Glassy character of DNA hydration water.

The coherent excitations of DNA hydration water at 100 K have been investigated by neutron scattering spectroscopy to extract the excess signal of D(2)O-hydrated DNA with respect to dry DNA samples. A structural characterization of the sample, through the analysis of the static structure factor, has suggested that DNA hydration water is largely in an amorphous state up to high hydration degree, with only a small contribution coming from slightly deformed crystalline ice. To describe the inelastic spectra of DNA hydration water, we exploited a phenomenological model already applied in similar disordered systems, such as bulk water (Sacchetti et al. Phys. Rev. E2004, 69, 061203; Petrillo et al. Phys. Rev. E2000, 62, 3611-3618; Sette et al. Phys. Rev. Lett.1996, 77, 83-86) and protein hydration water (Orecchini et al. J. Am. Chem. Soc.2009, 131, 4664-4669). Over the low-energy range, the coherent dynamics of DNA hydration water is characterized by a branch at about 7.5 meV, a value slightly larger than that of bulk water. An additional mode in the energy range 20-35 meV is found, with a wavevector dependence seemingly connected with the structural features of amorphous ice. The ensemble of the results supports the glassy nature of DNA hydration water.