[Ru(phen)₂DPPZ]²⁺ is in contact with DNA bases when it forms a luminescent complex with single-stranded oligonucleotides

The luminescence intensity of the Δ- and Λ-enantiomer of [Ru(phen)₂DPPZ]²⁺ ([Ru(phenanthroline)₂ dipyrido[3,2-a:2′,3′-c]phenazine]²⁺) complex enhanced upon binding to double stranded DNA, which has been known as "light switch effect". The enhancement of the luminescence required the intercalation of the large ligand between DNA base pairs. In this study, we report the enhancement in the luminescence intensity when the metal complexes bind to single stranded oligonucleotides, indicating that the "light switch effect" does not require intercalation of the large DPPZ ligand. Oligonucleotides may provide a hydrophobic cavity for the [Ru(phen) ₂DPPZ]²⁺ complex to prevent the quenching by the water molecule. In the cavity, the metal complex is in contact with DNA bases as is evidenced by the observation that the excited energy of the DNA bases transfer to the bound metal complex. However, the contact of the metal complex with DNA bases is different from the stacking of DPPZ in the intercalation pocket. In addition to the normal two luminescence lifetimes, a short lifetime in the range of 1-2 ns was found for both the Δ- and Λ-enantiomer of [Ru(phen)₂DPPZ]²⁺ when complexed with single stranded oligonucleotides, which may be assigned to the metal complex that is outside of the cavity, interacting with phosphate groups of DNA.