Intensities of DNA ion-phosphate modes in the low-frequency Raman spectra

The Raman intensities of counterion vibrations with respect to the phosphate groups of the double-helix backbone (ion-phosphate modes) in the low-frequency spectra (< 200 cm^-1) of B -DNA with different alkali metal counterions have been calculated using the model for DNA conformational vibrations and the valence-optic approach. The results have showed that the spectra of DNA with heavy counterions ( Rb⁺ and Cs⁺ differ from the spectra of DNA with light counterions ( Na⁺ and K⁺ . The calculated spectra of DNA with heavy counterions are characterized by intensive modes of ion-phosphate vibrations that form one united band near 115cm^-1. Ion-phosphate modes in the spectra of DNA with light counterions are characterized by higher frequencies (near 180cm^-1) and much lower intensity. Our calculations explain why the modes of ion-phosphate vibrations are observed in Cs-DNA spectra rather than in Na-DNA. The determined sensitivity of the intensities of DNA low-frequency spectra to the counterion type proves the existence of the ion-phosphate modes.