Ir and Raman spectroscopic studies on coulombic interaction between water‐soluble porphyrins and nucleic acids

We have shown previously that the N‐methylpridyl 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 + ‐CH 3 group of M(TMpy‐P4) and the PO 2 group of a nucleic acid. We have now turned over our attention to the effect of this coulombic interaction on the PO 2 group vibrations of nucleic acids. Using Fourier transform ir and Raman spectroscopy, we found that the ν a (PO 2 ) at 1221 cm −1 is shifted 12 ∼ 17 cm −1 to higher frequencies, whereas the ν s (PO 2 ) at 1087 cm −1 is shifted 18 ∼ 26 cm −1 to lower frequencies, when DNA is mixed with M(TMpy‐P4). These results indicate that the N + ‐CH 3 group of M(TMpy‐P4) interacts preferentially with one of the two oxygen atoms of the PO 2 group of the DNA duplex.