(57b) Simulating Water Sorption On Protein Matrices

Understanding the role that hydrating water plays in governing the structure, dynamics, and biological activity of proteins is crucial to identifying key mechanisms for maintenance and repair of biological substrates, mitigating the effects of extreme environmental conditions, and developing strategies for long-term preservation of proteins. Although a vast number of experimental studies demonstrate strong correlations between solvent nature and protein behavior, identification of specific mechanisms responsible for protein-solvent coupling remains a formidable challenge. A notable example is the putative protein dynamical transition¹ (PDT), which is often associated with abrupt changes in the magnitudes of atomic fluctuations at ~220 K as measured using a variety of techniques, including quasi-elastic neutron scattering^1,2, Mossbauer spectroscopy³, and terahertz dielectric response⁴.   Reducing the amount of adsorbed water hydrating the protein system suppresses the PDT⁵, shifting it to higher temperatures, until the signatures vanish completely under extreme dehydration conditions^2,5. However, despite great efforts to characterize the PDT, a consensus has not been reached regarding its mechanistic origins⁶. This is in part due to the limited ability of available empirical techniques to fully resolve molecular structure and motions of protein systems at the nanoscale⁶.  Our aim is to use advanced molecular simulation techniques to investigate the structural and dynamic changes that occur in protein systems upon rehydration at a resolution that is beyond the current capabilities of empirical methods. Our recent efforts have resulted in the development of a new simulation protocol to compute water sorption isotherms (hydration level vs. relative humidity) for protein systems, allowing for the first time, for a rigorous connection to be made between the simulated state conditions and those imposed during real experiments.   Using our new approach, we have calculated water sorption isotherms for crystals and powders of Ubiquitin and Lysozyme.   We find that the simulated isotherms and enthalpies of hydration are in good qualitative agreement with experimental measurements, suggesting that our approach provides a reasonable description of the hydration thermodynamics.   We also show that the dynamical properties of the proteins are altered during the hydration process, and that they exhibit glassy behavior at low hydration levels and gradually recover internal flexibility as water content is increased.