Molecular Dynamics Simulation of Water and Ions in Nanopores of Lysozyme Protein Crystal

Highly porous cross-linked protein crystals are a novel class of nanoporous materials with vast applications in biocatalysis and selective separation membranes. Long time equilibrium molecular dynamics (MD) simulations were performed to study the behavior of water and ions in the nanopores of lysozyme protein crystals. Pore size profile along different axes showed that the main pores lie along the z-axis consisting of an anisotropic structure. The morphology of pore network and pore sizes, influence water dynamics in protein crystals. Transport properties of water molecules were investigated under two diffusion regimes around protein surface, i.e. surface and core zone. Results showed that water molecules near the surface zone had the anomalous diffusion behavior while the behavior in the core zone was diffusive. Moreover, an anisotropic diffusion behavior was occurred along different axes in accordance to experimental predictions. Simulations demonstrated that nearly 16 percent of water molecules have the residence time above 100 ps at the first hydration layer around the protein crystal, while 3.3 percent of those remain in the cavities over a longer time of about 1400 ps. The behavior of chloride counter ions in the first hydration layer around the protein crystal or on the specific residues of the crystal was investigated as well. The simulation results were in a good agreement with the previous theoretical studies and experimental data. This study provides valuable insights into understanding the transport phenomena in the protein crystals in view of the nature of solvent-protein and ion-protein interactions.