Protein dynamics by neutron scattering Original Research Article

Neutron scattering techniques represent a powerful tool for characterizing both the structure and dynamical properties of bio-systems, for example, proteins and membranes interacting with their solvents. In this paper, Elastic Neutron Scattering (ENS) data collected at the Institut Laue-Langevin (Grenoble, France) on dry and D2O hydrated lysozyme by varying hydration level are presented, and compared with previously published data on the same protein system, also with the addition of bio-protectants. The data have been collected with three different spectrometers, i.e. IN13, IN10 and IN4. This set of ENS data gives direct access to the temperature behavior of both (i) the Mean Square Displacement (MSD) and (ii) the characteristic system relaxation time. As a result, an explicative hypothesis on the relationship between the so-called “protein dynamical transition” (PDT) and the “fragile-to-strong dynamical crossover” (FSC) is formulated. Furthermore, by taking into proper account the effect of the finite instrumental energy resolution of the used spectrometers, the vibrational MSD of dry and hydrated lysozyme is calculated. The vibrational MSD of the lysozyme in the dry state resulted to be higher than the one in the hydrated state; the latter reaches the former at a temperature value of T = 220 K that corresponds to the temperature at which the FSC occurs. As a result, a cage effect resulting from the hydration water on the protein surface is hypothesized and subsequently linked to the FSC.