Inelastic neutron scattering studies of oriented DNA

Abstract

The spectrum of self-correlation of DNA hydrogens has been measured by thermal neutron scattering for two orientations of the helical axis relative to the momentum transfer. Spectra typical for glass formers are observed for energy transfers below 6 meV in the temperature range 200 ≤ T ≤ 300 K. Analysis in terms of coupled oscillatory (phonons) and jump (pseudo-spins) motions results in a spin-phonon coupling energy of ≃ 8 meV and an average jump frequency rising from 10 to 140 μeV in this temperature range. High resolution measurements for 270 K ≤ T ≤ 350 K reveal in addition a much narrower quasielastic component of ≃ 20 μeV which is related to jump distances of ≃ S.4Å. The “glass spectra” are found to be essentially isotropic. Anisotropic contributions to the scattering are identified as being due to compressional waves along the helix and the low lying optical band observed in Raman scattering. Both modes show strong coupling to relaxational modes at the zone center (10-th layer line). It is argued that the observability of the optical band at this position is due to an antiparallel displacement of the base pair centers along the helix direction.