Enantioselective DNA binding of iron(II) complexes of methyl-substituted phenanthroline

The enantioselective binding of [Fe(4,7-dmp)3]2+ (dmp: 4,7-dimethyl-1,10-phenantroline) and [Fe(3,4,7,8-tmp)3]2+ (tmp: 3,4,7,8-tetramethyl-1,10-phenanthroline) to calf-thymus DNA (ct-DNA) has been systematically studied by monitoring the circular dichroism (CD) spectral profile of the iron(II) complexes in the absence and presence of ct-DNA. The effect of salt concentration and temperature on the degree of enantioselectivity of the ct-DNA binding of the iron(II) complexes, i.e. the molar ratio of Δ- to Λ-enantiomer in the solution or vice versa has been rigorously evaluated. It is noticeable that Δ-[Fe(4,7-dmp)3]2+ and Λ-[Fe(3,4,7,8-tmp)3]2+ are preferentially bound to ct-DNA as reflected in their opposite CD spectral profiles. The preferential binding of the Λ-enantiomer of [Fe(3,4,7,8-tmp)3]2+ to ct-DNA compared to that of the Δ-enantiomer is associated with the bulkiness of the ancillary ligands due to substitution of four hydrogen atoms in 1,10-phenanthroline for four methyl groups. The determination of enantiomeric inversion constant (Kinv) at various salt concentrations has revealed that the degree of enantioselectivity is salt concentration dependent, indicating that electrostatic interaction is involved in the enantioselective binding of the iron(II) complexes to ct-DNA. Although [Fe(4,7-dmp)3]2+ and [Fe(3,4,7,8-tmp)3]2+ exhibit an opposite pattern in the CD spectra, the degree of their enantioselectivity (Kinv) is not different from each other significantly. A thermodynamic study on the enantioselective binding of [Fe(4,7-dmp)3]2+ to ct-DNA using the van't Hoff plot of ln Kinv versus 1/T has demonstrated that the enthalpy change (ΔH°) in the inversion process from the Λ- to Δ-enantiomer of [Fe(4,7-dmp)3]2+ ct-DNA is positive, indicating that the process is endothermic and thus entropically driven.