Understanding the molecular dynamics and diffusivity of proteins at cryogenic temperatures is essential for optimizing cryopreservation techniques of biological materials, with applications ranging from biotechnology to food science. This knowledge is also relevant for organism living under extreme conditions, such as the sub-zero temperatures of the Arctic Sea. A key feature observed at cryogenic temperatures is the protein dynamic transition near T = 230 K, where proteins lose their flexibility and functionality. The nature of this transition is still elusive, also due to the experimental challenge posed from the crystallization of water at these low temperatures. We investigate the structural dynamics of proteins under supercooled conditions with two approaches: hydrated proteins, where the absence of bulk water prevents freezing, and cryoprotected protein solutions, where cryoprotectants lower the water melting point.

We employ existing X-ray scattering techniques, namely small- and wide- angle X-ray scattering. Additionally, we advance the development of X-ray Photon Correlation Spectroscopy for studying biological systems. In hydrated lysozyme, we observe water temperature-dependent structural changes with a crossover at T = 230 K. Notably, nanoscale dynamics of hydrated proteins reveal enhanced density fluctuations at the same temperature, consistent with the crossing of the hypothesized Widom line in bulk water. This finding suggest a clear link between the protein dynamic transition and the water properties. We extend these studies to cryoprotected ferritin solutions. We explore the collective dynamics of proteins at molecular length scales and observed anomalous diffusion, which was enhanced with increasing protein concentration. Furthermore, we detect a deviation from the Stokes-Einstein relation and a shift in the arrest temperature of the solvent to lower temperature, likely caused by the presence of proteins, which significantly alter the local solvent environment. These results suggests that protein mobility near glassy conditions and at supercooled temperatures may differ drastically from predictions based on solvent viscosity.