The two-step scenario of the protein dynamical transition

In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: a b s t r a c t a r t i c l e i n f o 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 H 2 O 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. The " protein dynamical transition " (PDT) denotes the abrupt loss of structural flexibility in response to the freezing of hydration water [1–3]. A thin layer of protein-adsorbed water remains liquid well below the freezing point and, instead of crystallizing, turns into an amorphous solid at T g ≈170 K. The term " liquid " implies viscous flow and long range diffusion in the hydration shell. The water-coupled protein degrees of freedom thus stay mobile at subzero temperatures, but become structurally arrested at T g. These motions comprise a particular class of fluctuations, which are controlled by polar interactions, essentially by hydrogen …