The microscopic picture of protein dynamics provides insight into the protein functionality. It was accepted earlier by many researchers that protein dynamics and activity are related to the solvent and its viscosity. However the detailed mechanism of the solvent-protein interactions is not fully understood. On the other hand, a connection between the appearance of measurable activity and the “dynamic transition” in proteins has been observed. The “dynamic transition” is marked by a sharp rise in the mean squared atomic displacement in proteins occurring at TD ~ 200-230 K. Many contradicting models have been proposed to describe the origin of this phenomenon including a recent idea that relates it to the sudden change in a property of the hydration water. After decades of studies, the origin of the dynamic transition and the role of solvent in protein dynamics remain a subject of active discussions.
Combining dielectric spectroscopy and neutron scattering techniques, we are able to follow protein dynamics over an extremely broad frequency and temperature range. We identify several relaxation processes in dielectric spectra of the hydrated lysozyme. We assign the main observed dielectric relaxation process to the structural relaxation of the protein-water coupled motion. Based on analysis of neutron spectroscopy and simulations results, we ascribe the slower dielectric relaxation process to a global large-scale motion of the protein.
We demonstrate that the sharp rise in mean squared atomic displacement is just a result of the protein-water coupled relaxation reaching the limit of the experimental frequency window of the neutron spectrometer. Our results show no sharp change in temperature variation of the structural relaxation of both the protein and its hydration water. Light scattering measurements of hydrated lysozyme indicate a broad glass transition at Tg ~ 180±15 K. We emphasize that the dynamic transition (as measured by mean squared atomic displacement) and the glass transition of the system happening at Tg ~ 180±15 K are not the same phenomena.
A strong coupling of protein activity and its dynamics has been observed in protein solutions in glycerol-water and in sucrose-water. The analysis, however, indicates that protein dynamics and its activity are not always coupled to the solvent viscosity. The microscopic mechanism of this decoupling remains unclear.