Dielectric investigation of the temperature dependence of the dynamics of a hydrated protein

We have investigated the dielectric properties, as a function of frequency and temperature, of the hydrated globular protein lysozyme. A large data set made up by measurements performed with a conventional capacitance bridge and with a dielectric broadband apparatus on the same sample has allowed us to identify two dielectric dispersions. The main dispersion is due to migrating protons along hydrogen bonded water molecules adsorbed on the protein surface. The dynamics of these charges has many analogies with that of proton glasses and other glass forming materials. Remarkably, the proton glass transition temperature, T-g, coincides with the temperature indicating the onset of hydrogen bond network dynamics at the protein water interface. On the other hand, the second dispersion does not show any glassy, behavior; this is attributed to small scale motions of protein side chains triggered by solvent, thus providing compelling evidence of the interplay between hydration water and protein dynamics, with possible implication for catalytic activity.