Water Dynamics in Protein Solutions
Zhe Zhang (Oak Ridge National Laboratory)
Water and water-containing systems are ubiquitous in nature. Generally, water can be categorized into two populations in water-containing systems: a) bulk water, which exists away from the solute, and therefore, the atomistic and molecular interactions can be ignored; and b) interfacial water, which is usually found close to the solute or interfaces having different properties from bulk water. The interfacial water molecules often demonstrate extraordinary structural and dynamical properties compared to those of the bulk water molecules, and the efforts are still being made to understand the structure and dynamics of interfacial water. Polyethylene oxide (PEO) and Pluronic block copolymer (e.g. L62) aqueous solution are used as model systems in our research. PEO has been widely used as polyelectrolytes in batteries. For PEO solution, we combined quasi-elastic neutron scattering experiments and atomistic molecular dynamics (MD) simulation to study the effect of lithium ions (Li+) on the dynamics of both bulk water and interfacial water. A detailed analysis of microstructure of PEO-Li+-Water complex indicated a cage conformation surrounding the ions. A further investigation with sodium and potassium ions has revealed that the stability of PEO cage is determined by the ionic radius of ions. The composition of L62 is PEO6-PPO34-PEO6, where PPO stands for the polypropylene oxide. The small-angle neutron scattering (SANS) experiments and MD simulation have been applied to understand the distribution of water molecules and water dynamics in different layers of lamellar membrane formed by L62. Water molecules at bulk area are following normal diffusion, while that at lamellar layers are following sub-diffusion.
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