Direct Experimental Characterization of Contributions from Self-Motion of Hydrogen and from Interatomic Motion of Heavy Atoms to Protein Anharmonicity

Decoupling of the onset of anharmonicity between a protein and its surface water around 200 K

The protein dynamical transition at ~200 K, where the biomolecule transforms from a harmonic, non-functional form to an anharmonic, functional state, has been thought to be slaved to the thermal activation of dynamics in its surface hydration water. Here, by selectively probing the dynamics of protein and hydration water using elastic neutron scattering and isotopic labeling, we found that the onset of anharmonicity in the two components around 200 K is decoupled. The one in protein is an intrinsic transition, whose characteristic temperature is independent of the instrumental resolution time, but varies with the biomolecular structure and the amount of hydration, while the one of water is merely a resolution effect.

An experimentally representative in-silico protocol for dynamical studies of lyophilised and weakly hydrated amorphous proteins

Characterization of biopolymers in both dry and weakly hydrated amorphous states has implications for the pharmaceutical industry since it provides understanding of the effect of lyophilisation on stability and biological activity. Atomistic Molecular Dynamics (MD) simulations probe structural and dynamical features related to system functionality. However, while simulations in homogenous aqueous environments are routine, dehydrated model assemblies are a challenge with systems investigated in-silico needing careful consideration; simulated systems potentially differing markedly despite seemingly negligible changes in procedure. Here we propose an in-silico protocol to model proteins in lyophilised and weakly hydrated amorphous states that is both more experimentally representative and routinely applicable. Since the outputs from MD align directly with those accessed by neutron scattering, the efficacy of the simulation protocol proposed is shown by validating against experimental neutron data for apoferritin and insulin. This work also highlights that without cooperative experimental and simulative data, development of simulative procedures using MD alone would prove most challenging.

Direct Observation of the Mutual Coupling Effect in the Protein-Water-Glycerol Mixture by Combining Neutron Scattering and Selective Deuteration

Numerous studies have discussed the impact of cosolvents on the structure, dynamics, and stability of proteins in aqueous solutions. However, the dynamics of cosolvents in the protein-water-cosolvent ternary system is largely unexplored in experiments due to technical difficulty. Consequently, a comprehensive understanding of the interplay among proteins, water, and cosolvents is still lacking. Here, we employed selective deuteration and neutron scattering techniques to characterize the individual motions of each component in the protein/water/glycerol (GLY) mixture across various temperatures. The consistent dynamic onset temperatures and the correlation between the MSD of the protein and the viscosity of solvents revealed the mutual coupling effects among the three components. Furthermore, our experimental and simulation results showed that the hydrogen bond relaxation energy barrier in the ternary system is ∼43 kJ/mol, whereas in the protein-water binary system it is merely ∼35 kJ/mol. Therefore, we suggest that GLY can enhance hydrogen bond interactions in the ternary system through the mutual coupling effect, thereby serving as one of the protective mechanisms of protein preservation by GLY.

Universal dynamical onset in water at distinct material interfaces

Interfacial water remains liquid and mobile much below 0 °C, imparting flexibility to the encapsulated materials to ensure their diverse functions at subzero temperatures. However, a united picture that can describe the dynamical differences of interfacial water on different materials and its role in imparting system-specific flexibility to distinct materials is lacking. By combining neutron spectroscopy and isotope labeling, we explored the dynamics of water and the underlying substrates independently below 0 °C across a broad range of materials. Surprisingly, while the function-related anharmonic dynamical onset in the materials exhibits diverse activation temperatures, the surface water presents a universal onset at a common temperature. Further analysis of the neutron experiment and simulation results revealed that the universal onset of water results from an intrinsic surface-independent relaxation: switching of hydrogen bonds between neighboring water molecules with a common energy barrier of ∼35 kJ mol⁻¹.

We demonstrated that the dynamical onset of interfacial water is an intrinsic property of water itself, resulting from a surface independent relaxation process in water with an approximately universal energy barrier of ∼35 kJ mol⁻¹.

Anomalous sub-diffusion of water in biosystems: From hydrated protein powders to concentrated protein solution to living cells

Water is essential to life and its translational motion in living systems mediates various biological processes, including transportation of function-required ingredients and facilitating the interaction between biomacromolecules. By combining neutron scattering and isotopic labeling, the present work characterizes translational motion of water on a biomolecular surface, in a range of systems: a hydrated protein powder, a concentrated protein solution, and in living Escherichia coli (E. coli) cells. Anomalous sub-diffusion of water is observed in all samples, which is alleviated upon increasing the water content. Complementary molecular dynamics simulations and coarse-grained numerical modeling demonstrated that the sub-diffusive behavior results from the heterogeneous distribution of microscopic translational mobility of interfacial water. Moreover, by comparing the experimental results measured on E. coli cells with those from a concentrated protein solution with the same amount of water, we show that water in the two samples has a similar average mobility, however the underlying distribution of motion is more heterogeneous in the living cell.