Effect of Axial Ligand on the Binding Mode of M-meso-Tetrakis(N-methylpyridinium-4-yl)porphyrin to DNA Probed by Circular and Linear Dichroism Spectroscopies

The binding modes of cationic porphyrins, namely M-meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (MTMPyP, where M = free base, Cu(II), Ni(II), Co(III), Mn(III), V(IV)=O, and Ti(IV)=O), to native DNA were systematically investigated by polarized light spectroscopies, viz, circular and linear dichroism spectroscopy (CD and LD, respectively). At low [porphyrin]/[DNA] ratio, planar porphyrins including TMPyP, CuTMPyP, and NiTMPyP exhibited a negative CD signal in the Soret region and large negative wavelength-dependent LDr signal which is characteristics of their intercalative binding mode. In the intercalation pocket, the molecular plane of porphyrin is skewed to a large extent as it was judged from a large wavelength-dependency of the LDr magnitude in the Soret region. As the mixing ratio was increased, a bisignate CD spectrum became apparent for all of the planar porphyrins, while the shape of LDr remained the same, indicating the coupling of the electric transition moments of the intercalated porphyrins. Thus, coupling can occur between the porphyrins when they are separated at least two DNA base-pairs according to the nearest neighboring site exclusion model. This coupling interaction was the weakest for NiTMPyP. The porphyrins with axial ligands, namely VOTMPyP, TiOTMPyP, MnTMPyP, and CoTMPyP, bind to the exterior of the DNA at a low [porphyrin]/[DNA] ratio, which is indicated by the positive CD signal in the Soret region. The absorption and LD spectra in the Soret region were treated as the sum of two transitions to calculate the angle between each of the electric transition moments of the porphyrin, i.e., the B-x and B-y transitions relative to the local DNA helix axis. The angle of one of the two electric transitions of the porphyrin is in the range of 56 degrees similar to 59 degrees, whereas the other is in the range of 59 degrees similar to 65 degrees. Increasing the porphyrin density resulted in the appearance of an interaction between the bound porphyrins, which was indicated by the distorted CD spectra. In contrast with the planar porphyrins, which are parallel to each other in the intercalation pocket when they interact by themselves, the molecular plane of the porphyrin with the axial ligand tilts more toward the local DNA helix axis as the population of porphyrins was increased. CoTMPyP exhibited complicated CD and LD spectra at high [porphyrin]/[DNA] ratios that were impossible to explain by the presence of two porphyrin species: those with a monomeric external binding and electric coupling. Since the CoTMPyP-DNA complex started to aggregate at relatively low [porphyrin]/[DNA] ratios, the complication may due to the involvement of extensive porphyrin aggregation.