N,N,O and N,O,N Meridional cis Coordination of Two Guanines to Copper(II) by d(CGCGCG)2

Many research groups study the generation of supramolecular n-dimensional arrays by combining metals with DNA building blocks. Most of the time, the natural nucleobases are modified to obtain higher-affinity metal binding sites. Using unmodified nucleobases avoids a potentially difficult synthesis; however, they have the possible disadvantage of a less defined and/or weaker coordination mode of the metal. Structural studies on the behavior of copper­(II) as a linking metal and guanine as the natural ligand for metals in unmodified DNA are reported. Previously, the ability of mono- and dinuclear metal complexes to induce Z-DNA has been explored [Medina-Molner, A.; Spingler, B. Chem. Commun. 2012, 48, 1961; Medina-Molner, A.; Rohner, M.; Pandiarajan, D.; Spingler, B. Dalton Trans. 2015, 44, 3664]. Herein, X-ray crystallographic studies of the structures resulting from the combination of copper­(II) ions with DNA hexamers of the general sequence d­(CG)3 are presented. Three different packing motifs were observed in three crystal structures with resolutions ranging from 2.15 to 1.45 Å. The motifs are dependent upon other cations being present and/or the crystallization conditions. The first examples of intramolecular O6,N7-chelates of a neutral purine nucleobase to copper­(II) were obtained as well as the first meridional N,N,O and N,O,N coordination modes of two guanines to copper­(II). The fascinating coordination chemistry of copper­(II) complexes generated by the Z-DNA oligonucleotides and the differences to simple nucleobases complexes with copper­(II) are discussed in detail.