X-ray scattering studies of supercooled hydrated proteins

by Maddalena Bin (Stockholm University)

Monday 13 Mar 2023, 13:00 → 15:00 Europe/Stockholm

Albano 3: 6228 - Mega (22 seats) (Albano Building 3)

The protein low-temperature crossover, also called protein dynamic transition, has been observed at ≈220 K in hydrated proteins. Below this transition temperature, proteins show reduced conformational flexibility and lose their biological function. To date, different views have been proposed to explain the origin of the low-temperature crossover. One hypothesis associates the transition with the intrinsic temperature dependence of the protein dynamics, which shows a crossover from harmonic to anharmonic regime. A second hypothesis, instead, proposes that the solvent, i.e. water, is responsible for the observed low-temperature crossover. It is postulated that water exhibits a fragile-to-strong transition in the same temperature range, in connection to the hypothesized low- and high-density liquid phases of water. Resolving the mechanism of the protein dynamic transition is essential for understanding protein dynamics and function influencing diverse biochemical mechanisms, such as enzymatic activity and protein folding. This can fundamentally change the way we view water as a specific requirement for life and can lead to applications in cryobiology and cryopreservation. 

For this purpose, we investigated the structural changes of hydrated proteins upon cooling utilizing a combination of X-ray diffraction and molecular dynamics simulations. The latter allows us to separate the contributions to the scattering intensity arising from the protein and its hydration water, finding that the protein scattering intensity is mainly temperature-independent, whereas hydration water shows changes with temperature. Secondly, we studied the dynamics of hydrated proteins in the nanometer length scale using coherent X-ray scattering, i.e. X-ray Photon Correlation Spectroscopy (XPCS). We found increased dynamical heterogeneity at 227 K, at which temperature the fluctuations observed in the two-time correlation functions are enhanced. Both studies are consistent with the proposed liquid-liquid critical point hypothesis of water, which has received experimental support in recent studies. However, hydration water and bulk water can exhibit different physical properties as hydration water is confined in the protein matrix. Therefore, in order to elucidate the mechanism behind the protein dynamic transition and the role of water still requires additional investigations. Future studies include the crowding effect of proteins as in vivo systems show a deviation from the normal diffusion due to the high concentration of biomolecules and increased viscosity. In these systems, intracellular water includes a significant proportion of hydration water, which needs to be further investigated in order to understand its role in life molecules.