Contiguous Heterobimetallic Arrays of Metal-Mediated Base Pairs

DNA duplexes bearing a central block of six phenanthroline-derived nucleobase analogs (P) with thymine (T) or cytosine (C) in the complementary position were investigated with respect to their ability to act as ligands for the selective binding of Hg2+ and Ag+ ions to give heterobimetallic arrays. In particular, P-Hg2+-T and P-Ag+-C base pairs were anticipated to form, so that predefined arrays of these metal ions should be accessible via suitable design of the oligonucleotide sequence. Binding stoichiometries were derived from thermal denaturation curves in the presence of varying amounts of metal ions and via ESI(-) mass spectrometry. Surprisingly, the data indicate that duplexes cannot contain more than two consecutive P-Hg2+-T base pairs, possibly prevented by a local accumulation of positive charge. When both Hg2+ and Ag+ ions are present, neighboring P-Hg2+-T base pairs are prevented entirely. This is achieved by Ag+ ions binding to all P:C sites and to as many P:T sites as necessary. In conclusion, DNA duplexes can be used to generate heterobimetallic arrays of metal ions by using appropriate metal-mediated base pairs, provided care is taken to prevent the accumulation of excessive positive charge along the helical axis.