For all MD simulations, the Amber package with ff03 forcefield was used . Each model was first optimized and then heated from 0 to 303 K with the NVT ensemble. Then 100 ps with NPT conditions were simulated to equilibrate the system. Finally, 60 nanoseconds of molecular dynamics simulation with periodic boundary conditions, controlled with a Langevin thermostat were conducted. A cutoff for nonbonding interactions was set to 8.0 Å.