Effects of flexible bridging ligands on DNA-binding of dinuclear ruthenium(II)-2,2′-bipyridine complexes

For reactions of [{RuCl(bpy)₂}₂(μ-BL)]²⁺ (bpy = 2,2′-bipyridine, BL = H₂N(CH₂)_nNH₂ (n = 4-8, 12), [Ru₂-BL]²⁺) with mononucleotides, the MLCT absorption bands of [Ru₂-BL]²⁺ blue-shifted with hyperchromism for GMP and hypochromism for TMP with time. Reactions of [Ru₂-BL]²⁺ with GMP or TMP proceed via initial Cl⁻ ions replacement by coordination to N7 of GMP and N3 of TMP, respectively. In competition binding experiments for [Ru₂-BL]²⁺ with GMP versus TMP, only GMP selectively coordinated to ruthenium(II). For reactions with calf thymus (CT) DNA, [Ru₂-BL]²⁺ complexes selectively bind to guanine residues of DNA. The higher degrees of binding of [Ru₂-BL]²⁺ to CT-DNA were observed with increasing n values for H₂N(CH₂)_nNH₂, which may be explained by the length of the bridging ligands. Studies on the inhibition of the restriction enzyme Acc I revealed that [Ru₂-BL]²⁺ complexes appear to be covalently favorable for the type of difunctional binding. In addition, it is very interesting to observe that circular dichroism spectroscopy of the supernatants obtained following the reactions of CT-DNA with racemic [Ru₂-BL]²⁺ show enrichments of the solutions in the ΔΔ isomers, demonstrating preferences of the ΛΛ isomers for covalent binding to CT-DNA.