The dynamic transition in proteins may have a simple explanation

Abstract

The transition that has been observed in the dynamics of hydrated proteins at low temperatures (180-230 K) is normally interpreted as a change from vibrational, harmonic motion at low temperatures to anharmonic motions as the temperature is raised. It is taken to be an intrinsic property of proteins and has been associated with the onset of protein functions. Examination of the dynamic behaviour of proteins in solution within a defined timescale window suggests that certain observations can be explained without the need to invoke a discontinuity in the dynamics of proteins with temperature, i.e. the existence of a dynamical transition is not required. This is discussed in the context of recent evidence that enzyme activity is independent of the activation of anharmonic picosecond dynamics and declines steadily with temperature through the apparent dynamic transition, in accordance with the Arrhenius relationship. That similar timescale dependent dynamical behaviour has been observed experimentally in chain polymers, and seen also in computer simulations of silica glasses, suggests that the phenomenon may be of wide general relevance in both simple glassy and more complex polymeric systems.

Enzyme hydration, activity and flexibility: A neutron scattering approach

“…This observation is also in accord with recent molecular dynamics simulation results which indicate that deviation from harmonic dynamics in a small protein in vacuo is apparent at temperatures as low as 120 K [36,37], and also with experimental results demonstrating the appearence of a transition at similarly-low temperatures in a membrane protein [38]. In experiments on hydrated powders or enzyme solutions the change in slope at 200-250 K was found to be dependent on the energy resolution [39,40], indicating that the apparent position of the dynamical transition depends on the timescale explored [31,30] consistent with molecular dynamics simulations that indicate that the transition does not involve an abrupt change from harmonic to anharmonic dynamics [41]. At a low enough temperature the quasielastic scattering should no longer be visible, as the associated relaxation time will have moved to slower timescales than can be detected on the instrument.…”

Slowdown of Interhelical Motions Induces a Glass Transition in RNA

“…We expect that dehydrated RNAs have a different sea structure, the dynamics of which remain superpositions of harmonic motions. This view is consistent with a model that collectively accounts for all evidence to date simply by assuming that motions of biomolecules in solution slow down continuously with temperature (68). In this model, the observed dynamical transition may be Biophysical Journal 108(12) 2876-2885 no more than the appearance of motions within the observational timescale window of the experimental apparatus used.…”

Broadened dielectric loss spectra and reduced dispersion strength of viscous glycerol in a connective tissue protein

“…However, a recent conductivity study of the lysozyme-glycerol system demonstrated that the ion transport dynamics are coupled with the structural relaxation of glycerol, i.e., no anomalies were found at and near T D [17]. Rather, on the basis of neutron scattering experiments, the existence of a dynamic protein transition has been questioned [18], thereby confirming reports that T D can depend on the experimental time window [19,20].…”