  The approach for calculation of the mode intensities of DNA conformational
vibrations in the Raman spectra is developed. It is based on the valence-optic
theory and the model for description of conformational vibrations of DNA with
counterions. The calculations for Na- and Cs-DNA low-frequency Raman spectra
show that the vibrations of DNA backbone chains near 15 cm-1 have the greatest
intensity. In the spectrum of Na-DNA at frequency range upper than 40 cm-1 the
modes of H-bond stretching in base pairs have the greatest intensities, while
the modes of ion-phosphate vibrations have the lowest intensity. In Cs-DNA
spectra at this frequency range the mode of ion-phosphate vibrations is
prominent. Its intensity is much higher than the intensities of Na-DNA modes of
this spectra range. Other modes of Cs-DNA have much lower intensities than in
the case of Na-DNA. The comparison of our calculations with the experimental
data shows that developed approach gives the understanding of the sensitivity
of DNA low-frequency Raman bands to the neutralization of the double helix by
light and heavy counterions.
