Far-infrared spectroscopy on oriented films of dry and hydrated DNA

Far-infrared measurements, between 3 and 450 ${\mathrm{cm}}^{\mathrm{-}1}$, of absorption spectra of highly oriented films of Li-DNA and Na-DNA in the temperature range 5–300 K are reported. Five low-frequency infrared-active vibrational modes are observed. The lowest infrared-active mode at 45 ${\mathrm{cm}}^{\mathrm{-}1}$ for Li-DNA and 41 ${\mathrm{cm}}^{\mathrm{-}1}$ for Na-DNA, as observed for the first time, is found to soften upon sample hydration. Studies of the hydration-induced absorption at the low-frequency end of the spectrum also show a pronounced absorption band at about 10 ${\mathrm{cm}}^{\mathrm{-}1}$ that is attributed to relaxation processes. Based on a simple lattice-dynamics model, the description of low-lying vibrational modes of DNA is presented. Eigenvectors of the lowest infrared-active and Raman-active modes are calculated. In addition, the model semiquantitatively describes the influence of hydration on mode frequencies.