The two-step scenario of the protein dynamical transition

The “protein dynamical transition” (PDT) characterizes the abrupt loss of structural flexibility at a particular temperature and time scale in response to the glass transition of protein hydration water. The water-coupled structural degrees of freedom interact with the protein via hydrogen bonds, causing fluctuations, which can be probed by dynamic neutron scattering experiments. To emphasize the properties of hydration water a perdeuterated protein C-PC hydrated with H₂O is investigated together with native myoglobin. The respective intermediate scattering function of hydration water displays a two-step decay involving fast local re-orientational fluctuations and a slow collective relaxation. The anharmonic onset in the mean squared displacements, which is generally used to identify the PDT, is derived from the properties of the intermediate scattering function at the time given by the resolution of the spectrometer. It is shown that the onset temperature depends on the shape of the relaxation time spectrum. A shape-independent transition temperature T_Δ is defined, associated with the main structural relaxation, which decreases with increasing resolution. A second onset is identified near the glass temperature T_g, which is related to the initial decay of the intermediate scattering function. This onset is independent of the instrumental resolution and causes a change in molecular elasticity and thermal expansion. With this approach a more precise definition of the PDT is given, providing answers to the critical questions about the nature and the mechanism of the effect.