Simultaneous Binding of Ruthenium(II) [(1,10-Phenanthroline) ₂dipyridophenazine]²⁺ and Minor Groove Binder 4′,6-Diamidino-2-Phenylindole to Poly[d(A-T)₂] at High Binding Densities: Observation of Fluorescence Resonance Energy Trasfer Across the DNA Stem

Photophysical properties of Δ- and Λ-[Ru(phenanthroline) ₂dipyrido[3,2-a:2′,3′-c]phenazine]²⁺ ([Ru(phen)₂DPPZ]²⁺) bound to poly[d(A-T)₂] in the presence and absence of 4′,6-diamidino-2-phenylindole (DAPI) has been studied. In the presence of DAPI, the minor groove of poly[d(A-T)₂] is blocked, allowing comparison of the binding mode of the ruthenium complexes. Absorption and circular dichroism (CD) as well as flow linear dichroism (LD) of the Λ-enantiomer are essentially unaffected when the minor groove is blocked by DAPI or not. However, with the Δ-enantiomer LD suggests a small but distinct change in binding geometry for both the DAPI and the ruthenium complex when DAPI is simultaneously bound to poly[d(A-T)₂]. In all cases, the DPPZ ligand of the metal complex is intercalated between the nucleobases. From these data, it is concluded that the ruthenium complex still binds to DNA with the Ru(phen)₂ moiety of both the δ- and δ-enantiomer located in the major groove when the minor groove is blocked. When DAPI and the ruthenium complex ions are simultaneously bound to poly[d(A-T)₂], the luminescence of DAPI is decreased and that of the ruthenium complex is increased. The intensity ratio of the excitation relative to absorption spectra of the ruthenium complex is similar to the absorption spectrum of DAPI, demonstrating that excitation energy of DAPI is transferred to the ruthenium complex across the stem of DNA.