Structure determination of an intercalating ruthenium dipyridophenazine complex which kinks DNA by semiintercalation of a tetraazaphenanthrene ligand

We describe a crystal structure, at atomic resolution (1.1 angstrom, 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 degrees kinking of the double helix. The complex cation Lambda-[Ru(1,4,5,8-tetraazaphenanthrene)(2)(dipyridophenazine)](2+) 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 semi-intercalation of one of the orthogonal tetraazaphenanthrene ligands into a second symmetrically equivalent duplex. The result is noncovalent cross-linking and marked kinking of DNA.