Thread insertion of a bis(dipyridophenazine) diruthenium complex into the DNA double helix by the extrusion of AT base pairs and cross-linking of DNA duplexes.

The crystal structure of the Δ,Δ enantiomer of the binuclear "light-switch" ruthenium complex [μ-(11,11'-bidppz)(1,10-phenanthroline)4  Ru2 ](4+) bound to the oligonucleotide d(CGTACG) shows that one dppz moiety of the dumbbell-like compound inserts into the DNA stack through the extrusion of an AT base pair. The second dppz moiety recruits a neighboring DNA molecule, and the complex thus cross-links two adjacent duplexes by bridging their major grooves.