Role of hydration water in dynamics of biological macromolecules

We present an overview of neutron scattering studies of the dynamics in hydrated and dry protein and RNA. We demonstrate that the difference observed in the dynamics of dry lysozyme and dry RNA is related to the methyl group contributions to the protein dynamics. Hydration of biological macromolecules leads to significant activation of conformational transitions. They appear in neutron scattering spectra as an additional slow relaxation process that exhibits a strongly stretched spectral shape and slightly non-Arrhenius temperature dependence of the characteristic relaxation time. Our analysis suggests that the appearance of this relaxation process in the experimentally accessible frequency window is the main cause for the sharp rise of the atomic mean-squared displacements (the so-called dynamic transition) that takes place in hydrated biomolecules at T ≈ 200-220 K.