Mixing ratio-dependent energy transfer from DNA-bound 4′,6-diamidino-2-phenylindole to [Ru(1,10-phenanthroline)₂dipyrido[3,2-a:2′,3′-c]phenazine]²⁺

The binding mode of Δ- and Λ-[Ru(1,10-phenanthroline)₂dipyrido[3,2-a:2′,3′-c]phenazine]²⁺ ([Ru(phen)₂DPPZ]²⁺) to DNA in the presence of 4′,6-diamidino-2-phenylindole (DAPI) at a low and high [DAPI]/[DNA base] ratio (0.02 and 0.20, respectively) was investigated using electric absorption and circular dichroism spectroscopy. The spectral properties of both the Δ- and Λ-[Ru(phen)₂DPPZ]²⁺ were not altered in the presence of DAPI disregarding the [DAPI]/[DNA] ratio, suggesting that the presence of DAPI in the minor groove of DNA does not affect the binding mode of the [Ru(phen)₂DPPZ]²⁺ complex to DNA. The transferring excited energy of DAPI to both Δ- and Λ-[Ru(phen)₂DPPZ]²⁺ occurs through Förster type resonance when they both spontaneously bound to DNA. At a high [DAPI]/[DNA] ratios, an upward bending curve in the Stern-Volmer plot, and a shortening the DAPI fluorescence decay time with increasing [Ru(phen)₂DPPZ]²⁺ concentration were found. These results indicate that the quenching of the DAPI's fluorescence occurs through both the static and dynamic mechanisms. In contrast, the quenching mechanism at a low [DAPI]/[DNA] ratios was found to be purely static. The static quenching constant decreased linearly with respect to the [DAPI]/[DNA] ratio. Decrease in quenching efficiency can be explained by the association constant of [Ru(phen)₂DPPZ]²⁺ to DNA while being within a quenchable distance from a DAPI molecule.