Dynamics of biopolymers and their hydration water studied by neutron and X-ray scattering

Abstract

Protein functions are intimately related to their dynamics. Moreover, protein hydration water is believed to have significant influence on the dynamics of proteins. One of the evidence is that both protein and its hydration water have the same dynamic transition temperature at around 220 K. This thesis intends to understand the dynamic coupling of biopolymers (mainly proteins) and their hydration water by means of neutron and X-ray scattering. We first approach this problem by studying the dynamics of hydration water and the dynamics of hydrated proteins respectively. We study the hydration water dynamics by using elastic neutron scattering (ENS) and quasielastic neutron scattering (QENS). We observe a fragile-to-strong crossover phenomenon at the temperature TL in supercooled water confined in substrates with different hydrophilicity and geometry. We find that water confined in hydrophobic double wall carbon nanotubes (DWNT) has a slightly lower TL than that confined in hydrophilic MCM-41-S (a porous silica material). We then observe an interesting phenomenon that TL of water confined in a hydrophobic mesoporous material CMK-1 occurs in-between the above two. Our results indicate that besides the obvious surface effect brought about by the hydrophobic confinements, the value of the crossover temperature is also dependent on the dimensionality of the confinement. This result provides a possible way of understanding the effect of pressure on protein-hydration-water system. The crossover temperature TL can be used as an indicator of the hydrophilicity of the protein surface. Meanwhile, we use three different methods to study the protein dynamics in the full time range. We study the protein softness in the long-time a-relaxation region by measuring the mean squared displacement (MSD) using ENS. We then use QENS to study the logarithmic decay of protein dynamics in the mid-time p-relaxation range. In addition, Inelastic X-ray scattering (IXS) is used to study the phonon dispersion relation (short-time dynamics) inside the protein molecules and thus help us to understand the intra-protein collective dynamics. Finally, the coupled dynamics of the hydration water and the protein is studied. A series of ENS and QENS experiments are performed at different temperatures and pressures in order to investigate this problem. We find that the dynamics of protein follows that of its hydration water and proteins remain soft at lower temperatures under pressure. We also relate this phenomenon to the existence of the second critical point in the hydration water. The comparison of experimental data with computer simulations (MC and MD) elucidates the physical origin of the coupling between the dynamics of protein and its hydration water.