Raman spectroscopic studies on interactions of water soluble cationic oxovanadyl (IV) meso-tetrakis(1-methylpyridium-4-yl) porphyrin with nucleic acids

Interactions of water soluble cationic oxovanadyl (IV) meso-tetrakis(1-methylpyridium-4-yl)porphyrin (OVIV(TMPyP)4+) with double stranded poly[d(A-T)2], poly[d(G-C)2] nucleotides and calf thymus DNA were studied by the Raman spectroscopic techniques as well as the polarization spectroscopic measurements. The ground state Raman bands of OVIV(TMPyP)4+ observed in the presence of poly[d(A-T)2] were almost the same as those observed in a pure water solution except the slightly up-shifted Raman band (963 cm−1) which is ascribed to the VO stretching mode of six-coordinated complex, OVIV(H2O)(TMPyP)4+. On the other hand, in the presence of poly[d(G-C)2], a markedly up-shifted Raman band was observed at 992 cm−1, indicating that OVIV(TMPyP)4+ interacts with poly[d(G-C)2] by losing the sixth axial ligand. In the presence of calf thymus DNA, the VO stretching band was observed to be resulted by combination of those observed in poly[d(A-T)2] and poly[d(G-C)2]. The polarization spectroscopic studies with these results imply that OVIV(TMPyP)4+ interacts with DNA in different groove binding patterns originated from the formation of five-coordinated adducts in the G-C pair-rich regions and six-coordinated adducts in the A-T pair-rich regions. The down-shifts of core-size sensitive transient Raman bands (ν2 and ν4 modes) in the presence of nucleic acids illustrate an increase of the core-size of porphyrin macrocycle in the excited triplet state. The transient Raman bands related to pyridine group were observed, and they are interpreted to be due to the excited-state charge transfer from porphyrin ring to peripheral substituents.