Dynamical Transition of Collective Motions in Dry Proteins

Water is widely assumed to be essential for protein dynamics and function. In particular, the well-documented ``dynamical'' transition at $\ensuremath{\sim}200\text{ }\text{ }\mathrm{K}$, at which the protein changes from a rigid, nonfunctional form to a flexible, functional state, as detected in hydrogenated protein by incoherent neutron scattering, requires hydration. Here, we report on coherent neutron scattering experiments on perdeuterated proteins and reveal that a transition occurs in dry proteins at the same temperature resulting primarily from the collective heavy-atom motions. The dynamical transition discovered is intrinsic to the energy landscape of dry proteins.