Sequence dependence of low-frequency Raman-active modes in nucleic acids

We have measured the low-frequency (<200 ${\mathrm{cm}}^{\mathrm{-}1}$) Raman activity of dehydrated fibers and films of polynucleotides and random-sequenced nucleic acids. The spectra exhibit a pronounced, unresolved band in the range 10–150 ${\mathrm{cm}}^{\mathrm{-}1}$. A nonlinear least-squares algorithm optimally fits the spectra from all samples with three low-frequency modes in the range 25–100 ${\mathrm{cm}}^{\mathrm{-}1}$ and a fourth mode near 200 ${\mathrm{cm}}^{\mathrm{-}1}$. A mode near 25 ${\mathrm{cm}}^{\mathrm{-}1}$ is sequence independent, but does shift to marginally higher frequency in RNA. The mode frequencies and oscillator strengths of two modes in the range 35–100 ${\mathrm{cm}}^{\mathrm{-}1}$ exhibit sequence dependence.