Vibrational modes of a DNA polymer at low temperature

The vibrational modes of a DNA homopolymer poly(dG)-poly(dC), where dG refers to repeating guanine bases on one strand and dC the repeating cytosine bases on the other strand, are studied at low temperature (below 77 K) by self-consistent phonon theory. We find that the modes at 84.3 and 123.9 ${\mathrm{cm}}^{\mathrm{-}1}$ at room temperature are shifted to 91.8 and 133.2 ${\mathrm{cm}}^{\mathrm{-}1}$ at low temperature. We believe these modes are important for the strand-separation melting of the DNA helix. Other modes are relatively unaffected by the same temperature change. These results are in agreement with recent infrared transmission experiments at 77 K in poly(dA)-poly(dT), where A and T stand for the adenine and thymine homopolymers.