DNA oligonucleotides can be used in order to assemble highly structured materials. Oligonucleotides with sticky ends can form long linear structures, whereas branching is required to form two- and three-dimensional nanostructures. In this paper, we show that when Ni2+ is attached to the N7 atom of guanine, it can also act as a branching point. Thus, we have found that the heptanucleotide d(GAATTCG) can assemble into long linear duplex structures, which cross in space to generate a cubic structure. The three-dimensional arrays are stabilized by phosphate-Ni2+-guanine interactions. For the first time, the crystallization of a B form DNA oligonucleotide in a cubic system is reported, space group I23. Large solvent cavities are found among the DNA duplexes.