Molecular Simulation of Water and Ion Motion in Lysozyme Crystals

The dynamics of water and chloride counter ions (Cl) in single unit cells of orthorhombic and tetragonal hen egg white lysozyme lattices are investigated by 5 ns molecular dynamics simulations. The electrostatic interaction inside the crystal channel significantly influences the diffusion of chloride ions. The corresponding diffusion constants of Cl calculated by averaging over all ions during the 5 ns time scale for the orthorhombic and tetragonal lysozyme unit cells are (1.604 ± 0.05)×10 and (0.95 ± 0.05)×10 cms, respectively. According to our results, water molecules close to the protein surface jump between hydration sites with a broad range of site residence time, and show a higher mobility than water molecules further away from the protein surface. The results are of interest to study ion and water transport through well-defined biological nanopores and may elucidate more features of water-protein and ion-protein interactions in protein crystals.