Protein dynamics in a broad frequency range: Dielectric spectroscopy studies

We present detailed dielectric spectroscopy studies of dynamics in two hydrated proteins, lysozyme and myoglobin. We emphasize the importance of explicit account for possible Maxwell-Wagner (MW) polarization effects in protein powder samples. Combining our data with earlier literature results, we demonstrate the existence of three major relaxation processes in globular proteins. To understand the mechanisms of these relaxations we involve literature data on neutron scattering, simulations and NMR studies. The faster process is ascribed to coupled protein-hydration water motions and has relaxation time ~ 10-50 ps at room temperature. The intermediate process is ~ 10²-10³ times slower than the faster process and might be strongly affected by MW polarizations. Based on the analysis of data obtained by different experimental techniques and simulations, we ascribe this process to large scale domain-like motions of proteins. The slowest observed process is ~ 10⁶-10⁷ times slower than the faster process and has anomalously large dielectric amplitude Δε ~ 10²-10⁴. The microscopic nature of this process is not clear, but it seems to be related to the glass transition of hydrated proteins. The presented results suggest a general classification of the relaxation processes in hydrated proteins.