Low‐frequency Raman scattering by anharmonic DNA modes with A⇌B character modeled with a classical double‐well oscillator

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Abstract
The optical modes of DNA that displace one strand against the other are modeled by the motion of an oscillator in an asymmetric quartic double‐well potential whose minima represent the A‐ and B‐conformations. Assuming that the variation of the polarizability during vibration derives mainly from the tilting of the base rings relative to the helix axis, the total polarizability tensor is shown to possess approximately ellipsoidal symmetry and to depend nonlinearly on the instantaneous displacement of the two strands. The Raman spectrum of a collection of randomly oriented molecules is calculated. It consists of one or more peaks with characteristic shape. The depolarization ratio is 3/4, independent of molecular conformation and frequency. The results are discussed in the light of existing experimental and theoretical information.