Chiral discrimination in the binding of tris(phenanthroline)ruthenium(II) to calf thymus DNA: an electrochemical study

Abstract

The binding of Δ-, Λ-, and rac-[Ru(phen)₃]²⁺ (phen = 1,10-phenanthroline) and Δ-, Λ-, and rac-[Ru(bpy)₃]²⁺ (bpy = 2,2'-bipyridyl) with calf thymus DNA has been examined by cyclic and differential pulse voltammetric techniques to obtain structural insight into the noncovalent binding of the enantiomers to DNA. The insignificant shift in Ru^II/Ru^III peak potentials on the addition of DNA suggests that both the oxidized and reduced forms bind to DNA to the same extent. Interestingly, DNA selectively decreases the peak currents of Δ-[Ru(phen)₃]²⁺ but not those of the Λ-enantiomer; rac-[Ru(phen)₃]²⁺ exhibits an intermediate behavior, thus suggesting that the Δ-form exhibits significant selectivity for B-DNA. The binding constants (K₂₊) and binding site sizes (s) have been determined from the decrease in the peak currents. The binding constant (K₂₊) of Δ-[Ru(phen)₃]²⁺ is on the order of 10⁴ M^-1 which is less than that for proven intercalators. In contrast, the electrochemical behavior of all three forms of [Ru(bpy)₃]²⁺ remains almost unaffected in the presence of DNA, suggesting that the complexes might reside on the hydrophilic coat of the DNA helix.