Description of base motion and role of excitations in the melting of poly(dG).poly(dC).

We study the contribution of various vibrational modes to the melting of poly(dG).poly(dC). We find that the principal contribution comes from the H-bond breathing modes that have been observed in Raman scattering and that we have associated with helix melting. We show the softening of these modes on approach to melting in agreement with the observed behavior. We also describe the contribution to melting from base rotation modes that others have suggested are important in melting.