AFM-correlated CSM-coupled Raman and fluorescence properties of water-soluble oxo-titanium (IV) porphyrin bound with DNA

The Raman and fluorescence spectroscopic properties of water-soluble oxo-titanium (IV) meso-tetrakis (1-methyl pyridium-4-yl) porphyrin (OTi(TMPyP)4+) bound with calf thymus DNA and artificial DNAs such as double stranded poly[d(A-T)2] and poly[d(G-C)2] have been investigated on the single DNA molecule basis by atomic force microscopy (AFM)-correlated confocal scanning microscope (CSM)-coupled Raman and fluorescence spectroscopic techniques as well as the ensemble-averaged spectroscopy. The ensemble-averaged spectroscopic studies imply that the porphyrin interacts with DNA in different groove binding patterns depending on the base pairs. AFM-images of the different DNAs bound with OTi(TMPyP)4+ were measured, and their morphologies are found to depend on kind of base pairs interacting with OTi(TMPyP)4+. Being correlated with the AFM images, the CSM-coupled Raman and fluorescence spectral properties of the three different single OTi(TMPyP)4+–DNA complexes were observed to be highly resolved and sensitive to base pair-dependent axial ligation of Ti–O bond as compared to the corresponding ensemble-averaged spectral properties, which affect the groove binding and its strength of the OTi(TMPyP)4+ with DNA. The axial ligation was found to be accompanied by vibration structural change of the porphyrin ring, leading to keep the shape of double stranded poly[d(A-T)2] rigid while poly[d(G-C)2] and calf thymus DNA flexible after binding with the oxo-titanyl porphyrin. The base pair dependence of the fluorescence decay times of the DNA-bound porphyrins was also observed, implying that an excited-state charge transfer takes place in the G-C rich major groove in calf thymus DNA. These results suggest that binding of OTi(TMPyP)4+ is more preferential with the G-C rich major groove than with the A-T rich minor groove in calf thymus DNA so that the morphology of DNA is changed.