Effects of ligand planarity on the interaction of polypyridyl Ru(II) complexes with DNA

Two new polypyridyl ligands containing substituent Br at different positions in the phenyl ring, OBIP {OBIP = 2-(2-bromophenyl)imidazo[4,5-f]-1,10-phenanthroline}, PBIP {PBIP = 2-(4-bromophenyl) imidazo[4,5-f]-1,10-phenanthroline} and their Ru(II) complexes, [Ru(dmp)(2)(OBIP)](2+) 1 and [Ru(dmp)(2)(PBIP)](2+) 2 (dmp = 2,9-dimethyl-1,10-phenanthroline), have been synthesized and characterized. The binding of the two complexes to calf thymus DNA (CT DNA) has been investigated by spectrophotometric methods, viscosity measurements, as well as equilibrium dialysis and circular dichroism spectroscopy. Theoretical calculations for the two complexes were also carried out applying the density functional theory (DFT) method. The structure of complex 1 has been determined by single-crystal X-ray diffraction techniques. The imidazo[4,5-f]-1,10-phenanthroline moiety is not coplanar with the 2-bromophenyl ring, having a dihedral angle of 48.6degrees in the OBIP ligand. The twisted conformation has been further confirmed by theoretical calculations, in which this dihedral angle is 48.2degrees. The theoretical calculations also suggest that the PBIP ligand in complex 2 is essentially planar (dihedral angle is 0.4degrees). The experimental results show that while complex 1 binds to CT DNA via a semi-intercalative mode, complex 2 strongly binds to CT DNA through intercalation. Complex 2 is thus a much better candidate as an enantioselective binder to CT DNA than complex 1. Some experimental regularities or trends have been reasonably explained by the theoretical results. These suggest that the planarity of the intercalated ligand has significant effects on the spectral properties and the DNA-binding behavior of the complexes, and that the DFT method can be used effectively to explain and predict some regularities or trends in the interaction of polypyridyl Ru(II) complexes with DNA.