Comparison of Binding Stoichiometry of [Ru(1,10-phenanthroline)2dipyrido [3,2-a:2',3'-c]phenazine]2+ and its Bis-derivative to DNA

A new bis-Ru(II) complex, in which two [Ru(1,10-phenanthroline)2dipyrido[3,2-a:2',3'-c]phenazine]2+ were tethered by a 1,3-bis(4-pyridyl)propane linker, was synthesized and its binding mode and stoichiometry to DNA was investigated by optical spectroscopy including linear dichroism (LD) and fluorescence intensity measurement. The magnitude of the negatively reduced LD signal of the bis-Ru(II) complex in the dipyrido[3,2-a:2',3'-c]phenazine (DPPZ) ligand absorption region appeared to be similar compared to that in the DNA absorption region, which is considered to be a diagnostic for DPPZ ligand intercalation. The binding stoichiometry measured from its LD magnitude and enhanced fluorescence intensity corresponds to one ligand per three DNA bases, effectively violating the nearest neighbouring site exclusion model for classical DNA intercalation. This observation is in contrast with monomer analogue [Ru(1,10-phenanthroline)2dipyrido[3,2-a:2',3'-c]phenazine]2+, which is saturated at the DPPZ ligand to DNA base ratio of 0.25, or one DPPZ ligand per four nucleobases.