The ruthenium complex [Ru(phen)(2)(dppz)](2+) (where phen is phenanthroline and dppz dipyridophenazine is known as a 'light switch' complex because its luminescence in solution is significantly enhanced in the presence of DNA. This property is poised to serve in diagnostic and therapeutic applications, but its binding mode with DNA needs to be elucidated further. Here, we describe the crystal structures of the Λ enantiomer bound to two oligonucleotide duplexes. The dppz ligand intercalates symmetrically and perpendicularly from the minor groove of the d(CCGGTACCGG)(2) duplex at the central TA/TA step, but not at the central AT/AT step of d(CCGGATCCGG)(2). In both structures, however, a second ruthenium complex links the duplexes through the combination of a shallower angled intercalation into the C(1)C(2)/G(9)G(10) step at the end of the duplex, and semi-intercalation into the G(3)G(4) step of an adjacent duplex. The TA/TA specificity of the perpendicular intercalation arises from the packing of phenanthroline ligands against the adenosine residue.

We describe a crystal structure, at atomic resolution (1.1 Å, 100 K), of a ruthenium polypyridyl complex bound to duplex DNA, in which one ligand acts as a wedge in the minor groove, resulting in the 51°kinking of the double helix. The complex cation Λ-½Ruð1,4,5,8-tetraazaphenanthreneÞ 2 ðdipyridophenazineÞ 2þ crystallizes in a 1∶1 ratio with the oligonucleotide d(TCGGCGCCGA) in the presence of barium ions. Each complex binds to one duplex by intercalation of the dipyridophenazine ligand and also by semiintercalation of one of the orthogonal tetraazaphenanthrene ligands into a second symmetrically equivalent duplex. The result is noncovalent cross-linking and marked kinking of DNA.DNA oligonucleotide | ruthenium complex | X-ray crystal structure

I-motif stability is enhanced by short loop lengths compared to long loop lengths.

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