations.
Crawford et al. (1980) studied this question.