Ruthenium(ii) complexes of 6-hydroxydipyrido[3,2-a:2′,3′-c]phenazine: self-association, and concentration-dependent acid–base and DNA-binding properties

Abstract

6-Hydroxydipyrido[3,2-a:2′,3′-c]phenazine (hdppz) and its ruthenium(II) complexes of [Ru(bpy)₂(hdppz)]X₂ (bpy = 2,2′-bipyridine and X = ClO₄⁻, PF₆⁻ and Cl⁻) have been synthesized and characterized. The ion association (self aggregation of the complex cation, and ion pairing) behaviors have been studied in both aqueous and nonaqueous (MeCN) media by UV-visible spectrophotometric measurements. The concentration-dependent ground- and excited-state acid–base properties of the perchlorate complex have been studied by pH spectrophotometric titrations. Calf thymus DNA binding properties of the perchlorate complex have been studied by UV-visible and luminescence titrations, steady-state emission quenching by [Fe(CN)₆]⁴⁻, DNA competitive binding with ethidium bromide, DNA melting experiments, and viscosity measurements. The results have shown that the ion association properties are markedly influenced by the counter ions and concentrations with the effects of ClO₄⁻ being the most prominent. The ion association in aqueous solution for the perchlorate complex moderately affects acidity ionization constants of the complex by a difference of over 0.2 pH units, but affects DNA binding constants significantly by a difference of over one order of magnitude, as studied at two [Ru(bpy)₂(hdppz)](ClO₄)₂ concentrations of 4 and 20 μM. [Ru(bpy)₂(hdppz)](ClO₄)₂ was demonstrated to act as a DNA intercalator and a rare “turn-on” type pH emission molecular switch driven by the deprotonation of the hydroxy group over the pH range 6.00–11.50 with an emission enhancement factor of 150.