The binding site of Δ- and Λ-[Ru(phenanthroline) 2L]2+ (L being phenanthroline (phen), dipyrido[3,2-a:2′3′-c] phenazine (DPPZ), and benzodipyrido[3,2-a:2′3′-c]phenazine (benzoDPPZ)), bound to poly[d(A-T)2] in the presence and absence of 4′ ,6-diamidino-2-phenylindole (DAPI) was investigated by circular dichroism and fluorescence techniques. DAPI binds at the minor groove of poly[d(A-T) 2] and blocks the groove. The circular dichroism spectrum of all Ru(II) complexes are essentially unaffected whether the minor groove of poly[d(A-T)2] is blocked by DAPI or not, indicating that the Ru(II) complexes are intercalated from the major groove. When DAPI and Ru(II) complexes simultaneously bound to poly[d(A-T)2], the fluorescence intensity of DAPI decreases upon increasing Ru(II) complex concentrations. The energy of DAPI at excited state transfers to Ru(II) complexes across the DNA via the Förster type resonance energy transfer. The efficiency of the energy transfer is similar for both [Ru(phen)2DPPZ]2+ and [Ru(phen)2benzoDPPZ]2+ complexes, whereas that of [Ru(phen)3]2+ is significantly lower. The distance between DAPI and [Ru(phen)3]2+ is estimated as 0.38 and 0.64 Förster distance, respectively, for the Δ- and Λ-isomer.
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