Effect of Axial Ligand on the Binding Mode of M-Meso-Tetrakis(N-Methylpyridinium-4-Yl)Porphyrin to DNA and their Efficiency as an Acceptor in DNA-Mediated Energy Transfer

Abstract

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. At low [porphyrin]/[DNA] ratio, planar porphyrins including TMPyP, CuTMPyP and NiTMPyP exhibited intercalative binding mode. In the intercalation pocket, the molecular plane of porphyrin is skewed to a large extent. As the mixing ratio was increased the coupling of the electric transition moments of the intercalated porphyrins was observed. Coupling can occur between the porphyrins when they are separated at least two DNA base. The porphyrins with axial ligands, namely VOTMPyP, TiOTMPyP, MnTMPyP and CoTMPyP bind to the exterior of the DNA at a low [porphyrin]/[DNA] ratio. The angle of one the two electric transitions of the porphyrin is in the range of 56°∼59°, while the other is in the range of 59°∼65°. Increasing the porphyrin density resulted in the appearance of an interaction between the bound porphyrins. All porphyrins were able to quench the fluorescence of intercalated ethidium. The mechanism behind the energy transfer is, at least in part, the Forster type resonance energy transfer (FRET). The minimum distance in base pairs between ethidium and porphyrin required to permit the excited ethidium to emit a photon was the longest for CoTMPyP being 17.6 base-pairs and was the shortest for CuTMPyP and NiTMPyP at 8.0 base-pairs. The variation in the distance was almost proportional to the extent of the spectral overlap, the common area under emission spectrum of ethidium and absorption spectrum of porphyrin, supporting the FRET mechanism, while the effect of the orientation factor which was considered by relative binding geometry was limited.