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

The binding mode of Delta- and Lambda-Ru(1,10-phenanthroline)(2)dipyrido3,2-a:2',3'-c]phenazine](2+) (Ru(phen)(2)DPPZ](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 Delta- and Lambda-Ru(phen)(2)DPPZ](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)(2)DPPZ](2+) complex to DNA. The transferring excited energy of DAPI to both Delta- and Lambda-Ru(phen)(2)DPPZ](2+) occurs through F?rster 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)(2)DPPZ](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)(2)DPPZ](2+) to DNA while being within a quenchable distance from a DAPI molecule.