IR and Raman spectroscopic studies on coulombic interaction between water-soluble porphyrins and nucleic acids

We have shown previously that the N-methylpyridyl group vibrations of water-soluble porphyrins, M(TMpy-P4), are shifted to lower frequencies (0.5-2.6 cm-1) as a result of coulombic interaction between the N(+)-CH3 group of M(TMpy-P4) and the PO2 group of a nucleic acid. We have now turned over our attention to the effect of this coulombic interaction on the PO2 group vibrations of nucleic acids. Using Fourier transform ir and Raman spectroscopy, we found that the va(PO2) at 1221 cm-1 is shifted 12 approximately 17 cm-1 to higher frequencies, whereas the vs(PO2) at 1087 cm-1 is shifted 18 approximately 26 cm-1 to lower frequencies, when DNA is mixed with M(TMpy-P4). These results indicate that the N(+)-CH3 group of M(TMpy-P4) interacts preferentially with one of the two oxygen atoms of the PO2 group of the DNA duplex.