Inelastic neutron scattering studies of water in DNA

Neutron vibrational spectroscopy is a powerful tool to study the structure and dynamics of water inside and around biomolecules, because the translational frequencies will change with the local structure of water around DNA/proteins and can be observed in the measured inelastic neutron spectrum with high accuracy even at very low concentrations of water, due to the large cross section of the hydrogen atom. On the other hand, the scattering from DNA and proteins in general is very weak and in a different energy-transfer range. We can observe the vibrations largely due to water-water interactions perturbed by the biomolecules. In this paper, we present a series of neutron-spectroscopy measurements for the structure of water around DNA molecules at different hydration levels in the energy range from 2 to 500 meV at ∼ 20 K using TOSCA and HET spectrometers at ISIS (UK). Spectra at low energy transfers show that the dynamics of water adsorbed on DNA is significantly different from normal bulk water. The different appearance in the translational region below 40meV and the librational region between 50 and 120meV suggests that there are differences in the local structure as well. The OH stretching modes of water at about 400 meV shift to lower energy transfers with increasing hydration level due to the formation of large water clusters and the increase of the degree of hydrogen bonding between water molecules.