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The dynamics of the DNA hydration shell at gigahertz frequencies
Author(s) -
Tao N. J.,
Lindsay S. M.,
Rupprecht A.
Publication year - 1987
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.360260202
Subject(s) - chemistry , relaxation (psychology) , phonon , solvation shell , arrhenius equation , anisotropy , softening , activation energy , condensed matter physics , molecular physics , analytical chemistry (journal) , nuclear magnetic resonance , optics , materials science , solvation , composite material , ion , physics , psychology , social psychology , organic chemistry , chromatography
Abstract We have used Brillouin scattering to measure the linewidths and frequencies of GHz acoustic phonons in Na‐ and Li‐DNA films as a function of temperature between 300 and 140 K for samples that were dry, lightly, and heavily hydrated. The linewidths decrease with falling temperature and water contents, indicating that coupling to a water relaxation is the main source of phonon damping. The strength of the relaxation was determined using measurements of the phonon linewidth as a function of frequency, and confirmed by comparison of measured and calculated spectral profiles. The relaxation strength is anisotropic, being greater for phonons propagating perpendicular to the helix axis. The hydrated DNA exhibits both a rapid relaxation (≤ 10 −11 s per radian) giving rise to a classical f 2 damping, and a slower motion with a relaxation time that varies from ∼ 4 × 10 −11 s per radian (primary hydration shell) to ∼ 2 × 10 −12 s per radian (secondary hydration shell) at room temperature. In the frequency interval that bounds these relaxation times (∼ 4 to 80 GHz) we expect degrees of freedom associated with the primary hydration shell to be important. The sample with primary hydration follows a simple Arrhenius behavior with Δ H ∼ 5 kcal mole −1 . The effective activation energy for the sample with secondary hydration is somewhat higher (indicating a more cooperative water relaxation) and varies strongly with temperature. The elastic moduli change much more than can be accounted for by relaxation, indicating the importance of water motion in softening interatomic potentials. The extent of the softening caused by the “unfreezing” of water motion is similar to the degree of softening caused by hydrating the sample.