A Rigid Dinuclear Ruthenium(II) Complex as an Efficient Photoactive Agent for Bridging Two Guanine Bases of a Duplex or Quadruplex Oligonucleotide

The rigid dinuclear [(tap) 2 Ru(tpac)Ru(tap) 2 ] 4+ complex ( 1 ) (TAP=1,4,5,8‐tetraazaphenanthrene, TPAC=tetrapyridoacridine) is shown to be much more efficient than the mononuclear bis‐TAP complexes at photodamaging oligodeoxyribonucleotides (ODNs) containing guanine (G). This is particularly striking with the G‐rich telomeric sequence d(T 2 AG 3 ) 4 . Complex 1 , which interacts strongly with the ODNs as determined by surface plasmon resonance (SPR) and emission anisotropy experiments, gives rise under illumination to the formation of covalent adducts with the G units of the ODNs. The yield of photocrosslinking of the two strands of duplexes by 1 is the highest when the G bases of each strand are separated by three to four base pairs. This corresponds with each Ru(tap) 2 moiety of complex 1 forming an adduct with the G base. This separation distance of the G units of a duplex could be determined thanks to the rigidity of complex 1 . On the basis of results of gel electrophoresis, mass spectrometry, and molecular modelling, it is suggested that such photocrosslinking can also occur intramolecularly in the human telomeric quadruplex d(T 2 AG 3 ) 4 .