DOI 10.5012/bkcs.2010.31.6.1615 Binding Modes of New Bis-Ru(II) Complexes to DNA: Effect of the Length of the Linker

Bis-[dipyrido[3,2-a:2',3'-c]phenazine)2(1,10-phenanthroline)2Ru2]2+ complexes (bis-Ru(II) complexes) tethered by lin-kers of various lengths were synthesized and their binding properties to DNA investigated by normal absorption and linear dichroism spectra, and fluorescence techniques in this study. Upon binding to DNA, the bis-Ru(II) complex with the longest linker (1,3-bis-(4-pyridyl)-propane), exhibited a negative LDr signal whose intensity was as large as that in the DNA absorption region, followed by a complicate LDr signal in the metal-to-ligand charge transfer region. The lu-minescence intensity of this bis-Ru(II) complex was enhanced. The observed LDr and luminescence results resembled that of the [Ru(1,10-phenanthroline)2dipyrido[3,2-a:2',3'-c]phenazine]2+ complex, whose dipyrido[3,2-a:2',3'-c]phena-zine (dppz) ligand has been known to intercalate between DNA bases. Hence, it is conclusive that both dppz ligands of the bis-Ru(II) complex intercalate. The binding stoichiometry, however, was a single intercalated dppz per ~ 2.3 bases, which violates the “nearest binding site exclusion ” model for intercalation. The length between the two Ru(II) complexes may be barely long enough to accommodate one DNA base between the two dppz ligands, but not for two DNA bases. When the linker was shorter (4,4'-bipyridine or 1,2-bis-(4-pyridyl)-ethane), the magnitude of the LD in the dppz absorp-tion region, as well as the luminescence intensity of both bis-Ru(II) complexes, was half that of the bis-Ru(II) complex bearing a long linker. This observation can be elucidated by a model whereby one of the dppz ligands intercalates whil