Dynamic coupling between DNA and its primary hydration shell studied by brillouin scattering

We have measured the dispersion of phonon line widths between frequencies of about 2 and 10 GHz in DNA films at relative humidities between 0 and 95%. The results show that the relaxation mode of the primary hydration shell retains its basic characteristics even in samples with very high water content. A modified mode coupling model is used to include both the collective nature of the sound wave and to describe the change in hydration explicitly. It enables us to describe the coupling between the phonons and the water relaxation mode at various water contents, and allows us to extract values for the primary shell relaxation time and coupling constants over the range of hydration studied. The primary shell relaxation time (∼ 40 ps) and coupling parameters remain nearly constant over the entire range of hydration. We have reanalyzed our earlier Brillouin data (taken as a function of temperature) in terms of two relaxation processes (primary plus a secondary shell contribution of about 2 ps at room temperature). This new analysis indicates that both processes follow a simple Arrhenius behavior with activation energies of 5 kcal mole⁻¹ for the primary relaxation and 7 kcal mole⁻¹ for the secondary relaxation. We also observe a rather broad central mode that can be fitted by a Lorentzian, and that may arise from direct (as opposed to coupled‐mode) scattering from the primary relaxation mode.