The dynamical transition in proteins
Wolfgang Doster, Technische Universität München, Physik Department E 13, D-85747 Garching, Germany
The onset of non-harmonic motion in proteins at a particular temperature has been interpreted as a dynamical transition from solid to liquid-like behaviour. In the literature several transitions have been invoked. In this contribution we discriminate between protein-internal molecular motions, which follow an Arrhenius law, and water-coupled displacements, which show a super-Arrhenius temperature dependence. Only in the latter case is it justified to talk about a dynamical transition in close analogy to the liquid-glass transition.
We introduce a moment analysis of the neutron scattering function in the time domain to reconstruct the displacement distribution function. Hydration water displays two dynamical components, related to librational motions and anomalous diffusion along the protein surface. Rotational transitions of side-chains, in particular of methyl groups, persist in the dehydrated and in the solvent-vitrified protein structure. The interaction with water induces further continuous protein motions on a small scale. Water acts as a plasticizier to protein displacements, which couple to functional processes such as open-closed transitions and ligand exchange.
Reference: W. Doster and M. Settles, Biochim.Biophys. Acta 1749 (2005) 173-186
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